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THE HARVARD CLASSICS

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THE HARVARD CLASSICS
EDITED BY CHARLES W. ELIOT, LL.D.

Scientific Papers

Physics • Chemistry
Astronomy • Geology

W//A Introductions and 'Notes
Volume 30

P. F. Collier & Son Corporation

NEW YORK

Copyright, 1910
By p. F. Collier & Son

manufactured in u. s. a.

CONTENTS

THE FORCES OF MATTER

Delivered before a Juvenile Auditory at the Royal Institution
OF Great Britain during the Christmas Holidays of 1859-60

PAGE

Lecture I. — The Force of Gravitation 7

Lecture IL — Gravitation — Cohesion 22

Lecture IIL — Cohesion — Chemical Affinity 37

Lecture IV. — Chemical Affinity — Heat 50

Lecture V. — Magnetism — Electricity 61

Lecture VI. — The Correlation of the Physical Forces 73

THE CHEMICAL HISTORY OF A CANDLE

Lecture I. — A Candle: The Flame — Its Sources — Structure

— Mobility — Brightness 86

Lecture II. — A Candle: Brightness of the Flame — Air Neces-
sary for Combustion — Production of Water .... loi

Lecture III. — Products: Water from the Combustion — Na-
ture OF Water — A Compound — Hydrogen 113

Lecture IV. — Hydrogen in the Candle — Burns into Water —
The Other Part of Water — Oxygen 128

Lecture V. — Oxygen Present in the Air — Nature of the
Atmosphere — Its Properties — Other Products from the
Candle — Carbonic Acid — Its Properties 141

Lecture VI. — Carbon or Charcoal — Coal-Gas — Respiration

and its Analogy to the Burning of a Candle — Conclusion 156

ON THE CONSERVATION OF FORCE

Introduction to a Series of Lectures Delivered at Carlsruhe

in the Winter of i 862-1 863 173

2 CONTENTS

ICE AND GLACIERS

PAGE

A Lecture Delivered at Frankfort-on-the-Main, and at

Heidelberg, in February, 1865 211

THE WAVE THEORY OF LIGHT

[A Lecture Delivered at the Academy of Music, Philadelphia,
Under the Auspices of the Franklin Institute, September
29TH, 1884] 251

THE TIDES

[Evening Lecture to the British Association at the Southamp-
ton Meeting, Friday, August 25TH, 1882] 274

THE EXTENT OF THE UNIVERSE 311

GEOGRAPHICAL EVOLUTION 325

THE FORCES OF MATTER
THE CHEMICAL HISTORY OF A CANDLE

BY

MICHAEL FARADAY

INTRODUCTORY NOTE

Michael Faraday was the son of a blacksmith, and was born at New-
ington Butts, near London, September 22, 1791. He began life as an
errand boy to a bookbinder and stationer, to whom he was later bound
apprentice. After eight years in this business, he was engaged by Sir
Humphry Davy as his laboratory assistant at the Royal Institution, and
in 1813-15 he traveled extensively on the Continent with his master, and
saw some of the most famous scientists of Europe. Shortly after his return
to the Royal Institution, he began to make contributions of his own to
science, his first paper appearing in 1816. He became director of the
laboratory in 1825, and professor of chemistry in 1833; rising rapidly,
through the number and importance of his discoveries, to a most dis-
tinguished position. But he was working at too great pressure, and in
1 84 1 his health gave way, so that for some three years he could not work
at all. He recovered, however, and made some of his most important
discoveries after this interruption; and was offered, but declined, the
presidency of both the Royal Society and the Royal Institution. He died
August 25, 1867.

It was characteristic of Faraday's devotion to the enlargement of the
bounds of human knowledge that on his discovery of magneto-electricity
he abandoned the commercial work by which he had added to his small
salary, in order to reserve all his energies for research. This financial
loss was in part made up later by a pension of /300 a year from the
British Government.

Faraday's parents were members of the obscure religious denomination
of the Sandemanians, and Faraday himself, shortly after his marriage, at
the age of thirty, joined the same sect, to which he adhered till his death.
Religion and science he kept strictly apart, believing that the data of
science were of an entirely different nature from the direct communica-
tions between God and the soul on which his religious faith was based.

The discoveries made by Faraday were so numerous, and often demand
so detailed a knowledge of chemistry and physics before they can be
understood, that it is impossible to attempt to describe or even enumerate
them here. Among the most important are the discovery of magneto-
electric induction, of the law of electro-chemical decomposition, of the
magnetization of light, and of diamagnetism. Round each of these are
grouped numbers of derivative but still highly important additions to
scientific knowledge, and together they form so vast an achievement as

6 INTRODUCTORY NOTE

to lead his successor, Tyndall, to say, "Taking him for all and all, I think
it will be conceded that Michael Faraday was the greatest experimental
philosopher the world has ever seen; and I will add the opinion, that the
progress of future research will tend, not to dim or to diminish, but to
enhance and glorify the labours of this mighty investigator."

In spite of the highly technical nature of his work in research, Faraday
was remarkably gifted as an expounder of science to popular audiences;
and his lectures at the Royal Institution, especially those to younger audi-
ences, were justly famous. The following examples are classics in the
department of clear and fascinating scientific exposition.

THE FORCES OF MATTER

DELIVERED BEFORE A JUVENILE AUDITORY AT THE
ROYAL INSTITUTION OF GREAT BRITAIN
DURING THE CHRISTMAS HOLI-
DAYS OF 1859-60

By Michael Faraday

LECTURE I
THE FORCE OF GRAVITATION

IT grieves me much to think that I may have been a cause of
disturbance to your Christmas arrangements,* for nothing is
more satisfactory to my mind than to perform what I under-
take; but such things are not always left to our own power, and we
must submit to circumstances as they are- appointed. I will to-day
do my best, and will ask you to bear with me if I am unable to give
more than a few words; and, as a substitute, I will endeavor to
make the illustrations of the sense I try to express as full as possible;
and if we find by the end of this lecture that we may be justified in
continuing them, thinking that next week our power shall be greater,
why then, with submission to you, we will take such course as you
may think fit, either to go on or discontinue them; and although I
now feel much weakened by the pressure of the illness (a mere cold)
upon me, both in facility of expression and clearness of thought, I
shall here claim, as I always have done on these occasions, the right
of addressing myself to the younger members of the audience; and
for this purpose, therefore, unfitted as it may seem for an elderly,
infirm man to do so, I will return to second childhood, and become,
as it were, young again among the young.

Let us now consider, for a little while, how wonderfully we stand
upon this world. Here it is we are born, bred, and live, and yet we
*The opening lecture was twice postponed on account of Dr. Faraday's illness.

7

8 FARADAY

view these things with an almost entire absence of wonder to our-
selves respecting the way in which all this happens. So small, indeed,
is our wonder, that we are never taken by surprise; and I do think
that, to a young person of ten, fifteen, or twenty years of age, per-
haps the first sight of a cataract or a mountain would occasion him
more surprise than he had ever felt concerning the means of his own
existence; how he came here; how he lives; by what means he stands
upright; and through what means he moves about from place to
place. Hence, we come into this world, we live, and depart from it,
without our thoughts being called specifically to consider how all this
takes place; and were it not for the exertions of some few inquiring
minds, who have looked into these things, and ascertained the very
beautiful laws and conditions by which we do live and stand upon
the earth, we should hardly be aware that there was any thing
wonderful in it. These inquiries, which have occupied philosophers
from the earliest days, when they first began to find out the laws by
which we grow, and exist, and enjoy ourselves, up to the present
time, have shown us that all this was effected in consequence of the
existence of certain forces, or abilities to do things, or powers, that
are so common that nothing can be more so; for nothing is com-
moner than the wonderful powers by which we are enabled to stand
upright: they are essential to our existence every moment.

It is my purpose to-day to make you acquainted with some of these
powers; not the vital ones, but some of the more elementary, and
what we call physical powers; and, in the outset, what can I do to
bring to your minds a notion of neither more nor less than that
which I mean by the word power or force? Suppose I take this
sheet of paper, and place it upright on one edge, resting against a
support before me (as the roughest possible illustration of something
to be disturbed), and suppose I then pull this piece of string which
is attached to it. I pull the paper over. I have therefore brought
into use a power of doing so — the power of my hand carried on
through this string in a way which is very remarkable when we
come to analyze it; and it is by means of these powers conjointly
(for there are several powers here employed) that I pull the paper
over. Again, if I give it a push upon the other side, I bring into
play a power, but a very different exertion of power from the former;

THE FORCE OF GRAVITATION 9

or, if I take now this bit of shell-lac [a stick of shell-lac about 12
inches long and i 1-2 in diameter], and rub it with flannel, and
hold it an inch or so in front of the upper part of this upright sheet,
the paper is immediately moved towards the shell-lac, and by now
drawing the latter away, the paper falls over without having been
touched by any thing. You see, in the first illustration I produced
an effect than which nothing could be commoner; I pull it over
now, not by means of that string or the pull of my hand, but by some
action in this shell-lac. The shell-lac, therefore, has a power where-
with it acts upon the sheet of paper; and, as an illustration of the
exercise of another kind of power, I might use gunpowder with
which to throw it over.

Now I want you to endeavor to comprehend that when I am
speaking of a power or force, I am speaking of that which I used just
now to pull over this piece of paper. I will not embarrass you at
present with the name of that power, but it is clear there was a
something in the shell-lac which acted by attraction, and pulled the
paper over; this, then, is one of those things which we call power, or
jorce; and you will now be able to recognize it as such in whatever
form I show it to you. We are not to suppose that there are so very
many different powers; on the contrary, it is wonderful to think
how few are the powers by which all the phenomena of nature are
governed. There is an illustration of another kind of power in that
lamp; there is a power of heat — a power of doing something, but
not the same power as that which pulled the paper over; and so, by
degrees, we find that there are certain other powers (not many) in
the various bodies around us; and thus, beginning with the simplest
experiments of pushing and pulling, I shall gradually proceed to
distinguish these powers one from the other, and compare the way
in which they combine together. This world upon which we stand
(and we have not much need to travel out of the world for illustra-
tions of our subject; but the mind of man is not confined like the
matter of his body, and thus he may and does travel outward, for
wherever his sight can pierce, there his observations can penetrate)
is pretty nearly a round globe, having its surface disposed in a
manner of which this terrestrial globe by my side is a rough model;
so much is land and so much is water, and by looking at it here we

10 FARADAY

see in a sort of map or picture how the world is formed upon its
surface. Then, when we come to examine farther, I refer you to this
sectional diagram of the geological strata of the earth, in which
there is a more elaborate view of what is beneath the surface of our
globe. And, when we come to dig into or examine it (as man does
for his own instruction and advantage, in a variety of ways), we
see that it is made up of different kinds of matter, subject to a
very few powers; and all disposed in this strange and wonderful
way, which gives to man a history — and such a history — as to what
there is in those veins, in those rocks, the ores, the water-springs, the
atmosphere around, and all varieties of material substances, held
together by means of forces in one great mass, 8,000 miles in diameter,
that the mind is overwhelmed in contemplation of the wonderful
history related by these strata (some of which are fine and thin like
sheets of paper), all formed in succession by the forces of which I
have spoken.

I now shall try to help your attention to what I may say by
directing, to-day, our thoughts to one kind of power. You see what
I mean by the term matter — any of these things that I can lay hold
of with the hand, or in a bag (for I may take hold of the air by
inclosing it in a bag) — they are all portions of matter with which we
have to deal at present, generally or particularly, as I may require
to illustrate my subject. Here is the sort of matter which we call
water — it is there ice [pointing to a block of ice upon the table],
there water — [pointing to the water boiling in a flask] — here vapor —
you see it issuing out from the top [of the flask]. Do not suppose
that that ice and that water are two entirely different things, or that
the steam rising in bubbles and ascending in vapor there is absolutely
different from the fluid water: it may be different in some par-
ticulars, having reference to the amounts of power which it con-
tains; but it is the same, nevertheless, as the great ocean of water
around our globe, and I employ it here for the sake of illustration,
because if we look into it we shall find that it supplies us with
examples of all the powers to which I shall have to refer. For
instance, here is water — it is heavy; but let us examine it with regard
to the amount of its heaviness or its gravity. I have before me a
little glass vessel and scales [nearly equipoised scales, one of which

THE FORCE OF GRAVITATION II

contained a half-pint glass vessel], and the glass vessel is at present
the lighter of the tw^o; but if I now take some water and pour it in,
you see that that side of the scales immediately goes down; that
shows you (using common language, which I will not suppose for
the present you have hitherto applied very strictly) that it is heavy,
and if I put this additional weight into the opposite scale, I should
not wonder if this vessel would hold water enough to weigh it
down. [The lecturer poured more water into the jar, which again
went down.] Why do I hold the bottle above the vessel to pour the
water into it.'' You will say, because experience has taught me that it
is necessary. I do it for a better reason — because it is a law of nature
that the water should fall toward the earth, and therefore the very
means which I use to cause the water to enter the vessel are those
which will carry the whole body of water down. That power is
what we call gravity, and you see there [pointing to the scales] a
good deal of water gravitating toward the earth. Now here [ex-
hibiting a small piece of platinum(')] is another thing which
gravitates toward the earth as much as the whole of that water. See
what a little there is of it; that little thing is heavier than so much
water [placing the metal in opposite scales to the water]. What a
wonderful thing it is to see that it requires so much water as that
[a half-pint vessel full] to fall toward the earth, compared with the
little mass of substance I have herel And again, if I take this metal
[a bar of aluminium(^) about eight times the bulk of the platinum],
we find the water will balance that as well as it did the platinum;
so that we get, even in the very outset, an example of what we want
to understand by the words forces or powers,

I have spoken of water, and first of all of its property of falling
downward : you know very well how the oceans surround the globe
— how they fall round the surface, giving roundness to it, clothing
it like a garment; but, besides that, there are other properties of
water. Here, for instance, is some quicklime, and if I add some water
to it, you will find another power and property in the water. (^) It is

^Platinum, with one exception the heaviest body known, is 2i!4 times heavier than
water. ^ Aluminium is 2 Yi times heavier than water.

' Power or property in water. This power — the heat by which the water is kept
in a jiuid state — is said, under ordinary circumstances, to be latent or insensible. When,
however, the water changes its form, and, by uniting with the lime or sulphate of
copper, becomes solid, the heat which retained it in a liquid state is evolved.

12 FARADAY

now very hot; it is steaming up; and I could perhaps light phos-
phorus or a lucifer-match with it. Now that could not happen
without a force in the water to produce the result; but that force
is entirely distinct from its power of falling to the earth. Again,
here is another substance [some anhydrous sulphate of copperC)]
which will illustrate another kind of power. [The lecturer here
poured some water over the white sulphate of copper, which imme-
diately became blue, evolving considerable heat at the same time.]
Here is the same water with a substance which heats nearly as much
as the lime does, but see how differently. So great indeed is this
heat in the case of lime, that it is sufficient sometimes (as you see
here) to set wood on fire; and this explains what we have sometimes
heard, of barges laden with quicklime taking fire in the middle of
the river, in consequence of this power of heat brought into play
by a leakage of the water into the barge. You see how strangely
different subjects for our consideration arise when we come to think
over these various matters — the power of heat evolved by acting upon
lime with water, and the power which water has of turning this salt
of copper from white to blue.

I want you now to understand the nature of the most simple
exertion of this power of matter called weight or gravity. Bodies
are heavy; you saw that in the case of water when I placed it in the
balance. Here I have what we call a weight [an iron half cwt.] — a
thing called a weight because in it the exercise of that power of
pressing downward is especially used for the purposes of weighing;
and I have also one of these little inflated India-rubber bladders,
which are very beautiful although very common (most beautiful
things are common), and I am going to put the weight upon it, to
give you a sort of illustration of the downward pressure of the iron,
and of the power which the air possesses of resisting that pressure;
it may burst, but we must try to avoid that. [During the last few
observations the lecturer had succeeded in placing the half cwt. in
a state of quiescence upon the inflated India-rubber ball, which
consequently assumed a shape very much resembling a flat cheese
with round edges.] There you see a bubble of air bearing half a

* Anhydrous sulphate of copper: sulphate of copper deprived of its water of crystal-
lization. To obtain it the blue sulphate is calcined in an earthen crucible.

THE FORCE OF GRAVITATION I3

hundred-weight, and you must conceive for yourselves what a won-
derful power there must be to pull this weight downward, to sink
it thus in the ball of air.

Let me now give you another illustration of this power. You
know what a pendulum is. I have one here (Fig. 1), and if I set it
swinging, it will continue to swing to and
fro. Now I wonder whether you can tell me
why that body oscillates to and fro — that J

pendulum bob, as it is sometimes called. /

Observe, if I hold the straight stick horizon- /

tally, as high as the position of the ball at the /

two ends of its journey, you see that the ball ^

is in a higher position at the two extremities
than it is when in the middle. Starting from
one end of the stick, the ball falls toward the centre, and then rising
again to the opposite end, it constantly tries to fall to the lowest point,
swinging and vibrating most beautifully, and with wonderful prop-
erties in other respects — the time of its vibration, and so on — but
concerning which we will not now trouble- ourselves.

If a gold leaf, or piece of thread, or any other substance were hung
where this ball is, it would swing to and fro in the same manner,
and in the same time too. Do not be startled at this statement; I
repeat, in the same manner and in the same time, and you will see
by-and-by how this is. Now that power which caused the water to
descend in the balance — which made the iron weight press upon and
flatten the bubble of air — which caused the swinging to and fro o£
the pendulum, that power is entirely due to the attraction which
there is between the falling body and the earth. Let us be slow and
careful to comprehend this. It is not that the earth has any particular
attraction toward bodies which fall to it, but, that all these bodies
possess an attraction every one toward the other. It is not that the
earth has any special power which these balls themselves have not;
for just as much power as the earth has to attract these two balls
[dropping two ivory balls], just so much power have they in pro-
portion to their bulks to draw themselves one to the other; and the
only reason why they fall so quickly to the earth is owing to its
greater size. Now if I were to place these two balls near together,

14 FARADAY

I should net be able, by the most delicate arrangement of apparatus,
to make you, or myself, sensible that these balls did attract one
another; and yet we know that such is the case, because if, instead
of taking a small ivory ball, we take a mountain, and put a ball
like this near it, we find that, owing to the vast size of the mountain
as compared with the billiard ball, the latter is drawn slightly toward
it, showing clearly that an attraction does exist, just as it did between
the shell-lac which I rubbed and the piece of paper which was over-
turned by it.

Now it is not very easy to make these things quite clear at the
outset and I must take care not to leave anything unexplained as I
proceed, and, therefore, I must make you clearly understand that all
bodies are attracted to the earth, or, to use a more learned term,
gravitate. You will not mind my using this word, for when I say
that this penny-piece gravitates, I mean nothing more nor less than
that it falls toward the earth, and, if not intercepted, it would go on
falling, falling, until it arrived at what we call the centre of gravity
of the earth, which I will explain to you by-and-by.

I want you to understand that this property of gravitation is never
lost; that every substance possesses it; that there is never any change
in the quantity of it; and, first of all, I will take as illustration a
piece of marble. Now this marble has weight, as you will see if I put
it in these scales; it weighs the balance down, and if I take it off,
the balance goes back again and resumes its equilibrium. I can
decompose this marble and change it in the same manner as I can
change ice into water and water into steam. I can convert a part of
it into its own steam easily, and show you that this steam from the
marble has the property of remaining in the same place at common
temperatures, which water steam has not. If I add a little liquid to
the marble and decompose it Q), I get that which you see — [the
lecturer here put several lumps of marble into a glass jar, and poured

* Add a little liquid to the marble and decompose it. Marble is composed of carbonic
acid and lime, and, in chemical language, is called carbonate of lime. When sulphuric
acid is added to it, the carbonic acid is set free, and the sulphuric acid unites with the
lime to form sulphate of lime.

Carbonic acid, under ordinary circumstances, is a colorless invisible gas, about half
as heavy again as air. Dr. Faraday first showed that under great pressure it could be
obtained in a liquid state. Thilorier, a French chemist, afterward found that it could
be sdidiiied.

THE FORCE OF GRAVITATION 1 5

water and then acid over them; the carbonic acid immediately com-
menced to escape with considerable effervescence] — the appearance
of boiling, which is only the separation of one part of the marble
from another. Now this [marble] steam, and that [water] steam,
and all other steams, gravitate just like any other substance does;
they all are attracted the one toward the other, and all fall toward the
earth, and what I want you to see is that this steam gravitates. I
have here (Fig. 2) a large vessel placed upon a balance, and the

Fig. 2

moment I pour this steam into it you see that the steam gravitates.
Just watch the index, and see whether it tilts over or not. [The
lecturer here poured the carbonic acid out of the glass in which it
was being generated into the vessel suspended on the balance, when
the gravitation of the carbonic acid was at once apparent.] Look
how it is going down. How pretty that is! I poured nothing in but
the invisible steam, or vapor, or gas which came from the marble,
but you see that part of the marble, although it has taken the shape
of air, still gravitates as it did before. Now will it weigh down that
bit of paper? [placing a piece of paper in the opposite scale.] Yes,
more than that; it nearly weighs down this bit of paper [placing
another piece of paper in]. And thus you see that other forms of
matter besides solids and liquids tend to fall to the earth; and,
therefore, you will accept from me the fact that all things gravitate,
whatever may be their form or condition. Now here is another
chemical test which is very readily applied. [Some of the carbonic
acid was poured from one vessel into another, and its presence in
the latter shown by introducing into it a lighted taper, which was
immediately extinguished.] You see from this result also that it

1 6 FARADAY

gravitates. All these experiments show you that, tried by the balance,
tried by pouring like water from one vessel to another, this steam,
or vapor, or gas is, like all other things, attracted to the earth.

There is another point I want in the next place to draw your
attention to. I have here a quantity of shot; each of these falls
separately, and each has its own gravitating power, as you perceive
when I let them fall loosely on a sheet of paper. If I put them into a
bottle, I collect them together as one mass, and philosophers have
discovered that there is a certain point in the middle of the whole
collection of shots that may be considered as the one point in which
all their gravitating power is centred, and that point they call the
centre of gravity; it is not at all a bad name, and rather a short one —
the centre of gravity. Now suppose I take a sheet of pasteboard, or
any other thing easily dealt with, and run a bradawl through it at

Fig. 3

one corner, A (Fig. 3), and Mr. Anderson holds that up in his hand
before us, and I then take a piece of thread and an ivory ball, and
hang that upon the awl, then the centre of gravity of both the paste-
board and the ball and string are as near as they can get to the
centre of the earth; that is to say, the whole of the attracting power
of the earth is, as it were, centred in a single point of the cardboard,
and this point is exactly below the point of suspension. All I have
to do, therefore, is to draw a line, A B, corresponding with the
string, and we shall find that the centre of gravity is somewhere in

THE FORCE OF GRAVITATION 1 7

that line. But where ? To find that out, all we have to do is to take
another place for the awl (Fig. 4) , hang the plumb-line, and make
the same experiment, and there [at the point C] is the centre of
gravity, — there where the two lines which I have traced cross each
other; and if I take that pasteboard and make a hole with the brad-
awl through it at that point, you will see that it will be supported
in any position in which it may be placed. Now, knowing that,
what do I do when I try to stand upon one leg.? Do you not see
that I push myself over to the left side, and quietly take up the
right leg, and thus bring some central point in my body over this
left leg? What is that point which I throw over.? You will know
at once that it is the centre of gravity — that point in me where the
whole gravitating force of my body is centred, and which I thus
bring in a line over my foot.

Here is a toy I happened to see the other day, which will, I think,
serve to illustrate our subject very well. That toy ought to lie some-
thing in this manner (Fig. 5), and would do so if it were uniform
in substance; but you see it does not; it will get up again. And now

Fig. 5

Fig. 6

philosophy comes to our aid, and I am perfectly sure, without look-
ing inside the figure, that there is some arrangement by which the
centre of gravity is at the lowest point when the image is standing
upright; and we may be certain, when I am tilting it over (see
Fig. 6), that I am Ufting up the centre of gravity (a), and raising
it from the earth. All this is effected by putting a piece of lead
inside the lower part of the image, and making the base of large
curvature, and there you have the whole secret. But what will

1 8 FARADAY

happen if I try to make the figure stand upon a sharp point? You
observe I must get that point exactly under the centre of gravity, or
it vv^ill fall over thus [endeavoring unsuccessfully to balance it] ; and

this, you see, is a difficult matter; I can
not make it stand steadily; but if I
embarrass this poor old lady with a
world of trouble, and hang this wire
with bullets at each end about her
neck, it is very evident that, owing to
there being those balls of lead hanging
down on either side, in addition to the
lead inside, I have lowered the centre
of gravity, and now she will stand
upon this point (Fig. 7), and, what is
more, she proves the truth of our phi-
losophy by standing sideways.

I remember an experiment which
puzzled me very much when a boy. I
read it in a conjuring book, and this
was how the problem was put to us: "How," as the book said, "how
to hang a pail of water, by means of a stick, upon the side of a table"
(Fig. 8). Now I have here a table, a piece of stick, and a pail, and the

Fig. 7

Fig. 8 Fig. 9

proposition is, how can that pail be hung to the edge of this table ? It
is to be done, and can you at all anticipate what arrangement I shall
make to enable me to succeed ? Why this. I take a stick, and put it in

THE FORCE OF GRAVITATION I9

the pail between the bottom and the horizontal piece of wood, and
thus give it a stiff handle, and there it is; and, what is more, the more
water I put into the pail, the better it will hang. It is very true that
before I quite succeeded I had the misfortune to push the bottoms
of several pails out; but here it is hanging firmly (Fig. 9), and you
now see how you can hang up the pail in the way which the con-
juring books require.

Again, if you are really so inclined (and I do hope all of you are),
you will find a great deal of philosophy in this [holding up a cork
and a pointed thin stick about a foot
long]. Do not refer to your toy-books,
and say you have seen that before. An-
swer me rather, if I ask you, have you
understood it before? It is an experiment
which appeared very wonderful to me
when I was a boy. I used to take a piece
of cork (and I remember I thought at
first that it was very important that it
should be cut out in the shape of a man,
but by degrees I got rid of that idea), and
the problem was to balance it on the
point of a stick. Now you will see I have only to place two sharp-
pointed sticks one on each side, and give it wings, thus, and you will
find this beautiful condition fulfilled.

We come now to another point. All bodies, whether heavy or
light, fall to the earth by this force which we call gravity. By obser-
vation, moreover, we see that bodies do not occupy the same time in
falling; I think you will be able to see that this piece of paper and
that ivory ball fall with different velocities to the table [dropping
them] ; and if, again, I take a feather and an ivory ball, and let them
fall, you see they reach the table or earth at different times; that is
to say, the ball falls faster than the feather. Now that should not be
so, for all bodies do fall equally fast to the earth. There are one or
two beautiful points included in that statement. First of all, it is
manifest that an ounce, or a pound, or a ton, or a thousand tons, all
fall equally fast, no one faster than another: here are two balls of
lead, a very light one and a very heavy one, and you perceive they

Fig. 10

20 FARADAY

both fall to the earth in the same time. Now if I were to put into a
little bag a number of these balls sufficient to make up a bulk equal
to the large one, they would also fall in the same time; for if an
avalanche fall from the mountains, the rocks, snow, and ice, together
falling toward the earth, fall with the same velocity, whatever be
their size.

I can not take a better illustration of this than that of gold leaf,
because it brings before us the reason of this apparent difference in
the time of the fall. Here is a piece of gold leaf. Now if I take a
lump of gold and this gold leaf, and let them fall through the air
together, you see that the lump of gold — the sovereign or coin — will
fall much faster than the gold leaf. But why ? They are both gold,
whether sovereign or gold leaf. Why should they not fall to the
earth with the same quickness? They would do so, but that the air
around our globe interferes very much where we have the piece of
gold so extended and enlarged as to offer much obstruction on falling
through it. I will, however, show you that gold leaf does fall as fast
when the resistance of the air is excluded; for if I take a piece of
gold leaf and hang it in the centre of a bottle so that the gold, and
the bottle, and the air within shall all have an equal chance of falling,
then the gold leaf will fall as fast as anything else. And if I suspend
the bottle containing the gold leaf to a string, and set it oscillating
like a pendulum, I may make it vibrate as hard as I please and the
gold leaf will not be disturbed, but will swing as steadily as a piece
of iron would do; and I might even swing it round my head with
any degree of force, and it would remain undisturbed. Or I can
try another kind of experiment: if I raise the gold leaf in this way
[pulling the bottle up to the ceiling of the theatre by means of a
cord and pulley, and then suddenly letting it fall within a few
inches of the lecture table], and allow it then to fall from the ceiling
downward (I will put something beneath to catch it, supposing I
should be maladroit), you will perceive that the gold leaf is not in
the least disturbed. The resistance of the air having been avoided,
the glass bottle and gold leaf all fall exactly in the same time.

Here is another illustration: I have hung a piece of gold leaf in
the upper part of this long glass vessel, and I have the means by a
little arrangement at the top, of letting the gold leaf loose. Before

THE FORCE OF GRAVITATION 21

we let it loose we will remove the air by means of an air-pump,
and, while that is being done, let me show you another experiment
of the same kind. Take a penny-piece, or a half crown, and a round
piece of paper a trifle smaller in diameter than the coin, and try
them side by side to see whether they fall at the same time [dropping
them]. You see they do not — the penny-piece goes down first. But,
now place this paper flat on the top of the coin, so that it shall not
meet with any resistance from the air, and upon then dropping them
you see they do both fall in the same time [exhibiting the effect]. I
dare say, if I were to put this piece of gold leaf, instead of the paper,
on the coin, it would do as well. It is very difficult to lay the gold leaf
so flat that the air shall not get under it and lift it up in falling,
and I am rather doubtful as to the success of this, because the gold
leaf is puckery, but will risk the experiment. There they go together!
[letting them fall] and you see at once that they both reach the
table at the same moment.

We have now pumped the air out of the vessel, and you will per-
ceive that the gold leaf will fall as quickly in this vacuum as the
coin does in the air. I am now going to let it loose, and you must
watch to see how rapidly it falls. There! [letting the gold loose]
there it is, falling as gold should fall.

I am sorry to see our time for parting is drawing so near. As we
proceed, I intend to write upon the board behind me certain words,
so as to recall to your minds what we have already examined; and
I put the word Forces as a heading, and I will then add beneath the
names of the special forces according to the order in which we con-
sider them; and although I fear that I have not sufficiently pointed
out to you the more important circumstances connected with the
force of Gravitation, especially the law which governs its attraction
(for which, I think, I must take up a little time at our next meeting),
still I will put that word on the board, and hope you will now
remember that we have in some degree considered the force of
gravitation — that force which causes all bodies to attract each other
when they are at sensible distances apart, and tends to draw them
together.

LECTURE II
GRAVITATION— COHESION

DO me the favor to pay me as much attention as you did at
our last meeting, and I shall not repent o£ that which I
have proposed to undertake. It will be impossible for us
to consider the Laws of Nature, and what they effect, unless we now
and then give our sole attention, so as to obtain a clear idea upon the
subject. Give me now that attention, and then I trust we shall not
part without our knowing something about those laws, and the
manner in which they act. You recollect, upon the last occasion, I
explained that all bodies attracted each other, and that this power we
called gravitation. I told you that when we brought these two bodies
[two equal-sized ivory balls suspended by threads] near together,
they attracted each other, and that we might suppose that the whole
power of this attraction was exerted between their respective centres
of gravity; and, furthermore, you learned from me that if, instead
of a small ball I took a larger one, like that [changing one of the
balls for a much larger one], there was much more of this attraction
exerted; or, if I made this ball larger and larger, until, if it were
possible, it became as large as the Earth itself — or I might take the
Earth itself as the large ball — that then the attraction would become
so powerful as to cause them to rush together in this manner [drop-
ping the ivory ball]. You sit there upright, and I stand upright here,
because we keep our centres of gravity properly balanced with re-
spect to the earth; and I need not tell you that on the other side of
this world the people are standing and moving about with their feet
toward our feet, in a reversed position as compared with us, and all
by means of this power of gravitation to the centre of the earth.

I must not, however, leave the subject of gravitation without
telling you something about its laws and regularity; and, first, as
regards its power with respect to the distance that bodies are apart.
If I take one of these balls and place it within an inch of the other,

GRAVITY 23

they attract each other with a certain power. If I hold it at a greater
distance oflf, they attract with less power; and if I hold it at a greater
distance still, their attraction is still less. Now this fact is of the
greatest consequence; for, knowing this law, philosophers have dis-
covered most wonderful things. You know that there is a planet,
Uranus, revolving round the sun with us, but eighteen hundred
millions of miles off, and because there is another planet as far off
as three thousand millions of miles, this law of attraction, or gravi-
tation, still holds good, and philosophers actually discovered this
latter planet, Neptune, by reason of the effects of its attraction at
this overwhelming distance. Now I want you clearly to understand
what this law is. They say (and they are right) that two bodies
attract each other inversely as the square of the distance — a sad
jumble of words until you understand them; but I think we shall
soon comprehend what this law is, and what is the meaning of the
"inverse square of the distance."

I have here (Fig. ii) a lamp. A, shining most intensely upon this
disc, B, C, D, and this light acts as a sun by which I can get a

Fig. II

shadow from this little screen B F (merely a square piece of card),
which, as you know, when I place it close to the large screen, just
shadows as much of it as is exactly equal to its own size; but now
let me take this card, E, which is equal to the other one in size, and
place it midway between the lamp and the screen; now look at the

24 FARADAY

size of the shadow B D — it is four times the original size. Here,
then, comes the "inverse square of the distance." This distance, A E,
is one, and that distance, A B, is two, but that size E being one, this
size B D of shadow is jour instead of two, which is the square of the
distance, and, if I put the screen at one-third of the distance from
the lamp, the shadow on the large screen would be nine times the
size. Again, if I hold this screen here, at B F, a certain amount of
light falls on it; and if I hold it nearer the lamp at E, more light
shines upon it. And you see at once how much — exactly the quantity
which I have shut off from the part of this screen, B D, now in
shadow; moreover, you see that if I put a single screen here, at G,
by the side of the shadow, it can only receive one-fourth of the pro-
portion of light which is obstructed. That, then, is what is meant by
the inverse of the square of the distance. This screen E is the bright-
est because it is the nearest, and there is the whole secret of this
curious expression, inversely as the square of the distance. Now if
you can not perfectly recollect this when you go home, get a candle
and throw a shadow of something — your profile, if you like — on the
wall and then recede or advance, and you will find that your shadow
is exactly in proportion to the square of the distance you are off the
wall; and then, if you consider how much light shines on you at one
distance, and how much at another, you get the inverse accordingly.
So it is as regards the attraction of these two balls; they attract
according to the square of the distance, inversely. I want you to try
and remember these words, and then you will be able to go into all
the calculations of astronomers as to the planets and other bodies,
and tell why they move so fast, and why they go round the sun
without falling into it and be prepared to enter upon many other
interesting inquiries of the like nature.

Let us now leave this subject which I have written upon the board
under the word Force — Gravitation — and go a step farther. All
bodies attract each other at sensible distances. I showed you the
electric attraction on the last occasion (though I did not call it so) ;
that attracts at a distance; and in order to make our progress a little
more gradual, suppose I take a few iron particles [dropping some
small fragments of iron on the table]. There! I have already told
you that in all cases where bodies fall it is the particles that are

COHESION 25

attracted. You may consider these, then, as separate particles magni-
fied, so as to be evident to your sight; they are loose from each other
— they all gravitate — they all fall to the earth — for the force of gravi-
tation never fails. Now I have here a centre of power which I will
not name at present, and when these particles are placed upon it,
see what an attraction they have for each other.

Here I have an arch of iron filings (Fig. 12) regularly built up like
an iron bridge, because I have put them within a sphere of action
which will cause them to attract each other. See! I could let a mouse
run through it; and yet, if I try to do the same thing with them
here [on the table], they do not attract each other at all. It is that
[the magnet] which makes them hold
together. Now just as these iron particles
hold together in the form of an elliptical
bridge, so do the different particles of
iron which constitute this nail hold to-
gether and make it one. And here is a
bar of iron; why, it is only because the
different parts of this iron are so wrought
as to keep close together by the attraction
between the particles that it is held to-
gether in one mass. It is kept together, in
fact, merely by the attraction of one
particle to another, and that is the point I want now to illustrate.
If I take a piece of flint, and strike it with a hammer, and break it
thus [breaking off a piece of the flint], I have done nothing more
than separate the particles which compose these two pieces so far
apart that their attraction is too weak to cause them to hold together,
and it is only for that reason that there are now two pieces in the
place of one. I will show you an experiment to prove that this
attraction does still exist in those particles; for here is a piece of glass
(for what was true of the flint and the bar of iron is true of the
piece of glass, and is true of every other solid — they are all held
together in the lump by the attraction between their parts), and I
can show you the attraction between its separate particles; for if I
take these portions of glass which I have reduced to very fine powder,
you see that I can actually build them up into a solid wall by pressure

Fig. 12

2.6 FARADAY

between two flat surfaces. The power which I thus have of build-
ing up this wall is due to the attraction of the particles, forming, as
it were, the cement which holds them together; and so in this case,
where I have taken no very great pains to bring the particles together,
you see perhaps a couple of ounces of finely pounded glass standing
as an upright wall: is not this attraction most wonderful? That bar
of iron one inch square has such power of attraction in its particles —
giving to it such strength — that it will hold up twenty tons' weight
before the little set of particles in the small space equal to one
division across which it can be pulled apart will separate. In this
manner suspension bridges and chains are held together by the
attraction of their particles, and I am going to make an experiment
which will show how strong is this attraction of the particles. [The
lecturer here placed his foot on a loop of wire fastened to a support
above, and swung with his whole weight resting upon it for some
moments.] You see, while hanging here, all my weight is supported
by these little particles of the wire, just as in pantomimes they some-
times suspend gentlemen and damsels.

How can we make this attraction of the particles a little more
simple? There are many things which, if brought together properly,
will show this attraction. Here is a boy's experiment (and I like a
boy's experiment). Get a tobacco-pipe, fill it with lead, melt it, and
then pour it out upon a stone, and thus get a clean piece of lead
(this is a better plan than scraping it; scraping alters the condition
of the surface of the lead) . I have here some pieces of lead which I
melted this morning for the sake of making them clean. Now these
pieces of lead hang together by the attraction of their particles, and
if I press these two separate pieces close together, so as to bring their
particles within the sphere of attraction, you will see how soon they
become one. I have merely to give them a good squeeze, and draw
the upper piece slightly round at the same time, and here they are
as one, and all the bending and twisting I can give them will not
separate them again; I have joined the lead together, not with solder,
but simply by means of the attraction of the particles.

This, however, is not the best way of bringing those particles
together; we have many better plans than that; and I will show you
one that will do very well for juvenile experiments. There is some

COHESION 27

alum crystallized very beautifully by nature (for all things are far
more beautiful in their natural than their artificial form), and here
I have some of the same alum broken into fine powder. In it I have
destroyed that force of which I have placed the name on this board
— Cohesion, or the attraction exerted between the particles of bodies
to hold them together. Now I am going to show you that if we take
this powdered alum and some hot water, and mix them together, I
shall dissolve the alum; all the particles will be separated by the
water far more completely than they are here in the powder; but
then, being in the water, they will have the opportunity as it cools
(for that is the condition which favors their coalescence) of uniting
together again and forming one mass (^) .

Now, having brought the alum into solution, I will pour it into
this glass basin, and you will, to-morrow, find that these particles of
alum which I have put into the water, and so separated that they are
no longer solid, will, as the water cools, come together and cohere,
and by to-morrow morning we shall have a great deal of the alum
crystallized out — that is to say, come back to the solid form. [The
lecturer here poured a little of the hot solution of alum into the glass
dish, and when the latter had thus been made warm, the remainder
of the solution was added.] I am now doing that which I advise
you to do if you use a glass vessel, namely, warming it slowly and
gradually; and in repeating this experiment, do as I do — pour the
liquid out gently, leaving all the dirt behind in the basin; and
remember that the more carefully and quietly you make this experi-
ment at home, the better the crystals. To-morrow you will see the
particles of alum drawn together; and if I put two pieces of coke in
some part of the solution (the coke ought first to be washed very
clean, and dried), you will find to-morrow that we shall have a
beautiful crystallization over the coke, making it exactly resemble a
natural mineral.

Now how curiously our ideas expand by watching these conditions
of the attraction of cohesion! how many new phenomena it gives

* Crystallization of alum. The solution must be saturated — that is, it must contain
as much alum as can possibly be dissolved. In making the solution, it is best to add
powdered alum to hot water as long as it dissolves; and when no more is taken up,
allow the solution to stand a few minutes, and then pour it off from the dirt and
undissolved alum.

28 FARADAY

us beyond those of the attraction of gravitation! See how it gives us
great strength. The things we deal with in building up the struc-
tures on the earth are of strength — we use iron, stone, and other
things of great strength; and only think that all those structures you
have about you — think of the Great Eastern, if you please, which is
of such size and power as to be almost more than man can manage —
are the result of this power of cohesion and attraction.

I have here a body in which I believe you will see a change taking
place in its condition of cohesion at the moment it is made. It is at
first yellow; it then becomes a fine crimson red. Just watch when I
pour these two liquids together — both colorless as water. [The
lecturer here mixed together solutions of perchloride of mercury
and iodide of potassium, when a yellow precipitate of biniodide of
mercury fell down, which almost immediately became crimson red.]
Now there is a substance which is very beautiful, but see how it is
changing color. It was reddish-yellow at first, but it has now become
red ('). I have previously prepared a little of this red substance,
which you see formed in the liquid, and have put some of it upon
paper [exhibiting several sheets of paper coated with scarlet biniodide
of mercury (*)]. There it is — the same substance spread upon paper;
and there, too, is the same substance; and here is some more of it
[exhibiting a piece of paper as large as the other sheets, but having
only very little red color on it, the greater part being yellow] — a
little more of it, you will say. Do not be mistaken; there is as much
upon the surface of one of these pieces of paper as upon the other.
What you see yellow is the same thing as the red body, only the
attraction of cohesion is in a certain degree changed, for I will take
this red body, and apply heat to it (you may perhaps see a little

''Red precipitate of biniodide of mercury. A little care is necessary to obtain this
precipitate. The solution of iodide of potassium should be added to the solution of
perchloride of mercury (corrosive sublimate) very gradually. The red precipitate
which first falls is redissolved when the liquid is stirred: when a litrie more of the
iodide of potassium is added a pale red precipitate is formed, which, on the farther
addition of the iodide, changes into the brilliant scarlet biniodide of mercury. If
too much iodide of potassium is added, the scarlet precipitate disappears, and a
colorless solution is left.

^ Paper coated with scarlet biniodide of mercury. In order to fix the biniodide on
paper, it must be mixed with a litde weak gum water, and then spread over the
paper, which must be dried without heat.

Biniodide of mercury is said to be dimorphous; that is, is able to assume two different
forms.

COHESION 29

smoke arise, but that is of no consequence), and if you look at it it
will first of all darken — but see how it is becoming yellow. I have
now made it all yellow, and, what is more, it will remain so; but if I
take any hard substance, and rub the yellow part with it, it will
immediately go back again to the red condition [exhibiting the ex-
periment]. There it is. You see the red is not put bac\, but brought
bac}{ by the change in the substance. Now [warming it over the
spirit lamp] here it is becoming yellow again, and that is all because
its attraction of cohesion is changed. And what will you say to me
when I tell you that this piece of common charcoal is just the same
thing, only differently coalesced, as the diamonds which you wear.?
(I have put a specimen outside of a piece of straw which was
charred in a particular way — ^it is just like black lead.) Now this
charred straw, this charcoal, and these diamonds, are all of them the
same substance, changed but in their properties as respects the force
of cohesion.

Here is a piece of glass [producing a piece of plate-glass about two
inches square]. (I shall want this afterward to look to and examine

Fig. 13

Fig. 14

its internal condition), and here is some of the same sort of glass

differing only in its power of cohesion, because while yet melted it

had been dropped into cold water [exhibiting a "Prince Rupert's

drop,"(°) (Fig. 13)], and if I take one of these little tear-like pieces

and break off ever so little from the point, the whole will at once

burst and fall to pieces. I will now break off a piece of this. [The

^"Prince Rupert's Drops." These are made by pouring drops of melted green
glass into cold water. They were not, as is commonly supposed, invented by Prince
Rupert, but were first brought to England by him in 1660. They excited a great
deal o£ curiosity, and were considered "a kind of miracle in nature."

30 FARADAY

lecturer nipped off a small piece from the end of one of Rupert's
drops, whereupon the whole immediately fell to pieces.] There!
you see the solid glass has suddenly become powder, and more than
that, it has knocked a hole in the glass vessel in which it was held.
I can show the effect better in this bottle of water, and it is very
likely the whole bottle will go. [A 6-oz. vial was filled with water,
and a Rupert's drop placed in it with the point of the tail just pro-
jecting out; upon breaking the tip off, the drop burst, and the shock,
being transmitted through the water to the sides of the bottle,
shattered the latter to pieces.]

Here is another form of the same kind of experiment. I have
here some more glass which has not been annealed [showing some
thick glass vessels('°) (Fig. 14)], and if I take one of these glass
vessels and drop a piece of pounded glass into it (or I will take some
of these small pieces of rock crystal; they have the advantage of
being harder than glass), and so make the least scratch upon the
inside, the whole bottle will break to pieces — it can not hold together.
[The lecturer here dropped a small fragment of rock crystal into one
of these glass vessels, when the bottom immediately came out and
fell upon the plate.] There!, it goes through, just as it would through
a sieve.

Now I have shown you these things for the purpose of bringing
your minds to see that bodies are not merely held together by this
power of cohesion, but that they are held together in very curious
ways. And suppose I take some things that are held together by this
force, and examine them more minutely. I will first take a bit of
glass, and if I give it a blow with a hammer I shall just break it to
pieces. You saw how it was in the case of the flint when I broke
the piece off; a piece of a similar kind would come off, just as you
would expect; and if I were to break it up still more, it would be,
as you have seen, simply a collection of small particles of no definite
shape or form. But supposing I take some other thing— this stone,
for instance (Fig. 15) [taking a piece of mica(")], and if I hammer
this stone I may batter it a great deal before I can break it up. I may
even bend it without breaking it — that is to say, I may bend it in

" Thick, glass vessels. They are called Proofs or Bologna phials.
" Mica. A silicate of alumina and magnesia. It has a bright metallic lustre; hence
its name, from mico, to shine.

COHESION 31

one particular direction without breaking it much, although I feel in
my hands that I am doing it some injury. But now, if I take it by
the edges, I find that it breaks up into leaf after leaf in a most
extraordinary manner. Why should it break up like that? Not
because all stones do, or all crystals; for there is some salt (Fig. 16) —
you know what common salt is ('^) ; here is a piece of this salt, which
by natural circumstances has had its particles so brought together

Fig. 15 Fig. 16 Fig. 17

that they have been allowed free opportunity of combining or coa-
lescing, and you shall see what happens if I take this piece of salt
and break it. It does not break as flint did, or as the mica did, but
with a clean sharp angle and exact surfaces, beautiful and glittering
as diamonds [breaking it by gentle blows with a hammer]; there is
a square prism which I may break up into a square cube. You see
these fragments are all square; one side may be longer than the
other, but they will only split up so as to form square or oblong pieces
with cubical sides. Now I go a little farther, and I find another
stone (Fig. 17) [Iceland or calc-spar] (") which I may break in a
similar way, but not with the same result. Here is a piece which I
have broken off, and you see there are plain surfaces perfectly
regular with respect to each other, but it is not cubical — it is what
we call a rhomboid. It still breaks in three directions most beauti-
fully and regularly with polished surfaces, but with sloping sides,
not like the salt. Why not.? It is very manifest that this is owing
to the attraction of the particles one for the other being less in the
direction in which they give way than in other directions. I have
on the table before me a number of little bits of calcareous spar, and

'^ Common salt or chloride of sodium crystallizes in the form of solid cubes, which,
aggregated together, form a mass, which may be broken up into the separate cubes.
'' Iceland or calc-spar. Native carbonate of lime in its primitive crystalline form.

32 FARADAY

I recommend each of you to take a piece home, and then you can
take a knife and try to divide it in the direction of any of the
surfaces already existing. You will be able to do it at once; but
if you try to cut it across the crystals, you can not; by hammering you
may bruise and break it up, but you can only divide it into these
beautiful little rhomboids.

Now I want you to understand a little more how this is, and for
this purpose I am going to use the electric Hght again. You see we
can not look into the middle of a body like this piece of glass. We
perceive the outside form and the inside form, and we look through
it, but we can not well find out how these forms become so, and I

Fig. 1 8

want you, therefore, to take a lesson in the way in which we use a
ray of light for the purpose of seeing what is in the interior of bodies.
Light is a thing which is, so to say, attracted by every substance that
gravitates (and we do not know any thing that does not). AH
matter affects light more or less by what we may consider as a kind
of attraction, and I have arranged (Fig. i8) a very simple experiment
upon the floor of the room for the purpose of illustrating this. I
have put into that basin a few things which those who are in the
body of the theatre will not be able to see, and I am going to make
use of this power which matter possesses of attracting a ray of light.
If Mr. Anderson pours some water, gently and steadily, into the
basin, the water will attract the rays of light downward, and the
piece of silver and the sealing-wax will appear to rise up into
the sight of those who were before not high enough to see over the
side of the basin to its bottom. [Mr. Anderson here poured water
into the basin, and upon the lecturer asking whether any body could

COHESION 33

see the silver and sealing-wax, he was answered by a general affirm-
ative.] Now I suppose that every body can see that they are not at all
disturbed, while from the way they appear to have risen up you
would imagine the bottom of the basin and the articles in it were
two inches thick, although they are only one of our small silver
dishes and a piece of sealing-wax which I have put there. The light
which now goes to you from that piece of silver was obstructed by
the edge of the basin when there was no water there, and you were
unable to see anything of it; but when we poured in water the rays
were attracted down by it over the edge of the basin, and you were
thus enabled to see the articles at the bottom.

I have shown you this experiment first, so that you might under-
stand how glass attracts light, and might then see how other sub-
stances like rock-salt and calcareous spar, mica, and other stones,
would affect the light; and, if Dr. Tyndall will be good enough to
let us use his light again, we will first of all show you how it may
be bent by a piece of glass (Fig. 19). [The electric lamp was again

Fig. 19

lit, and the beam of parallel rays of light which it emitted was bent
about and decomposed by means of the prism.] Now, here you see,
if I send the light through this piece of plain glass, A, it goes straight
through without being bent (unless the glass be held obliquely, and
then the phenomenon becomes more complicated) ; but if I take this
piece of glass, B [a prism], you see it will show a very different
effect. It no longer goes to that wall, but it is bent to this screen, C,
and how much more beautiful it is now [throwing the prismatic
spectrum on the screen]. This ray of light is bent out of its course
by the attraction of the glass upon it; and you see I can turn and
twist the rays to and fro in different parts of the room, just as I
please. Now it goes there, now here. [The lecturer projected the

34 FARADAY

prismatic spectxum about the theatre.] Here I have the rays once
more bent on to the screen, and you see how wonderfully and beauti-
fully that piece of glass not only bends the light by virtue of its
attraction, but actually splits it up into different colors. Now I
want you to understand that this piece of glass [the prism], being
perfectly uniform in its internal structure, tells us about the action
of these other bodies which are not uniform — which do not merely
cohere, but also have within them, in different parts, different de-
grees of cohesion, and thus attract and bend the light with varying
powers. We will now let the light pass through one or two of these
things which I just now showed you broke so curiously; and, first
of all, I will take a piece of mica. Here, you see, is our ray of light :
we have first to make it what we call polarized; but about that you
need not trouble yourselves; it is only to make our illustration more
clear. Here, then, we have our polarized ray of light, and I can so
adjust it as to make the screen upon which it is shining either light
or dark, although I have nothing in the course of this ray of light
but what is perfectly transparent [turning the analyzer round]. I
will now make it so that it is quite dark, and we will, in the first
instance, put a piece of common glass into the polarized ray so as to
show you that it does not enable the light to get through. You see
the screen remains dark. The glass, then, internally, has no effect
upon light. [The glass was removed and a piece of mica introduced.]
Now there is the mica which we split up so curiously into leaf after
leaf, and see how that enables the light to pass through to the screen,
and how, as Dr. Tyndall turns it round in his hand, you have those
different colors, pink, and purple, and green, coming and going
most beautifully; not that the mica is more transparent than the
glass, but because of the different manner in which its particles are
arranged by the force of cohesion.

Now we will see how calcareous spar acts upon this light — that
stone which split up into rhombs, and of which you are each of you
going to take a little piece home. [The mica was removed, and a
piece of calc-spar introduced at A.] See how that turns the light
round and round, and produces these rings and that black cross
(Fig. 20). Look at those colors: are they not most beautiful for you
and for me? (for I enjoy these things as much as you do). In what

COHESION 35

a wonderful manner they open out to us the internal arrangement of
the particles of this calcareous spar by the force of cohesion.

And now I will show you another experiment. Here is that piece
of glass which before had no action upon the light. You shall see
what it will do when we apply pressure to it. Here, then, we have
our ray of polarized light, and I will first of all show you that the

f

Fig. 20

glass has no effect upon it in its ordinary state; when I place it in
the course of the light, the screen still remains dark. Now Dr.
Tyndall will press that bit of glass between three little points, one
point against two, so as to bring a strain upon the parts, and you
will see what a curious effect that has. [Upon the screen two white
dots gradually appeared.] Ah! these points show the position of
the strain; in these parts the force of cohesion is being exerted in a
different degree to what it is in the other parts, and hence it allows
the light to pass through. How beautiful that is! how it malces the
light come through some parts and leaves it dark in others, and all
because we weaken the force of cohesion between particle and
particle. Whether you have this mechanical power of straining, or
whether we take other means, we get the same result; and, indeed,
I will show you by another experiment that if we heat the glass in
one part, it will alter its internal structure and produce a similar
effect. Here is a piece of common glass, and if I insert this in the
path of the polarized ray, I believe it will do nothing. There is the
common glass [introducing it]. No light passes through; the screen
remains quite dark; but I am going to warm this glass in the lamp,
and you know yourselves that when you pour warm water upon

36 FARADAY

glass you put a strain upon it sufficient to break it sometimes — some-
thing like there was in the case of the Prince Rupert's drops. [The
glass was warmed in the spirit lamp, and again placed across the
ray of light.] Now you see how beautifully the light goes through
those parts which are hot, making dark and light lines just as the
crystal did, and all because of the alteration I have effected in its
internal condition; for these dark and light parts are a proof of the
presence of forces acting and dragging in different directions within
the solid mass.

LECTURE III

COHESION— CHEMICAL AFFINITY

WE will first return for a few minutes to one of the experi-
ments made yesterday. You remember what we put
together on that occasion — ^powdered alum and warm
water. Here is one of the basins then used. Nothing has been done
to it since; but you will find, on examining it, that it no longer
contains any powder, but a number of beautiful crystals. Here also
are the pieces of coke which I put into the other basin; they have a
fine mass of crystals about them. That other basin I will leave as
it is. I will not pour the water from it, because it will show you that
the particles of alum have done something more than merely crystal-
lize together. They have pushed the dirty matter from them, laying
it around the outside or outer edge of the lower crystals — squeezed
out, as it were, by the strong attraction which the particles of alum
have for each other.

And now for another experiment. We have already gained a
knowledge of the manner in which the particles of bodies — of solid
bodies — attract each other, and we have learned that it makes cal-
careous spar, and so forth, crystallize in these regular forms. Now
let me gradually lead your minds to a knowledge of the means we
possess of making this attraction alter a little in its force; either of
increasing, or diminishing, or, apparently, of destroying it alto-
gether. I will take this piece of iron [a rod of iron about two feet
long and a quarter of an inch in diameter]. It has at present a great
deal of strength, due to its attraction of cohesion; but if Mr. Ander-
son will make part of this red-hot in the fire, we shall then find that
it will become soft, just as sealing-wax will when heated, and we
shall also find that the more it is heated the softer it becomes. Ah!
but what does soft mean? Why, that the attraction between the
particles is so weakened that it is no longer sufficient to resist the
power we bring to bear upon it. [Mr. Anderson handed to the

37

38 FARADAY

lecturer the iron rod, with one end red-hot, which he showed could
be easily twisted about with a pair of pliers.] You see I now find no
difhculty in bending this end about as I like, whereas I can not bend
the cold part at all. And you know how the smith takes a piece of
iron and heats it in order to render it soft for his purpose: he acts
upon our principle of lessening the adhesion of the particles,
although he is not exactly acquainted with the terms by which we
express it.

And now we have another point to examine, and this water is
again a very good substance to take as an illustration (as philoso-
phers we call it all water, even though it be in the form of ice or
steam). Why is this water hard? [pointing to a block of ice];
because the attraction of the particles to each other is sufficient to
make them retain their places in opposition to force applied to it.
But what happens when we make the ice warm? Why, in that
case we diminish to such a large extent the power of attraction that
the solid substance is destroyed altogether. Let me illustrate this: I
will take a red-hot ball of iron [Mr. Anderson, by means of a pair
of tongs, handed to the lecturer a red-hot ball of iron, about two
inches in diameter], because it will serve as a convenient source of
heat [placing the red-hot iron in the centre of the block of ice]. You
see I am now melting the ice where the iron touches it. You see the
iron sinking into it; and while part of the solid water is becoming
liquid, the heat of the ball is rapidly going off. A certain part of
the water is actually rising in steam, the attraction of some of the
particles is so much diminished that they can not even hold together
in the liquid form, but escape as vapor. At the same time, you see
I can not melt all this ice by the heat contained in this ball. In
the course of a very short time I shall find it will have become quite
cold.

Here is the water which we have produced by destroying some of
the attraction which existed between the particles of the ice, for
below a certain temperature the particles of water increase in their
mutual attraction and become ice; and above a certain temperature
the attraction decreases and the water becomes steam. And exactly
the same thing happens with platinum, and nearly every substance
in nature; if the temperature is increased to a certain point it becomes

COHESION 39

liquid and a farther increase converts it into a gas. Is it not a glorious
thing for us to look at the sea, the rivers, and so forth, and to know
that this same body in the northern regions is all solid ice and ice-
bergs, while here, in a warmer climate, it has its attraction of
cohesion so much diminished as to be liquid water ? Well, in dimin-
ishing this force of attraction between the particles of ice, we made
use of another force, namely, that of heat; and I want you now to
understand that this force of heat is always concerned when water
passes from the solid to the liquid state. If I melt ice in other ways
I can not do without heat (for we have the means of making ice
liquid without heat — that is to say, without using heat as a direct
cause) . Suppose, for illustration, I make a vessel out of this piece of
tinfoil [bending the foil up into the shape of a dish]. I am making
it metallic, because I want the heat which I am about to deal with to
pass readily through it; and I am going to pour a little water on
this board, and then place the tin vessel on it. Now if I put some of
this ice into the metal dish, and then proceed to make it liquid by
any of the various means we have at our command, it still must
take the necessary quantity of heat from something, and in this case
it will take the heat from the tray, and from the water underneath,
and from the other things round about. Well, a little salt added to
the ice has the power of causing it to melt, and we shall very shortly
see the mixture become quite fluid, and you will then find that the
water beneath will be frozen — frozen because it has been forced to
give up that heat which is necessary to keep it in the liquid state
to the ice on becoming liquid. I remember once, when I was a boy,
hearing of a trick in a country ale-house: the point was how to
melt ice in a quart pot by the fire and freeze it to the stool. Well,
the way they did it was this : they put some pounded ice in a pewter
pot, and added some salt to it, and the consequence was that when
the salt was mixed with it, the ice in the pot melted (they did not
tell me any thing about the salt and they set the pot by the lire, just
to make the result more mysterious), and in a short time the pot and
the stool were frozen together, as we shall very shortly find it to be
the case here, and all because salt has the power of lessening the
attraction between the particles of ice. Here you see the tin dish is
frozen to the board; I can even lift the little stool up by it.

40 FARADAY

This experiment can not, I think, fail to impress upon your minds
the fact that whenever a soHd body loses some of that force of attrac-
tion by means of which it remains solid, heat is absorbed; and if,
on the other hand, we convert a liquid into a solid, e. g., water into
ice, a corresponding amount of heat is given out. I have an experi-
ment showing this to be the case. Here (Fig. 21) is a bulb, A, filled

Fig. 21

with air, the tube from which dips into some colored liquid in the
vessel B. And I dare say you know that if I put my hand on the
bulb A, and warm it, the colored liquid which is now standing in
the tube at C will travel forward. Now we have discovered a means,
by great care and research into the properties of various bodies, of
preparing a solution of a salt (") which, if shaken or disturbed, will
at once become a solid; and as I explained to you just now (for what
is true of water is true of every other liquid), by reason of its
becoming solid heat is evolved, and I can make this evident to you
by pouring it over this bulb; there! it is becoming solid; and look
at the colored liquid, how it is being driven down the tube, and
how it is bubbling out through the water at the end; and so we learn
this beautiful law of our philosophy, that whenever we diminish
the attraction of cohesion we absorb heat, and whenever we increase
that attraction heat is evolved. This, then, is a great step in advance,
for you have learned a great deal in addition to the mere circum-
stance that particles attract each other. But you must not now
suppose that because they are liquid they have lost their attraction

^* Solution of a salt. Acetate of soda. A solution saturated, or nearly so, at the
boiling point, is necessary, and it must be allowed to cool, and remain at rest until
the experiment is made.

COHESION 41

of cohesion; for here is the fluid mercury, and if I pour it from one
vessel into another, I find that it will form a stream from the bottle
down to the glass — a continuous rod of fluid mercury, the particles
of which have attraction sufficient to make them hold together all
the way through the air down to the glass itself; and if I pour water
quietly from a jug, I can cause it to run in a continuous stream in
the same manner. Again: let me put a little water on this piece
of plate glass, and then take another plate of glass and put it on the
water; there! the upper plate is quite free to move, gliding about
on the lower one from side to side; and yet, if I take hold of the
upper plate and lift it up straight, the cohesion is so great that the
lower one is held up by it. See how it runs about as I move the
upper one, and this is all owing to the strong attraction of the
particles of the water. Let me show you another experiment. If I
take a little soap and water — not that the soap makes the particles
of the water more adhesive one for the other, but it certainly has
the power of continuing in a better manner the attraction of the
particles (and let me advise you, when about to experiment with
soap-bubbles, to take care to have every thing clean and soapy). I
will now blow a bubble, and that I may be able to talk and blow a
bubble too, I will take a plate with a little of the soapsuds in it, and
will just soap the edges of the pipe and blow a bubble on to the
plate. Now there is our bubble. Why does it hold together in this
manner? Why, because the water of which it is composed has an
attraction of particle for particle — so great, indeed, that it gives to
this bubble the very power of an India-rubber ball; for you see, if I
introduce one end of this glass tube into the bubble, that it has the
power of contracting so powerfully as to force enough air through
the tube to blow out a light (Fig. 22) ; the light is blown out. And
look! see how the bubble is disappearing — see how it is getting
smaller and smaller.

There are twenty other experiments I might show you to illustrate
this power of cohesion of the particles of liquids. For instance, what
would you propose to me if, having lost the stopper out of this
alcohol bottle, I should want to close it speedily with something near
at hand. Well, a bit of paper would not do, but a piece of linen cloth
would, or some of this cotton wool which I have here. I will put a

42 FARADAY

tuft of it into the neck of the alcohol bottle, and you see, when I
turn it upside down, that it is perfectly well stoppered so far as the
alcohol is concerned; the air can pass through, but the alcohol can
not. And if I were to take an oil vessel this plan would do equally
well, for in former times they used to send us oil from Italy in
flasks stoppered only with cotton wool (at the present time the cotton
is put in after the oil has arrived here, but formerly it used to be
sent so stoppered). Now if it were not for the particles of liquid
cohering together, this alcohol would run out; and if I had time I

Fig. 22

Fig. 23

could have shown you a vessel with the top, bottom, and sides
altogether formed like a sieve, and yet it would hold water, owing
to the cohesion.

You have now seen that the solid water can become fluid by the
addition of heat, owing to this lessening the attractive force between
its particles, and yet you see that there is a good deal of attractive
force remaining behind. I want now to take you another step
beyond. We saw that if we continued applying heat to the water
(as indeed happened with our piece of ice here), that we did at last
break up that attraction which holds the liquid together, and I am
about to take some other (any other liquid would do, but ether
makes a better experiment for my purpose) in order to illustrate
what will happen when this cohesion is broken up. Now this liquid
ether, if exposed to a very low temperature, will become a solid; but
if we apply heat to it, it becomes vapor; and I want to show you the
enormous bulk of the substance in this new form: when we make
ice into water, we lessen its bulk; but when we convert water into
steam, we increase it to an enormous extent. You see it is very clear
that as I apply heat to the liquid I diminish its attraction of co-

COHESION 43

hesion; it is now boiling, and I will set fire to the vapor, so that you
may be enabled to judge of the space occupied by the ether in this
form by the size of its flame; and you now see what an enormously
bulky flame I get from that small volume of ether below. The heat
from the spirit lamp is now being consumed, not in making the
ether any warmer, but in converting it into vapor; and if I desired
to catch this vapor and condense it (as I could without much diffi-
culty), I should have to do the same as if I wished to convert steam
into water and water into ice: in either case it would be necessary to
increase the attraction of the particles by cold or otherwise. So largely
is the bulk occupied by the particles increased by giving them this
diminished attraction, that if I were to take a portion of water
a cubic inch in bulk (A, Fig. 23), I should produce a volume of
steam of that size, B [1,700 cubic inches; nearly a cubic foot], so
greatly is the attraction of cohesion diminished by heat; and yet it
still remains water. You can easily imagine the consequences which
are due to this change in volume by heat — the mighty powers of
steam and the tremendous explosions which are sometimes pro-
duced by this force of water. I want you now to see another experi-
ment, which will perhaps give you a better illustration of the bulk
occupied by a body when in the state of vapor. Here is a substance
which we call iodine, and I am about to submit this solid body to
the same kind of condition as regards heat that I did the water and
the ether [putting a few grains of iodine into a hot glass globe,
which immediately became filled with the violet vapor], and you
see the same kind of change produced. Moreover, it gives us the
opportunity of observing how beautiful is the violet-colored vapor
from this black substance, or rather the mixture of the vapor with
air (for I would not wish you to understand that this globe is
entirely filled with the vapor of iodine) .

If I had taken mercury and converted it into vapor (as I could
easily do), I should have a perfectly colorless vapor; for you must
understand this about vapors, that bodies in what we call the vapor-
ous or the gaseous state are always perfectly transparent, never
cloudy or smoky; they are, however, often colored, and we can
frequently have colored vapors or gases produced by colorless par-
ticles themselves mixing together, as in this case [the lecturer here

44 FARADAY

inverted a glass cylinder full of binoxide of nitrogen (") over a
cylinder of oxygen, when the dark red vapor of hyponitrous acid
was produced]. Here also you see a very excellent illustration of the
efifect of a power of nature which we have not as yet come to, but
which stands next on our list — Chemical Affinity. And thus you
see we can have a violet vapor or an orange vapor, and different
other kinds of vapor, but they are always perfectly transparent, or
else they would cease to be vapors.

I am now going to lead you a step beyond this consideration of
the attraction of the particles for each other. You see we have come
to understand that, if we take water as an illustration, whether it be
ice, or water, or steam, it is always to be considered by us as water.
Well, now prepare your minds to go a little deeper into the subject.
We have means of searching into the constitution of water beyond
any that are afforded us by the action of heat, and among these one
of the most important is that force which we call voltaic electricity,
which we used at our last meeting for the purpose of obtaining
light, and which we carried about the room by means of these
wires. This force is produced by the battery behind me, to which,
however, I will not now refer more particularly; before we have
done we shall know more about this battery, but it must grow up
in our knowledge as we proceed. Now here (Fig. 24) is a portion of
water in this little vessel, C, and besides the water there are two
plates of the metal platinum, which are connected with the wires
(A and B) coming outside, and I want to examine that water, and
the state and the condition in which its particles are arranged. If I
were to apply heat to it you know what we should get; it would
assume the state of vapor, but it would nevertheless remain water,
and would return to the liquid state as soon as the heat was removed.
Now by means of these wires (which are connected with the battery
behind me, and come under the floor and up through the table) we
shall have a certain amount of this new power at our disposal. Here

'^ Binoxide of nitrogen and hyponitrous acid. Binoxide of nitrogen is formed when
nitric acid and a little water are added to some copper turnings. It produces deep
red fumes as soon as it comes in contact with the air, by combining with the oxygen
of the latter to form hyponitrous acid. Binoxide of nitrogen is composed of two parts
of oxygen and one part of nitrogen; hyponitrous acid is composed of one part of
nitrogen and three parts of oxygen.

CHEMICAL AFFINITY 45

you see it is [causing the ends of the wires to touch] — that is the
electric Hght we used yesterday, and by means of these wires we
can cause water to submit itself to this power; for the moment I put
them into metallic connection (at A and B), you see the water boil-
ing in that little vessel (C), and you hear the bubbling of the gas
that is going through the tube (D). See how I am converting the
water into vapor; and if I take a httle vessel (E), and fill it with.
water, and put it into the trough over the end of the tube (D),

Fig. 24

there goes the vapor ascending into the vessel. And yet that is not
steam, for you know that if steam is brought near cold water, if
would at once condense, and return back again to water; this, then,
can not be steam, for it is bubbling through the cold water in this
trough; but it is a vaporous substance, and we must therefore
examine it carefully, to see in what way the water has been changed.
And now, in order to give you a proof that it is not steam, I am
going to show you that it is combustible; for if I take this small
vessel to a light, the vapor inside explodes in a manner that steam
could never do.

I will now fill this large bell-jar (F) with water; and I propose
letting the gas ascend into it, and I will then show you that we can
reproduce the water back again from the vapor or air that is there.
Here is a strong glass vessel (G), and into it we will let the gas
(from F) pass. We will there fire it by the electric spark, and then,

46 FARADAY

after the explosion, you will find that we have got the water back
again; it will not be much, however, for you will recollect that I
showed you how small a portion of water produced a very large
volume of vapor. Mr. Anderson will now pump all the air out of
this vessel (G), and when I have screwed it on to the top of ouf
jar of gas (F), you will see, upon opening the stop-cocks (H H H),
the water will jump up, showing that some of the gas has passed
into the glass vessel. I will now shut these stop-cocks, and we shall
be able to send the electric spark through the gas by means of the
wires (I, K) in the upper part of the vessel, and you will see it burn
with a most intense flash. [Mr. Anderson here brought a Ley den
jar, which he discharged through the confined gas by means of the
wires I, K.] You saw the flash, and now that you may see that there
is no longer any gas remaining, if I place it over the jar and open
the stop-cocks again, up will go the gas, and we can have a second
combustion; and so I might go on again and again, and I should
continue to accumulate more and more of the water to which the
gas has returned. Now is not this curious.'' In this vessel (C) we
can go on making from water a large bulk of permanent gas, as we
call it, and then we can reconvert it into water in this way. [Mr.
Anderson brought in another Ley den jar, which, however, from
some cause, would not ignite the gas. It was therefore recharged,
when the explosion took place in the desired manner.] How beau-
tifully we get our results when we are right in our proceedings! It
is not that Nature is wrong when we make a mistake. Now I will
lay this vessel (G) down by my right hand, and you can examine
it by-and-by; there is not very much water flowing down, but there
is quite sufficient for you to see.

Another wonderful thing about this mode of changing the con-
dition of the water is this : that we are able to get the separate parts
of which it is composed at a distance the one from the other, and
to examine them, and see what they are like, and how many of them
there are; and for this purpose I have here some more water in a
slightly different apparatus to the former one (Fig. 25), and if I place
this in connection with the wires of the battery (at A, B), I shall get
a similar decomposition of the water at the two platinum plates.

Fig. 25

CHEMICAL AFFINITY 47

Now I will put this little tube (O) over there, and that will collect
the gas together that comes from this side (A), and this tube (H)
will collect the gas that comes from the other side (B), and I think
we shall soon be able to see a difference. In this apparatus the wires
are a good way apart from each other,
and it now seems that each of them is
capable of drawing off particles from the
water and sending them off, and you see
that one set of particles (H) is coming off
twice as fast as those collected in the other
tube (O). Something is coming out of
the water there (at H) which burns [set-
ting fire to the gas]; but what comes out
of the water here (at O), although it will
not burn, will support combustion very
vigorously. [The lecturer here placed a match with a glowing tip in
the gas, when it immediately rekindled.]

Here, then, we have two things, neither of them being water
alone, but which we get out of the water. Water is therefore com-
posed of two substances different to itself, which appear at separate
places when it is made to submit to the force which I have in these
wires; and if I take an inverted tube of water and collect this gas
(H), you will see that it is by no means the same as the one we
collected in the former apparatus (Fig. 24). That exploded with a
loud noise when it was lighted, but this will burn quite noiselessly : it
is called hydrogen; and the other we call oxygen — that gas which so
beautifully brightens up all combustion, but does not burn of itself.
So now we see that water consists of two kinds of particles attracting
each other in a very different manner to the attraction of gravitation
or cohesion, and this new attraction we call chemical affinity, or the
force of chemical action between different bodies; we are now no
longer concerned with the attraction of iron for iron, water for water,
wood for wood, or like bodies for each other, as we were when deal-
ing with the force of cohesion; we are dealing with another kind
of attraction — the attraction between particles of a different nature
one to the other. Chemical affinity depends entirely upon the energy

48 FARADAY

with which particles of different kinds attract each other. Oxygen
and hydrogen are particles of different kinds, and it is their attrac-
tion to each other which makes them chemically combine and pro-
duce water.

I must now show you a little more at large what chemical affinity
is. I can prepare these gases from other substances as well as from
water; and we will now prepare some oxygen: here is another sub-
stance which contains oxygen — chlorate of potash; I will put some of
it into this glass retort, and Mr. Anderson will apply heat to it:
we have here different jars filled with water, and when, by the
application of heat, the chlorate of potash is decomposed, we will
displace the water, and fill the jars with gas.

Now, when water is opened out in this way by means of the
battery, which adds nothing to it materially, which takes nothing
from it materially (I mean no matter; I am not speaking of force),
which adds no matter to the water, it is changed in this way — the gas
which you saw burning a little while ago, called hydrogen, is evolved
in large quantity, and the other gas, oxygen, is evolved in only half
the quantity; so that these two areas represent water, and these are
always the proportions between the two gases.

Oxygen 88.9

Hydrogen .... 11. i

Water loo.o

But oxygen is sixteen times the weight of the other — eight times
as heavy as the particles of hydrogen in the water; and you therefore
know that water is composed of nine parts by weight — one of hydro-
gen and eight of oxygen; thus:

Hydrogen 46.2 cubic inches =1 grain

Oxygen 23.1 " " =8 grains

Water {steam) 69.3 " " =9 grains

Now Mr. Anderson has prepared some oxygen, and we will pro-

I
Hydrogen

8
Oxygen

9

CHEMICAL AFFINITY 49

ceed to examine what is the character of this gas. First of all, you
remember I told you that it does not burn, but that it affects the
burning of other bodies. I will just set fire to the point of this little
bit of wood, and then plunge it into the jar of oxygen, and you will
see what this gas does in increasing the brilliancy of the combustion.
It does not burn, it does not take fire as the hydrogen would; but
how vividly the combustion of the match goes on! Again, if I were
to take this wax taper and light it, and turn it upside down in the
air, it would, in all probability, put itself out, owing to the wax run-
ning down into the wick. [The lecturer here
turned the lighted taper upside down, when in a
few seconds it went out.] Now that will not
happen in oxygen gas; you will see how differently
it acts (Fig. 26). [The taper was again lighted,
turned upside down, and then introduced into a
jar of oxygen.] Look at that! See how the very
wax itself burns, and falls down in a dazzling'
stream of fire, so powerfully does the oxygen sup- ^ig. 26

port combustion. Again, here is another experiment which will serve
to illustrate the force, if I may so call it, of oxygen. I have here a
circular flame of spirit of wine, and with it I am about to show you
the way in which iron burns, because it will serve very well as a
comparison between the effect produced by air and oxygen. If I take
this ring flame, I can shake, by means of a sieve, the fine particles of
iron filings through it, and you will see the way in which they burn.
[The lecturer here shook through the flame some iron filings, which
took fire and fell through with beautiful scintillations.] But if I now
hold the flame over a jar of oxygen [the experiment was repeated
over a jar of oxygen, when the combustion of the filings as they fell
into the oxygen became almost insupportably brilliant], you see how
wonderfully different the effect is in the jar, because there we have
oxygen instead of common air.

LECTURE IV
CHEMICAL AFFINITY-HEAT

WE shall have to pay a little more attention to the forces
existing in water before we can have a clear idea on the
subject. Besides the attraction which there is between its
particles to make it hold together as a liquid or a solid, there is also
another force, different from the former — one which, yesterday, by
means of the voltaic battery, we overcame, drawing from the water
two different substances, which, when heated by means of the electric
spark, attracted each other, and rushed into combination to repro-
duce water. Now I propose to-day to continue this subject, and trace
the various phenomena of chemical affinity; and for this purpose, as
we yesterday considered the character of oxygen, of which I have
here two jars (oxygen being those particles derived from the water
which enable other bodies to burn), we will now consider the other
constituent of water, and, without embarrassing you too much with
the way in which these things are made, I will proceed now to show
you our common way of making hydrogen. (I called it hydrogen
yesterday: it is so called because it helps to generate water.)* I put
into this retort some zinc, water, and oil of vitriol, and immediately
an action takes place, which produces an abundant evolution of gas,
now coming over into this jar, and bubbling up in appearance
exactly like the oxygen we obtained yesterday (Fig. 27).

The processes, you see, are very different, though the result is the
same, in so far as it gives us certain gaseous particles. Here, then, is
the hydrogen. I showed you yesterday certain qualities of this gas;
now let me exhibit you some other properties. Unlike oxygen, which
is a supporter of combustion and will not burn, hydrogen itself is
combustible. There is a jar full of it; and if I carry it along in this
manner and put a light to it, I think you will see it take fire (Fig. 27)
— not with a bright light; you will, at all events, hear it if you do not

*'TSa)p, "water," and ytwahi, "I generate."
50

CHEMICAL AFFINITY 5 1

see it. Now that is a body entirely diflerent from oxygen; it is ex-
tremely light; for, although yesterday you saw twice as much of this
hydrogen produced on the one side as on the other by the voltaic

Fig. 27

battery, it was only one-eighth the weight of the oxygen. I carry this
jar upside down. Why.? Because I know that it is a very light body,
and that it will continue in this jar upside down quite as effectually
as the water will in that jar which is not upside down; and just as
I can pour water from one vessel into another in the right position to
receive it, so can I pour this gas from one
jar into another when they are upside
down. See what I am about to do. There
is no hydrogen in this jar at present, but I
will gently turn this jar of hydrogen up
under this other jar (Fig. 28), and then
we will examine the two. We shall see,
on applying a light, that the hydrogen
has left the jar in which it was at first,
and has poured upward into the other, and there we shall find it.
You now understand that we can have particles of very different
kinds, and that they can have different bulks and weights; and there
are two or three very interesting experiments which serve to illus-
trate this. For instance, if I blow soap bubbles with the breath from
my mouth, you will see them fall, because I fill them with common
air, and the water which forms the bubble carries it down. But now,
if I inhale hydrogen gas into my lungs (it does no harm to the lungs,
although it does no good to them), see what happens. [The lecturer
inhaled some hydrogen, and, after one or two ineffectual attempts,

Fig. 28

52 FARADAY

succeeded in blowing a splendid bubble, which rose majestically and
slowly to the ceiling of the theatre, where it burst.] That shows you
very well how light a substance this is; for, notwithstanding all the
heavy bad air from my lungs, and the weight of the bubble, you saw
how it was carried up. I want you now to consider this phenomenon
of weight as indicating how exceedingly different particles are one

Jljdr

Platinum.

Water:

o

from the other; and I will take as illustrations these very common
things, air, water, the heaviest body, platinum, and this gas, and
observe how they differ in this respect; for if I take a piece of plat-
inum of that size (Fig. 29), it is equal to the weight of portions of
water, air, and hydrogen of the bulks I have represented in these
spheres; and this illustration gives you a very good idea of the extraor-
dinary difference with regard to the gravity of the articles having
this enormous difference in bulk. [The following tabular statement
having reference to this illustration appeared on the diagram board.]

Hydrogen

I

Air

I

^^

Water

11943

829

I

Platinum

256774

1 7831

21-5

Whenever oxygen and hydrogen unite together they produce
water, and you have seen the extraordinary difference between the

CHEMICAL AFFINITY 53

bulk and appearance of the water so produced and the particles of
which it consists chemically. Now we have never yet been able to
reduce either oxygen or hydrogen to the liquid state; and yet their
first impulse, when chemically combined, is to take up first this
liquid condition and then the solid condition. We never combine
these different particles together without producing water; and it is
curious to think how often you must have
made the experiment of combining oxygen
and hydrogen to form water without know-
ing it. Take a candle, for instance, and a
clean silver spoon (or a piece of clean tin
will do), and, if you hold it over the flame,
you immediately cover it with dew — not a
smoke — which presently evaporates. This,
perhaps, will serve to show it better. Mr.
Anderson will put a candle under that jar, ''°' ^°

and you will see how soon the water is produced (Fig. 30) . Look at
that dimness on the sides of the glass, which will soon produce drops,
and trickle down into the plate. Well, that dimness and these drops
are water, formed by the union of the oxygen of the air with the
hydrogen existing in the wax of which that candle is formed.

And now, having brought you, in the first place, to the consider-
ation of chemical attraction, I must enlarge your ideas so as to
include all substances which have this attraction for each other; for
it changes the character of bodies, and alters them in this way and
that way in the most extraordinary manner, and produces other
phenomena wonderful to think about. Here is some chlorate of
potash, and there some sulphuret of antimony ('*). We will mix
these two different sets of particles together and I want to show you,
in a general sort of way, some of the phenomena which take place
when we make different particles act together. Now I can make
these bodies act upon each other in several ways. In this case I am
going to apply heat to the mixture; but if I were to give a blow
with a hammer, the same result would follow. [A lighted match

" chlorate of potash and sulphuret of antimony. Great care must be taken in
mixing these substances, as the mixture is dangerously explosive. They must be
powdered separately and mixed together with a feather on a sheet ot paper, or by
passing them several times through a small sieve.

54 FARADAY

was brought to the mixture, which immediately exploded with a
sudden flash, evolving a dense white smoke.] There you see the
result of the action of chemical affinity overcoming the attraction of
cohesion of the particles. Again, here is a little sugar ("), quite a
different substance from the black sulphuret of antimony, and you
shall see what takes place when we put the two together. [The
mixture was touched with sulphuric acid, when it took fire, and
burnt gradually and with a brighter flame than in the former
instance.] Observe this chemical affinity traveling about the mass,
and setting it on fire, and throwing it into such wonderful agitation!

I must now come to a few circumstances which require careful
consideration. We have already examined one of the effects of this
chemical affinity, but, to make the matter more clear, we must
point out some others. And here are two salts dissolved in water('*).
They are both colorless solutions, and in these glasses you can not
see any difference between them. But if I mix them, I shall have
chemical attraction take place. I will pour the two together into
this glass, and you will at once see, I have no doubt, a certain amount
of change. Look, they are already becoming milky, but they are
sluggish in their action — not quick as the others were — for we have
endless varieties of rapidity in chemical action. Now, if I mix them
together, and stir them so as to bring them properly together, you
will soon see what a different result is produced. As I mix them they
get thicker and thicker, and you see the liquid is hardening and
stiffening, and before long I shall have it quite hard; and before
the end of the lecture it will be a solid stone — a wet stone, no doubt,
but more or less solid — in consequence of the chemical affinity. Is not
this changing two liquids into a solid body a wonderful manifesta-
tion of chemical affinity ?

There is another remarkable circumstance in chemical affinity,
which is, that it is capable of either waiting or acting at once. And
this is very singular, because we know of nothing of the kind in
the forces either of gravitation or cohesion. For instance: here are

"The mixture of chlorate of potash and sugar does not require the same pre-
cautions. They may be rubbed together in a pestle and mortar without fear. One
part of chlorate of potash and three parts of sugar will answer. The mixture need
only be touched with a glass rod dipped in oil of vitriol.

'* Two salts dissolved in water. Sulphate of soda and chloride of calcium. The
solutions must be saturated for the experiment to succeed well.

CHEMICAL AFFINITY 55

some oxygen particles, and here is a lump of carbon particles. I am
going to put the carbon particles into the oxygen; they can act, but
they do not — they are just Uke this unhghted candle. It stands here
quietly on the table, waiting until we want to light it. But it is not
so in this other case: here is a substance, gaseous like the oxygen,
and if I put these particles of metal into it the two combine at once.
The copper and the chlorine unite by their power of chemical affinity,
and produce a body entirely unlike either of the substances used.
And in this other case, it is not that there is any deficiency of affinity
between the carbon and oxygen, for the moment I choose to put
them in a condition to exert their affinity, you will see the difference.
[The piece of charcoal was ignited, and introduced into the jar of
oxygen, when the combustion proceeded with vivid scintillations.]
Now this chemical action is set going exactly as it would be if I
had lighted the candle, or as it is when the servant puts coals on and
lights the fire: the substances wait until we do something which is
able to start the action. Can any thing be more beautiful than this
combustion of charcoal in oxygen? You must understand that each
of these little sparks is a portion of the charcoal, or the bark of the
charcoal thrown off white hot into the oxygen, and burning in it
most brilliantly, as you see. And now let me tell you another thing,
or you will go away with a very imperfect notion of the powers
and effects of this affinity. There you see some charcoal burn-
ing in oxygen. Well, a piece of lead will burn in oxygen just
as well as the charcoal does, or indeed better, for absolutely that
piece of lead will act at once upon the oxygen as the copper did in
the other vessel with regard to the chlorine. And here, also, a piece
of iron — if I light it and put it into the oxygen, it will burn away
just as the carbon did. And I will take some lead, and show you that
it will burn in the common atmospheric oxygen at the ordinary
temperature. These are the lumps of lead which you remember we
had the other day — the two pieces which clung together. Now
these pieces, if I take them to-day and press them together, will not
stick, and the reason is that they have attracted from the atmosphere
a part of the oxygen there present, and have become coated as with
a varnish by the oxide of lead, which is formed on the surface by a
real process of combustion or combination. There you see the iron

56 FARADAY

burning very well in oxygen, and I will tell you the reason why those
scissors and that lead do not take fire while they are lying on the
table. Here the lead is in a lump, and the coating of oxide remains
on its surface, while there you see the melted oxide is clearing itself
off from the iron, and allowing more and more to go on burning.
In this case, however, [holding up a small glass tube containing
lead pyrophorus(")], the lead has been very carefully produced in
fine powder, and put into a glass tube, and hermetically sealed so as
to preserve it, and I expect you will see it take fire at once. This has
been made about a month ago, and has thus had time to sink down
to its normal temperature; what you see, therefore, is the result
of chemical affinity alone. [The tube was broken at the end, and
the lead poured out on to a piece of paper, whereupon it immediately
took fire.] Look! look at the lead burning! Why, it has set fire to
the paper! Now that is nothing more than the common affinity
always existing between very clean lead and the atmospheric oxygen;
and the reason why this iron does not burn until it is made red hot
is because it has got a coating of oxide about it, which stops the action
of the oxygen — putting a varnish, as it were, upon its surface, as we
varnish a picture — absolutely forming a substance which prevents
the natural chemical affinity between the bodies from acting.

I must now take you a little farther in this kind of illustration, or
consideration I would rather call it, of chemical affinity. This attrac-
tion between different particles exists also most curiously in cases
where they are previously combined with other substances. Here
is a little chlorate of potash containing the oxygen which we found
yesterday could be procured from it; it contains the oxygen there
combined and held down by its chemical affinity with other things,
but still it can combine with sugar, as you saw. This affinity can thus
act across substances, and I want you to see how curiously what we
call combustion acts with respect to this force of chemical affinity.
If I take a piece of phosphorus and set fire to it, and then place a
jar of air over the phosphorus, you see the combustion which we
are having there on account of chemical affinity (combustion being
in all cases the result of chemical affinity). The phosphorus is

'* Lead pyrophorus. This is tartrate of lead which has been heated in a glass tube
to dull redness as long as vapors are emitted. As soon as they cease to be evolved
the end of the tube is sealed, and it is allowed to cool.

CHEMICAL AFFINITY 57

escaping in that vapor, whicii will condense into a snowlike mass at
the close of the lecture. But suppose I limit the atmosphere, what
then? why, even the phosphorus will go out. Here is a piece o£
camphor, which will burn very well in the atmosphere, and even on
water it will float about and burn, by reason of some of its particles
gaining access to the air. But if I limit the quantity of air by placing
a jar over it, as I am now doing, you will soon find the camphor
will go out. Well, why does it go out ? not for want of air, for there
is plenty of air remaining in the jar. Perhaps you will be shrewd
enough to say for want of oxygen.

This, therefore, leads us to the inquiry as to whether oxygen can
do more than a certain amount of work. The oxygen there (Fig. 30)
can not go on burning an unlimited quantity of candle, for that has
gone out, as you see; and its amount of chemical attraction or
affinity is just as strikingly limited: it can no more be fallen short
of or exceeded than can the attraction of gravitation. You might
as soon attempt to destroy gravitation, or weight, or all things that
exist, as to destroy the exact amount of force exerted by this oxygen.
And when I pointed out to you that eight by weight of oxygen to
one by weight of hydrogen went to form water, I meant this, that
neither of them would combine in different proportions with the
other, for you can not get ten of hydrogen to combine with six of
oxygen, or ten of oxygen to combine with six of hydrogen; it must
be eight of oxygen and one of hydrogen. Now suppose I limit the
action in this way : this piece of cotton wool burns, as you see, very
well in the atmosphere; and I have known of cases of cotton-mills
being fired as if with gunpowder through the very finely divided
particles of cotton being diffused through the atmosphere in the
mill, when it has sometimes happened that a flame has caught these
raised particles, and it has run from one end of the mill to the
other and blown it up. That, then, is on account of the affinity
which the cotton has for the oxygen; but suppose I set fire to this
piece of cotton v/hich is rolled up tightly; it does not go on burning,
because I have limited the supply of oxygen, and the inside is pre-
vented from having access to the oxygen, just as it was in the case
of the lead by the oxide. But here is some cotton which has been
imbued with oxygen in a certain manner. I need not trouble you

58 FARADAY

now with the way it is prepared; it is called guncottonC""). See how
that burns [setting fire to a piece]; it is very different from the
other, because the oxygen which must be present in its proper
amount is put there beforehand. And I have here some pieces of
paper which are prepared like the guncotton(^'), and imbued with
bodies containing oxygen. Here is some which has been soaked in
nitrate of strontia: you will see the beautiful red color of its flame;
and here is another which I think contains baryta, which gives that
fine green light; and I have here some more which has been soaked
in nitrate of copper : it does not burn quite so brightly, but still very
beautifully. In all these cases the combustion goes on independent
of the oxygen of the atmosphere. And here we have some gun-
powder put into a case, in order to show that it is capable of burning
under water. You know that we put it into a gun, shutting off the
atmosphere with shot, and yet the oxygen which it contains supplies
the particles with that without which chemical action could not
proceed. Now I have a vessel of water here, and am going to make
the experiment of putting this fuse under the water, and you will
see whether that water can extinguish it; here it is burning out of
the water, and there it is burning under the water; and so it will
continue until exhausted, and all by reason of the requisite amount
of oxygen being contained within the substance. It is by this kind
of attraction of the different particles one to the other that we are
enabled to trace the laws of chemical affinity, and the wonderful
variety of the exertions of these laws.

Now I want you to observe that one great exertion of this power
which is known as chemical affinity is to produce Heat and light;
you know, as a matter of fact, no doubt, that when bodies burn they
give out heat, but it is a curious thing that this heat does not con-
tinue; the heat goes away as soon as the action stops, and you see,
thereby, that it depends upon the action during the time it is going

^^ Guncotton is made by immersing cotton wool in a mixture of sulphuric acid and
the strongest nitric acid or of sulphuric acid and nitrate of potash.

2' Paper prepared lik.e guncotton. It should be bibulous paper, and must be soaked
for ten minutes in a mixture of ten parts, by measure, of oil of vitriol with five parts
of strong fuming nitric acid. The paper must afterward be thoroughly washed with
warm distilled water, and then carefully dried at a gentle heat. The paper is then
saturated with chlorate of strontia, or chlorate of baryta, or nitrate of copper, by im-
mersion in a warm solution of these salts. (See Chemical News, vol. i., p. 36.)

CHEMICAL AFFINITY 59

on. It is not so with gravitation; this force is continuous, and is just
as effective in making that lead press on the table as it was when it
first fell there. Nothing occurs there which disappears when the
action of falling is over; the pressure is upon the table, and will
remain there until the lead is removed; whereas, in the action of
chemical affinity to give light and heat, they go away immediately
the action is over. This lamp seems to evolve heat and light con-
tinuously, but it is owing to a constant stream of air coming into it
on all sides, and this work of producing light and heat by chemical
affinity will subside as soon as the stream of air is interrupted. What,
then, is this curious condition of heat? Why, it is the evolution of
another power of matter — of a power new to us, and which we must
consider as if it were now for the very first time brought under our
notice. What is heat ? We recognize heat by its power of liquefying
solid bodies and vaporizing liquid bodies; by its power of setting
in action, and very often overcoming, chemical affinity. Then how
do we obtain heat? We obtain it in various ways; most abundantly
by means of the chemical affinity we have just before been speaking
about, but we can also obtain it in many other ways. Friction will
produce heat. The Indians rub pieces of wood together until they
make them hot enough to take fire; and such things have been
known as two branches of a tree rubbing together so hard as to set
the tree on fire. I do not suppose I shall set these two pieces of wood
on fire by friction, but I can readily produce heat enough to ignite
some phosphorus. [The lecturer here rubbed two pieces of cedar
wood strongly against each other for a minute, and then placed on
them a piece of phosphorus, which immediately took fire.] And if
you take a smooth metal button stuck on a cork, and rub it on a
piece of soft deal wood, you will make it so hot as to scorch wood
and paper, and burn a match.

I am now going to show you that we can obtain heat, not by
chemical affinity alone, but by the pressure of air. Suppose I take a
pellet of cotton and moisten it with a little ether, and put it into a
glass tube (Fig. 31), and then take a piston and press it down sud-
denly, I expect I shall be able to burn a little of that ether in the
vessel. It wants a suddenness of pressure, or we shall not do what
we require. [The piston was forcibly pressed down, when a flame,

6o FARADAY

due to the combustion of the ether, was visible in the lower part of
the syringe.] All we want is to get a little ether in vapor, and give
fresh air each time, and so we may go on again and again, getting
heat enough by the compression of air to fire the ether-vapor.

This, then, I think, will be sufficient, accompanied with all you
have previously seen, to show you how we procure heat. And now
for the effects of this power. We need not consider many of
them on the present occasion, because, when you have seen its
power of changing ice into water and water into steam, you
have seen the two principal results of the application of heat.
I want you now to see how it expands all bodies — all bodies
but one, and that under limited circumstances. Mr. Anderson
will hold a lamp under that retort, and you will see, the
moment he does so, that the air will issue abundantly from
the neck which is under water, because the heat which he
^' applies to the air causes it to expand. And here is a brass rod
(Fig. 32) which goes through that hole, and fits also accurately into
this gauge; but if I make it warm with this spirit lamp, it will only
go in the gauge or through the hole with difficulty; and if I were to
put it into boiling water it would not go through at all. Again, as
soon as the heat escapes from bodies, they collapse: see how the air

3)

( ? -Q

Fig. 32

is contracting in the vessel now that Mr. Anderson has taken away
his lamp; the stem of it is filling with water. Notice too, now, that
although I cannot get the tube through this hole or into the gauge,
the moment I cool it, by dipping it into water, it goes through with
perfect facility, so that we have a perfect proof of this power of heat
to contract and expand bodies.

LECTURE V
MAGNETISM— ELECTRICITY

I WONDER whether we shall be too deep to-day or not. Remem-
ber that we spoke of the attraction by gravitation of all bodies
to all bodies by their simple approach. Remember that we
spoke of the attraction of particles of the same kind to each other —
that power which keeps them together in masses — iron attracted to
iron, brass to brass, or water to water. Remember that we found, on
looking into water, that there were particles of two different kinds
attracted to each other; and this was a great step beyond the first
simple attraction of gravitation, because here we deal with attraction
between different kinds of matter. The hydrogen could attract the
oxygen and reduce it to water, but it could not attract any of its
own particles, so that there we obtained a first indication of the
existence of two attractions.

To-day we come to a kind of attraction even more curious than
the last, namely, the attraction which we find to be of a double
nature — of a curious and dual nature. And I want, first of all, to
make the nature of this doubleness clear to you. Bodies are some-
times endowed with a wonderful attraction, which is not found in
them in their ordinary state. For instance, here is a piece of shellac,
having the attraction of gravitation, having the attraction of cohesion,
and if I set fire to it, it would have the attraction of chemical affinity
tb the oxygen in the atmosphere. Now all these powers we find in
it as if they were parts of its substance; but there is another property
which I will try and make evident by means of this ball, this bubble
of air [a light India-rubber ball, inflated and suspended by a thread].
There is no attraction between this ball and this shellac at present;
there may be a little wind in the rooms slightly moving the ball
about, but there is no attraction. But if I rub the shellac with a
piece of flannel [rubbing the shellac, and then holding it near the
ball], look at the attraction which has arisen out of the shellac simply

6i

62 FARADAY

by this friction, and which I may take away as easily by drawing
it gently through my hand. [The lecturer repeated the experiment
of exciting the shellac, and then removing the attractive power by
drawing it through his hand.] Again, you will see I can repeat this
experiment with another substance; for if I take a glass rod, and
rub it with a piece of silk covered with what we call amalgam, look
at the attraction which it has; how it draws the ball toward it; and
then, as before, by quietly rubbing it through the hand, the attraction
will be all removed again, to come back by friction with this silk.

But now we come to another fact. I will take this piece of shellac,
and make it attractive by friction; and remember that, whenever
we get an attraction of gravity, chemical affinity, adhesion, or elec-
tricity (as in this case), the body which attracts is attracted also, and
just as much as that ball was attracted by the shellac, the shellac was
attracted by the ball. Now I will suspend this piece of excited shellac
in a little paper stirrup, in this way (Fig. 33), in order to make it
move easily, and I will take another piece of shellac, and, after

rubbing it with flannel, will bring them near together: you will
think that they ought to attract each other; but now what happens.''
It does not attract; on the contrary, it very strongly repels, and I
can thus drive it round to any extent. These, therefore, repel each
other, although they are so strongly attractive — repel each other to
the extent of driving this heavy piece of shellac round and round
in this way. But if I excite this piece of shellac as before, and take
this piece of glass and rub it with silk, and then bring them near,

ELECTRICITY 63

what think you will happen? [The lecturer held the excited glass
near the excited shellac, when they attracted each other strongly.]
You see, therefore, what a difference there is between these two
attractions; they are actually two \inds of attraction concerned in
this case, quite different to any thing we have met with before, but
the force is the same. We have here, then, a double attraction — a
dual attraction or force — one attracting and the other repelling.

Again, to show you another experiment which will help to make
this clear to you: Suppose I set up this rough indicator again [the

Fig. 34

excited shellac suspended in the stirrup]: it is rough, but delicate
enough for my purpose; and suppose I take this other piece of
shellac, and take away the power, which I can do by drawing it
gently through the hand; and suppose I take a piece of flannel
(Fig. 34), which I have shaped into a cap for it and made dry. I
will put this shellac into the flannel, and here comes out a very
beautiful result. I will rub this shellac and the flannel together
(which I can do by twisting the shellac round), and leave them in
contact; and then if I ask, by bringing them near our indicator,
what is the attractive force.? it is nothing; but if I take them apart,
and then ask what will they do when they are separated? why, the
shellac is strongly repelled, as it was before, but the cap is strongly
attractive; and yet, if I bring them both together again, there is no
attraction; it has all disappeared [the experiment was repeated].

64 FARADAY

Those two bodies, therefore, still contain this attractive power; when
they were parted, it was evident to your senses that they had it,
though they do not attract when they are together.

This, then, is sufficient, in the outset, to give you an idea o£ the
nature of the force which we call Electricity. There is no

Fig. 35

end to the things from which you can evolve this power. When you
go home, take a stick of sealing-wax — I have rather a large stick, but
a smaller one will do — and make an indicator of this sort (Fig. 35).
Take a watch-glass (or your watch itself will do; you only want
something which shall have a round face) ; and now, if you place
a piece of flat glass upon that, you have a very easily moved centre;
and if I take this lath and put it on the flat glass (you see I am
searching for the centre of gravity of this lath; I want to balance it
upon the watch-glass), it is very easily moved round; and if I take
this piece of sealing-wax and rub it against my coat, and then try
whether it is attractive [holding it near the lath], you see how strong
the attraction is; I can even draw it about. Here, then, you have a
very beautiful indicator, for I have, with a small piece of sealing-wax

and my coat, pulled round a plank of
that kind, so you need be in no want
of indicators to discover the presence
of this attraction. There is scarcely a
substance which we may not use.
Here are some indicators (Fig. 36). I
^°' ^ bend round a strip of paper into a

hoop, and we have as good an indicator as can be required. See how
it rolls along, traveling after the sealing-wax! If I make them smaller,
of course we have them running faster, and sometimes they are actu-
ally attracted up into the air. Here, also, is a Httle collodion balloon.
It is so electrical that it will scarcely leave my hand unless to go to the

ELECTRICITY 65

Other. See how curiously electrical it is; it is hardly possible for me
to touch it without making it electrical; and here is a piece which
clings to any thing it is brought near, and which it is not easy to lay
down. And here is another substance, gutta-percha, in thin strips:
it is astonishing how, by rubbing this in your hands, you make it
electrical; but our time forbids us to go farther into this subject at
present; you see clearly there are two kinds of electricities which
may be obtained by rubbing shellac with flannel or glass with silk.
Now there are some curious bodies in nature (of which I have
two specimens on the table) which are called magnets or loadstones;
ores of iron, of which there is a great deal sent from Sweden. They
have the attraction of gravitation, and attraction of cohesion, and
certain chemical attraction; but they also have a great attractive
power, for this little key is held up by this stone. Now that is not
chemical attraction; it is not the attraction of chemical affinity, or
of aggregation of particles, or of cohesion, or of electricity (for it will
not attract this ball if I bring it near it), but it is a separate and
dual attraction, and, what is more, one which is not readily removed
from the substance, for it has existed in it for ages and ages in the
bowels of the earth. Now we can make artificial magnets (you
will see me to-morrow make artificial magnets of extraordinary
power) . And let us take one of these artificial magnets and examine
it, and see where the power is in the mass, and whether it is a dual
power. You see it attracts these keys, two or three in succession,
and it will attract a very large piece of iron. That, then, is a very
different thing indeed to what you saw in the case of the shellac, for
that only attracted a light ball, but here I have several ounces of iron
held up. And if we come to examine this attraction a little more
closely, we shall find it presents some other remarkable differences;
first of all, one end of this bar (Fig. 37) attracts this key, but the
middle does not attract. It is not, then, the whole of the substance
which attracts. If I place this little key in the middle it does not
adhere; but if I place it there, a little nearer the end, it does, though
feebly. Is it not, then, very curious to find that there is an attractive
power at the extremities which is not in the middle — to have thus
in one bar two places in which this force of attraction resides ? If I
take this bar and balance it carefully on a point, so that it will be

66 FARADAY

free to move round, I can try what action this piece of iron has on it.
Well, it attracts one end, and it also attracts the other end, just as
you saw the shellac and the glass did, with the exception of its not
attracting in the middle. But if now, instead of a piece of iron, I
take a magnet, and examine it in a similar way, you see that one of
its ends repels the suspended magnet; the force, then, is no longer
attraction, but repulsion; but, if I take the other end of the magnet
and bring it near, it shows attraction again.

You will see this better, perhaps, by another kind of experiment.
Here (Fig. 38) is a little magnet, and I have colored the ends difler-

FiG. 37 Fig. 38

ently, so that you may distinguish one from the other. Now this
end (S) of the magnet (Fig. 37) attracts the uncolored end of the
little magnet. You see it pulls toward it with great power; and, as I
carry it round, the uncolored end still follows. But now, if I gradu-
ally bring the middle of the bar magnet opposite the uncolored end
of the needle, it has no effect upon it, either of attraction or
repulsion, until, as I come to the opposite extremity (N), you
see that it is the colored end of the needle which is pulled toward it.
We are now, therefore, dealing with two kinds of power, attracting
different ends of the magnet — a double power, already existing in
these bodies, which takes up the form of attraction and repulsion.
And now, when I put up this label with the word Magnetism, you
will understand that it is to express this double power.

Now with this loadstone you may make magnets artificially.
Here is an artificial magnet (Fig. 39) in which both ends have been
brought together in order to increase the attraction. This mass will
lift that lump of iron, and, what is more, by placing this \eeper,
as it is called, on the top of the magnet, and taking hold of the

Fig. 39

ELECTRICITY 67

handle, it will adhere sufficiently strongly to allow itself to be lifted
up, so wonderful is its power of attraction. If you take a needle,
and just draw one of its ends along one extremity of the magnet,
and then draw the other end along the other extremity, and then
gently place it on the surface of some water (the needle
will generally float on the surface, owing to the slight
greasiness communicated to it by the fingers), you will
be able to get all the phenomena of attraction and repul-
sion by bringing another magnetized needle near to it.

I want you now to observe that, although I have shown
you in these magnets that this double power becomes evi-
dent principally at the extremities, yet the whole of the
magnet is concerned in giving the power. That will at
first seem rather strange; and I must therefore show you
an experiment to prove that this is not an accidental matter, but that
the whole of the mass is really concerned in this force, just as in
falling the whole of the mass is acted upon by the force of gravita-
tion. I have here (Fig. 40) a steel bar, and I am going to make it a
magnet by rubbing it on the large magnet- (Fig. 39). I have now
made the two ends magnetic in opposite ways. I do not at present
know one from the other, but we can soon find out. You see, when
I bring it near our magnetic needle (Fig. 38), one end repels and
the other attracts; and the middle will neither attract nor repel — ^it
can not, because it is halj way between the two ends. But now, if I
break out that piece («, s), and then examine it, see how strongly
one end («) pulls at this end (S, Fig. 38), and how it repels the
other end (N). And so it can be shown that every part of the magnet
contains this power of attraction and repul-
sion, but that the power is only rendered evi-
dent at the end of the mass. You will

N

Fig. 40

understand all this in a little while; but what
you have now to consider is that every part of this steel is in itself a
magnet. Here is a little fragment which I have broken out of the
very centre of the bar, and you will still see that one end is attrac-
tive and the other is repulsive. Now is not this power a most won-
derful thing? And very strange, the means of taking it from one
substance and bringing it to other matters. I can not make a piece

68 FARADAY

of iron or any thing else heavier or lighter than it is; its cohesive
power it must and does have; but, as you have seen by these experi-
ments, we can add or subtract this power of magnetism, and almost
do as we like with it.

And now we will return for a short time to the subject treated of
at the commencement of this lecture. You see here (Fig. 41) a large
machine arranged for the purpose of rubbing glass with silk, and

Fig. 41

for obtaining the power called electricity; and the moment the
handle of the machine is turned a certain amount of electricity is
evolved, as you will see by the rise of the little straw indicator (at A) .
Now I know, from the appearance of repulsion of the pith ball at
the end of the straw, that electricity is present in those brass con-
ductors (BB), and I want you to see the manner in which that
electricity can pass away [touching the conductor (B) with his
finger, the lecturer drew a spark from it, and the straw electrometer
immediately fell]. There, it has all gone; and that I have really
taken it away you shall see by an experiment of this sort. If I hold
this cylinder of brass by the glass handle, and touch the conductor
with it, I take away a little of the electricity. You see the spark in
which it passes, and observe that the pith-ball indicator has fallen
a little, which seems to imply that so much electricity is lost; but it
is not lost; it is here in this brass, and I can take it away and carry
it about, not because it has any substance of its own, but by some

ELECTRICITY 69

Strange property which we have not before met with as belonging
to any other force. Let us see whether we have it here or not. [The
lecturer brought the charged cylinder to a jet from which gas was
issuing; the spark was seen to pass from the cylinder to the jet, but
the gas did not light.] Ah! the gas did not light, but you saw the
spark; there is, perhaps, some draught in the room which blew the
gas on one side, or else it would light; we will try this experiment
afterward. You see from the spark that I can transfer the power
from the machine to this cylinder, and then carry it away and give
it to some other body.

You know very well, as a matter of experiment, that we can trans-
fer the power of heat from one thing to another; for if I put my
hand near the fire it becomes hot. I can show you this by placing
before us this ball, which has just been brought red-hot from the
fixe. If I press this wire to it some of the heat will be transferred
from the ball, and I have only now to touch this piece of gun-cotton
with the hot wire, and you see how I can transfer the heat from the
ball to the wire, and from the wire to the cotton. So you see that some
powers are transferable, and others are not. Observe how long the
heat stops in this ball. I might touch it with the wire or wdth my
finger, and if I did so quickly I should merely burn the surface of
the skin; whereas, if I touch that cylinder, however rapidly, with my
finger, the electricity is gone at once — dispersed on the instant, in a
manner wonderful to think of.

I must now take up a little of your time in showing you the
manner in which these powers are transferred from one thing to
another; for the manner in which force may be conducted or trans-
mitted is extraordinary, and most essential for us to understand.
Let us see in what manner these powers travel from place to place.
Both heat and electricity can be conducted; and here is an arrange-
ment I have made to show how the former can travel. It consists of
a bar of copper (Fig. 42) ; and if I take a spirit lamp (this is one way
of obtaining the power of heat) and place it under that little chim-
ney, the flame will strike against the bar of copper and keep it hot.
Now you are aware that power is being transferred from the flame
of that lamp to the copper, and you will see by-and-by that it is being
conducted along the copper from particle to particle; for, inasmuch

70 FARADAY

as I have fastened these wooden balls by a little wax at particular
distances from the point where the copper is first heated, first one
ball will fall and then the more distant ones, as the heat travels
along, and thus you will learn that the heat travels gradually through
the copper. You will see that this is a very slow conduction of power
as compared with electricity. If I take cylinders of wood and metal,
joined together at the ends, and wrap a piece of paper round, and
then apply the heat of this lamp to the place where the metal and
wood join, you will see how the heat will accumulate where the

Fig. 42

wood is, and burn the paper with which I have covered it; but where
the metal is beneath, the heat is conducted away too fast for the
paper to be burned. And so, if I take a piece of wood and a piece
of metal joined together, and put it so that the flame shall play
equally both upon one and the other, we shall soon find that the
metal will become hot before the wood; for if I put a piece of phos-
phorus on the wood and another piece on the copper, you will find
that the phosphorus on the copper will take fire before that on the
wood is melted; and this shows you how badly the wood conducts
heat. But with regard to the traveUng of electricity from place to
place, its rapidity is astonishing. I will, first of all, take these pieces
of glass and metal, and you will soon understand how it is that the
glass does not lose the power which it acquired when it is rubbed
by the silk; by one or two experiments I will show you. If I take
this piece of brass and bring it near the machine, you see how the
electricity leaves the latter and passes to the brass cyUnder. And
again: if I take a rod of metal and touch the machine with it, I
lower the indicator; but when I touch it with a rod of glass, no

ELECTRICITY 7I

power is drawn away, showing you that the electricity is conducted
by the glass and the metal in a manner entirely different; and, to
make you see that more clearly, we will take one of our Leyden
jars. Now I must not embarrass your minds with this subject too
much, but if I take a piece of metal and bring it against the knob
at the top and the metallic coating at the bottom, you will see the
electricity passing through the air as a brilliant spark. It takes no
sensible time to pass through this; and if I were to take a long
metallic wire, no matter what the length, at least as far as we are
concerned, and if I make one end of it touch the outside, and the
other touch the knob at the top, see how
the electricity passes! It has flashed in-
stantaneously through the whole length
of this wire. Is not this different from
the transmission of heat through this
copper bar (Fig. 42) which has taken a
quarter of an hour or more to reach the
first ball ?

Here is another experiment for the
purpose of showing the conductibility of
this power through some bodies and not
through others. Why do I have this
arrangement made of brass? [pointing
to the brass work of the electrical ma-
chine, Fig. 41]. Because it conducts elec-
tricity. And why do I have these col-
umns made of glass? Because they
obstruct the passage of electricity. And
why do I put that paper tassel (Fig. 43)
at the top of the pole, upon a glass rod,
and connect it with this machine by
means of a wire ? You see at once that
as soon as the handle of the machine is
turned, the electricity which is evolved travels along this wire and up
the wooden rod, and goes to the tassel at the top, and you see the
power of repulsion with which it has endowed these strips of paper,
each spreading outward to the ceiling and sides of the room. The out-

FiG. 43

72 FARADAY

side of that wire is covered with gutta-percha; it would not serve
to keep the force from you when touching it with your hands,
because it would burst through; but it answers our purpose for the
present. And so you perceive how easily I can manage to send this
power of electricity from place to place by choosing the materials
which can conduct the power. Suppose I want to fire a portion of
gunpowder, I can readily do it by this transferable power of elec-
tricity. I will take a Leyden jar, or any other arrangement which
gives us this power, and arrange wires so that they may carry the
power to the place I wish; and then placing a little gunpowder on
the extremities of the wires, the moment I make the connection by
this discharging rod I shall fire the gunpowder [the connection was
made and the gunpowder ignited]. And if I were to show you a
stool like this, and were to explain to you its construction, you could
easily understand that we use glass legs because these are capable
of preventing the electricity from going away to the earth. If, there-
fore, I were to stand on this stool, and receive the electricity through
this conductor, I could give it to anything that I touched. [The
lecturer stood upon the insulating stool, and placed himself in con-
nection with the conductor of the machine.] Now I am electrified;
I can feel my hair rising up, as the paper tassel did just now. Let us
see whether I can succeed in lighting gas by touching the jet with
my finger. [The lecturer brought his finger near a jet from which
gas was issuing, when, after one or two attempts, the spark which
came from his iinger to the jet set fire to the gas.] You now see how
it is that this power of electricity can be transferred from the matter
in which it is generated, and conducted along wires and other bodies,
and thus be made to serve new purposes, utterly unattainable by the
powers we have spoken of on previous days; and you will not now
be at a loss to bring this power of electricity into comparison with
those which we have previously examined, and to-morrow we shall
be able to go farther into the consideration of these transferable
powers.

LECTURE VI
THE CORRELATION OF THE PHYSICAL FORCES

WE have frequently seen, during the course of these lectures,
that one of those powers or forces of matter, of which I
have written the names on that board, has produced results
which are due to the action of some other force. Thus you have
seen the force of electricity acting in other ways than in attracting;
you have also seen it combine matters together or disunite them by
means of its action on the chemical force; and in this case, therefore,
you have an instance in which these two powers are related. But
we have other and deeper relations than these; we have not merely
to see how it is that one power affects another — how the force of
heat affects chemical affinity, and so forth, but we must try and
comprehend what relation they bear to each other, and how these
powers may be changed one into the other; and it will to-day require
all my care, and your care too, to make this clear to your minds.
I shall be obliged to confine myself to one or two instances, because
to take in the whole extent of this mutual relation and conversion
of forces would surpass the human intellect.

In the first place, then, here is a piece of fine zinc foil, and if I cut
it into narrow strips and apply to it the power of heat, admitting the
contact of air at the same time, you will find that it burns; and then,
seeing that it burns, you will be prepared to say that there is chemi-
cal action taking place. You see all I have to do is to hold the piece
of zinc at the side of the flame, so as to let it get heated, and yet to
allow the air which is flowing into the flame from all sides to have
access to it; there is the piece of zinc burning just like a piece of
wood, only brighter. A part of the zinc is going up into the air
in the form of that white smoke, and part is falling down on to the
table. This, then, is the action of chemical affinity exerted between
the zinc and the oxygen of the air. I will show you what a curious
kind of affinity this is by an experiment which is rather striking

73

74 FARADAY

when seen for the first time. I have here some iron fiUngs and gun-
powder, and will mix them carefully together, with as little rough
handling as possible; now we will compare the combustibility, so to
speak, of the two. I will pour some spirit of wine into a basin and
set it on fire; and, having our flame, I will drop this mixture of
iron filings and gunpowder through it, so that both sets of particles
will have an equal chance of burning. And now tell me which of
them it is that burns? You see a plentiful combustion of the iron
filings; but I want you to observe that, though they have equal
chances of burning, we shall find that by far the greater part of the
gunpowder remains untouched; I have only to drain off this spirit
of wine, and let the powder which has gone through the flame dry,
which it will do in a few minutes, and I will then test it with a
lighted match. So ready is the iron to burn, that it takes, under
certain circumstances, even less time to catch fire than gunpowder.
[As soon as the gunpowder was dry, Mr. Anderson handed it to the
lecturer, who apphed a lighted match to it, when a sudden flash
showed how large a proportion of gunpowder had escaped com-
bustion when falling through the flame of alcohol.]

These are all cases of chemical affinity, and I show them to make
you understand that we are about to enter upon the consideration of
a strange kind of chemical affinity, and then to see how far we are
enabled to convert this force of affinity into electricity or magnetism,
or any other of the forces which we have discussed. Here is some
zinc (I keep to the metal zinc, as it is very useful for our purpose),
and I can produce hydrogen gas by putting the zinc and sulphuric
acid together, as they are in that retort; there you see the mixture
which gives us hydrogen — the zinc is pulling the water to pieces and
setting free hydrogen gas. Now we have learned by experience that
if a little mercury is spread over that zinc, it does not tal{e away its
power of decomposing the water, but modifies it most curiously. See
how that mixture is now boiling; but when I add a little mercury
to it the gas ceases to come off. We have now scarcely a bubble of
hydrogen set free, so that the action is suspended for the time. We
have not destroyed the power of chemical affinity, but modified it
in a wonderful and beautiful manner. Here are some pieces of zinc
covered with mercury exactly in the same way as the zinc in that

THE CORRELATION OF THE PHYSICAL FORCES 75

retort is covered; and if I put this plate into sulphuric acid I get no
gas, but this most extraordinary thing occurs, that i£ I introduce
along with the zinc another metal which is not so combustible, then
I reproduce all the action. I am now going to put to the amalga-
mated zinc in this retort some portions of copper wire (copper not
being so combustible a metal as the zinc), and observe how I get
hydrogen again, as in the first instance; there, the bubbles are com-
ing over through the pneumatic trough, and ascending faster and
faster in the jar; the zinc now is acting by reason of its contact with
the copper.

Every step we are now taking brings us to a knowledge of new
phenomena. That hydrogen which you now see coming off so
abundantly does not come from the zinc, as it did before, but from
the copper. Here is a jar containing a solution of copper. If I put a
piece of this amalgamated zinc into it, and leave it there, it has
scarcely any action; and here is a plate of platinum which I will
immerse in the same solution, and might leave it there for hours,
days, months, or even years, and no action would take place; but,
by putting them both together, and allowing them to touch (Fig. 44),
you see what a coating of copper there is immediately thrown down
on the platinum. Why is this } The platinum has no power of itself
to reduce that metal from that fluid, but it has, in some mysterious
way, received this power by its contact with the metal zinc. Here,
then, you see a strange transfer of chemical force from one metal to
another; the chemical force from the zinc is transferred and made
over to the platinum by the mere association of the two metals. I
might take, instead of the platinum, a piece of copper or of silver,
and it would have no action of its own on this solution, but the
moment the zinc was introduced and touched the other metal, then
the action would take place, and it would become covered with
copper. Now is not this most wonderful and beautiful to see? We
still have the identical chemical force of the particles of zinc acting,
and yet, in some strange manner, we have power to make that
chemical force, or something it produces, travel from one place to
another; for we do make the chemical force travel from the zinc to
the platinum by this very curious experiment of using the two
metals in the same fluid in contact with each other.

'](> FARADAY

Let us now examine these phenomena a Uttle more closely. Here
is a drawing (Fig. 45) in which I have represented a vessel con-
taining the acid liquid and the slips of zinc and platinum or copper,
and I have shown them touching each other outside by means of
a wire coming from each of them (for it matters not whether they
touch in the fluid or outside; by pieces of metal attached, they still,
by that communication between them, have this power transferred
from one to the other) . Now if, instead of only using one vessel, as
I have shown there, I take another, and another, and put in zinc

Fig. 44

Fig. 45

and platinum, zinc and platinum, zinc and platinum, and connect the
platinum of one vessel with the zinc of another, the platinum of this
vessel with the zinc of that, and so on, we should only be using a
series of these vessels instead of one. This we have done in that
arrangement which you see behind me. I am using what we call a
Grove's voltaic battery, in which one metal is zinc and the other plat-
inum; and I have as many as forty pairs of these plates all exercising
their force at once in sending the whole amount of chemical power
there evolved through these wires under the floor and up to these two
rods coming through the table. We need do no more than just bring
these two ends in contact, when the spark shows us what power is
present; and what a strange thing it is to see that this force is brought
away from the battery behind me, and carried along through these
wires! I have here an apparatus (Fig. 46) which Sir Humphry Davy
constructed many years ago, in order to see whether this power from
the voltaic battery caused bodies to attract each other in the same
manner as the ordinary electricity did. He made it in order to
experiment with his large voltaic battery, which was the most power-

THE CORRELATION OF THE PHYSICAL FORCES 77

ful then in existence. You see there are in this glass jar two leaves of

gold, which I can cause to move to and fro by this rack-work. I will

connect each of these gold leaves with separate ends of this battery,

and if I have a sufficient number of plates in the battery, I shall

be able to show you that there will be some attraction between those

leaves even before they come in contact; if I bring them sufficiently

near when they are in communication with the

ends of the battery, they will be drawn gently

together; and you will know when this takes

place, because the power will cause the gold

leaves to burn away, which they could only do

when they touched each other. Now I am

going to cause these two leaves of gold to

approach gradually, and I have no doubt that

some of you will see that they approach before

they burn, and those who are too far off to see

them approach will see by their burning that "^^"^

they have come together. Now they are attract- ' ''

ing each other, long before the connection is complete, and there

they go! burnt up in that brilliant flash, so strong is the force. You

thus see, from the attractive force at the two ends of this battery, that

these are really and truly electrical phenomena.

Now let us consider what is this spark. I take these two ends
and bring them together, and there I get this glorious spark like the
sunlight in the heavens above us. What is this ? It is the same thing
which you saw when I discharged the large electrical machine, when
you saw one single bright flash; it is the same thing, only continued,
because here we have a more effective arrangement. Instead of hav-
ing a machine which we are obliged to turn for a long time together,
we have here a chemical power which sends forth the spark; and it
is wonderful and beautiful to see how this spark is carried about
through these wires. I want you to perceive, if possible, that this
very spark and the heat it produces (for there is heat) is neither
more nor less than the chemical force of the zinc — its very force
carried along wires and conveyed to this place. I am about to take a
portion of the zinc and burn it in oxygen gas for the sake of show-
ing you the kind of light produced by the actual combustion in

yS FARADAY

oxygen gas of some of this metal. [A tassel of zinc-foil was ignited
at a spirit lamp and introduced into a jar of oxygen, when it burnt
with a brilliant light.] That shows you what the affinity is when we
come to consider it in its energy and power. And the zinc is being
burned in the battery behind me at a much more rapid rate than you
see in that jar, because the zinc is there dissolving and burning,
and produces here this great electric light. That very same power
which in that jar you saw evolved from the actual combustion of
the zinc in oxygen, is carried along these wires and made evident
here; and you may, if you please, consider that the zinc is burning
in those cells, and that this is the light of that burning [bringing
the two poles in contact and showing the electric light]; and we
might so arrange our apparatus as to show that the amounts of power
evolved in either case are identical. Having thus obtained power
over the chemical force, how wonderfully we are able to convey it
from place to place! When we use gunpowder for explosive pur-
poses, we can send into the mine chemical
affinity by means of this electricity; not
having provided fire beforehand, we can
send it in at the moment we require it.
Now here (Fig. 47) is a vessel containing
two charcoal points, and I bring it forward
as an illustration of the wonderful power
of conveying this force from place to place.
I have merely to connect these by means of
wires to the opposite ends of the battery,
and bring the points in contact. See what
an exhibition of force we have! We have
exhausted the air so that the charcoal can
not burn, and therefore the light you see is
really the burning of the zinc in the cells
behind me; there is no disappearance of the
carbon, although we have that glorious electric light; and the moment
I cut off the connection it stops. Here is a better instance to enable
some of you to see the certainty with which we can convey this force,
where under ordinary circumstances, chemical affinity would not act.
We may absolutely take these two charcoal poles down under water,

Fig. 47

THE CORRELATION OF THE PHYSICAL FORCES 79

and get our electric light there. There they are in the water, and
you observe, when I bring them into connection, we have the same
light as we had in that glass vessel.

Now besides this production of light, we have all the other effects
and powers of burning zinc. I have a few wires here which are not
combustible, and I am going to take one of them, a small platinum
wire, and suspend it between these two rods which are connected
with the battery, and when contact is made at the battery see what

Fig. 48

heat we get (Fig. 48). Is not that beautiful? . It is a complete bridge
of power. There is metallic connection all the way round in this
arrangement, and where I have inserted the platinum, which offers
some resistance to the passage of the force, you see what an amount
of heat is evolved; this is the heat which the zinc would give if burnt
in oxygen; but, as it is being burnt in the voltaic battery, it is giving
it out at this spot. I will now shorten this wire for the sake of show-
ing you that, the shorter the obstructing wire is, the more and more
intense is the heat, until at last our platinum is fused and falls down,
breaking off the circuit.

Here is another instance. I will take a piece of the metal silver,
and place it on charcoal connected with one end of the battery, and
lower the other charcoal pole on to it. See how brilliantly it burns!
(Fig. 49.) Here is a piece of iron on the charcoal: see what a com-
bustion is going on; and we might go on in this way, burning
almost every thing we place between the poles. Now I want to
show you that this power is still chemical affinity; that if we call the
power which is evolved at this point heat, or electricity, or any other
name referring to its source, or the way in which it travels, we still

80 FARADAY

shall find it to be chemical action. Here is a colored liquid which
can show by its change of color the effects of chemical action; I will
pour part of it into this glass, and you will find that these wires have
a very strong action. I am not going to show you any effects of com-

FiG. 49

bustion or heat, but I will take these two platinum plates, and fasten
one to the one pole and the other to the other end, and place them
in this solution, and in a very short time you will see the blue color
will be entirely destroyed. See, it is colorless now! I have merely
brought the end of the wires into the solution of indigo, and the
power of electricity has come through these wires and made itself
evident by its chemical action. There is also another curious thing
to be noticed now we are dealing with the chemistry of electricity,
which is, that the chemical power which destroys the color is only

due to the action on one side. I will
pour some more of this sulphindigotic
acid(") into a flat dish, and will then
make a porous dike of sand separating
the two portions of fluid into two parts
(Fig. 50), and now we shall be able to
see whether there is any difference in
the two ends of the battery, and which
it is that possesses this peculiar action. You see it is the one on my
right hand which has the power of destroying the blue, for the
portion on that side is thoroughly bleached, while nothing has
apparently occurred on the other side. I say apparently, for you

^'^Sulphindigotic acid. A mixture of one part of indigo and fifteen parts of con-
centrated oil of vitriol. It is bleached on the side at which hydrogen gas is evolved
in consequence of the liberated hydrogen withdrawing oxygen from the indigo, thereby
forming a colorless deoxidized indigo. In making the experiment, only enough of the
sulphindigotic acid must be added to give the water a decided blue color.

Fig. 50

THE CORRELATION OF THE PHYSICAL FORCES 8 1

must not imagine that because you can not perceive any action none
has taken place.

Here we have another instance of chemical action. I take these
platinum plates again and immerse them in this solution of copper,
from which we formerly precipitated some of the metal, when the
platinum and zinc were both put in it together. You see that these
two platinum plates have no chemical action of any kind; they might
remain in the solution as long as I liked, without having any power
of themselves to reduce the copper; but the moment I bring the two
poles of the battery in contact with them, the chemical action which
is there transformed into electricity and carried along the wires again
becomes chemical action at the two platinum poles, and now we
shall have the power appearing on the left-hand side, and throwing
down the copper in the metallic state on the platinum plate; and in
this way I might give you many instances of the extraordinary way
in which this chemical action or electricity may be carried about.
That strange nugget of gold, of which there is a model in the other
room, and which has an interest of its own in the natural history
of gold, and which came from Ballarat, and was worth ^(^ 8,000 or
^^9,000 when it was melted down last November, was brought
together in the bowels of the earth, perhaps ages and ages ago, by
some such power as this. And there is also another beautiful result
dependent upon chemical affinity in that fine lead-tree(^^), the lead
growing and growing by virtue of this power. The lead and the
zinc are combined together in a little voltaic arrangement in a man-
ner far more important than the powerful one you see here, because
in nature these minute actions are going on forever, and are of
great and wonderful importance in the precipitation of metals and
formation of mineral veins, and so forth. These actions are not for
a limited time, like my battery here, but they act forever in small
degrees, accumulating more and more of the results.

I have here given you all the illustrations that time will permit me

^' Lead tree. To make a lead tree, pass a bundle of brass wires through the cork
of a bottle, and fasten a plate of zinc round them just as they issue from the cork,
so that the zinc may be in contact with every one of the wires. Make the wires to
diverge so as to form a sort of cone, and, having filled the bottle quite full of a
solution of sugar of lead, insert the wires and cork, and seal it down, so as to perfectly
exclude the air. In a short time the metallic lead will begin to crystallize around the
divergent wires, and form a beautiful object.

82 FARADAY

to show you of chemical affinity producing electricity, and electricity
again becoming chemical affinity. Let that suffice for the present;
and now let us go a little deeper into the subject of this chemical
force, or this electricity — which shall I name first? — the one pro-
ducing the other in a variety of ways. These forces are also won-
derful in their power of producing another of the forces we have
been considering, namely, that of magnetism; and you know that
it is only of late years, and long since I was born, that the discovery
of the relations of these two forces of electricity and chemical
affinity to produce magnetism has become known. Philosophers
had been suspecting this affinity for a long time, and had long had
great hopes of success; for in the pursuit of science we first start
with hopes and expectations; these we realize and estabUsh, never
again to be lost, and upon them we found new expectations of farther
discoveries, and so go on pursuing, realizing, establishing, and
founding new hopes again and again.

Now observe this: here is a piece of wire which I am about to
make into a bridge of force, that is to say, a communicator between

4W) *-+

m-* fl

f

rBi r—

^J

u>=

ZnX

i

Fig. 51

the two ends of the battery. It is copper wire only, and is therefore
not magnetic of itself. We will examine this wire with our magnetic
needle (Fig. 51), and, though connected with one extreme end of
the battery, you see that before the circuit is completed it has no
power over the magnet. But observe it when I make contact; watch
the needle; see how it is swung round; and notice how indifferent
it becomes if I break contact again; so, you see, we have this wire
evidently affecting the magnetic needle under these circumstances.
Let me show you that a little more strongly. I have here a quantity
of wire which has been wound into a spiral, and this will affect the

THE CORRELATION OF THE PHYSICAL FORCES 83

magnetic needle in a very curious manner, because, owing to its
shape, it will act very like a real magnet. The copper spiral has
no power over that magnetic needle at present; but if I cause the
electric current to circulate through it, by bringing the two ends of
the battery in contact with the ends of the wire which forms the
spiral, what will happen? Why, one end of the needle is most
powerfully drawn to it; and if I take the other end of the needle,
it is repelled; so, you see, I have produced exactly the same phe-
nomena as I had with the bar magnet, one end attracting and the
other repelling. Is not this, then, curious to see, that we can con-
struct a magnet of copper? Furthermore, if I take an iron bar, and
put it inside the coil, so long as there is no electric current circulating
round, it has no attraction, as you will observe if I bring a little iron
filings or nails near the iron. But now, if I make contact with the
battery, they are attracted at once. It becomes at once a powerful
magnet, so much so that I should not wonder if these magnetic
needles on different parts of the table pointed to it. And I will show
you, by another experiment, what an attraction it has. This piece

and that piece of iron,
and many other pieces,
are now strongly at-
tracted (Fig. 52); but,
as soon as I break con-
tact, the power is all
gone, and they fall.

rWf What, then, can be a better or a stronger proof than this
T of the relation of the powers of magnetism and electricity ?
^ Again: here is a little piece of iron which is not mag-
netized. It will not, at present, take up any one of these
Fig. 52 nails; but I will take a piece of wire and coil it round the
iron (the wire being covered with cotton in every part it
does not touch the iron), so that the current must go round in this
spiral coil; I am, in fact, preparing an electro-magnet (we are
obliged to use such terms to express our meaning, because it is a
magnet made by electricity — because we produce by the force of
electricity a magnet of far greater power than a permanent steel
one). It is now completed, and I will repeat the experiment which

84 FARADAY

you saw the other day, of building up a bridge of iron nails: the con-
tact is now made and the current is going through; it is now a
powerful magnet; here are the iron nails which we had the other
day, and now I have brought this magnet near them they are

clinging so hard that I can scarcely move
them with my hand (Fig. 53) . But when
the contact is broken, see how they fall.
What can show you better than such an
experiment as this the magnetic attrac-
tion with which we have endowed these
portions of iron? Here, again, is a fine
illustration of this strong power of mag-
netism. It is a magnet of the same sort as
the one you have just seen. I am about
to make the current of electricity pass
through the wires which are round this
iron for the purpose of showing you what
powerful effects we get. Here are the poles of the magnet; and let us
place on one of them this long bar of iron. You see, as soon as contact
is made, how it rises in position (Fig. 54) ; and if I take such a piece
as this cylinder, and place it on, woe be to me if I get my finger

Fig. 53

between; I can roll it over, but if I try to pull it off, I might lift up
the whole magnet, but I have no power to overcome the magnetic
power which is here evident. I might give you an infinity of illus-
trations of this high magnetic power. There is that long bar of
iron held out, and I have no doubt that if I were to examine the

THE CORRELATION OF THE PHYSICAL FORCES 85

Other end I should find that it was a magnet. See what power it
must have to support not only these nails, but all those lumps of iron
hanging on to the end. What, then, can surpass these evidences o£
the change of chemical force into electricity, and electricity into
magnetism? I might show you many other experiments whereby I
could obtain electricity and chemical action, heat and light from a
magnet, but what more need I show you to prove the universal
correlation of the physical forces of matter, and their mutual con-
version one into another.?

And now let us give place as juveniles to the respect we owe to
our elders, and for a time let me address myself to those of our
seniors who have honored me with their presence during these lec-
tures. I wish to claim this moment for the purpose of tendering our
thanks to them, and my thanks to you all for the way in which you
have borne the inconvenience that I at first subjected you to. I hope
that the insight which you have here gained into some of the laws
by which the universe is governed, may be the occasion of some
among you turning your attention to these subjects; for what study
is there more fitted to the mind of man than that of the physical
sciences ? And what is there more capable of giving him an insight
into the actions of those laws, a knowledge of which gives interest to
the most trifling phenomenon of nature, and makes the observing
student find

"Tongues in trees, books in the running brooks,
Sermons in stones, and good in every thing?"

THE CHEMICAL HISTORY
OF A CANDLE

A COURSE OF LECTURES DELIVERED BEFORE A

JUVENILE AUDIENCE AT THE ROYAL

INSTITUTION

LECTURE I

A CANDLE: THE FLAME— ITS SOURCES— STRUCTURE-
MOBILITY— BRIGHTNESS

I PURPOSE, in return for the honor you do us by coming to see
what are our proceedings here, to bring before you, in the course
of these lectures, the Chemical History of a Candle. I have
taken this subject on a former occasion, and, were it left to my own
will, I should prefer to repeat it almost every year, so abundant is
the interest that attaches itself to the subject, so wonderful are the
varieties of outlet which it oflfers into the various departments of
philosophy. There is not a law under which any part of this uni-
verse is governed which does not come into play and is touched upon
in these phenomena. There is no better, there is no more open door
by which you can enter into the study of natural philosophy than by
considering the physical phenomena of a candle. I trust, therefore, I
shall not disappoint you in choosing this for my subject rather than
any newer topic, which could not be better, were it even so good.
And, before proceeding, let me say this also: that, though our
subject be so great, and our intention that of treating it honestly,
seriously, and philosophically, yet I mean to pass away from all those
who are seniors among us. I claim the privilege of speaking to juve-
niles as a juvenile myself. I have done so on former occasions, and,
if you please, I shall do so again. And, though I stand here with the
knowledge of having the words I utter given to the world, yet that

86

CHEMICAL HISTORY OF A CANDLE 87

shall not deter me from speaking in the same familiar way to those
whom I esteem nearest to me on this occasion.

And now, my boys and girls, I must first tell you of what candles
are made. Some are great curiosities. I have here some bits of tim-
ber, branches of trees particularly famous for their burning. And
here you see a piece of that very curious substance, taken out of some
of the bogs in Ireland, called candle-wood; a hard, strong, excellent
wood, evidently fitted for good work as a register of force, and yet,
withal, burning so well that where it is found they make splinters
of it, and torches, since it burns like a candle, and gives a very good
light indeed. And in this wood we have one of the most beautiful
illustrations of the general nature of a candle that I can possibly give.
The fuel provided, the means of bringing that fuel to the place of
chemical action, the regular and gradual supply of air to that place
of action — heat and light — all produced by a little piece of wood of
this kind, forming, in fact, a natural candle.

But we must speak of candles as they are in commerce. Here are
a couple of candles commonly called dips. They are made of lengths
of cotton cut off, hung up by a loop, dipped into melted tallow,
taken out again and cooled, then redipped, until there is an accumu-
lation of tallow round the cotton. In order that you may have an
idea of the various characters of these candles, you see these which
I hold in my hand — they are very small and very curious. They are,
or were, the candles used by the miners in coal mines. In olden
times the miner had to find his own candles, and it was supposed
that a small candle would not so soon set fire to the fire-damp in the
coal mines as a large one; and for that reason, as well as for economy's
sake, he had candles made of this sort — 20, 30, 40, or 60 to the pound.
They have been replaced since then by the steel-mill, and then by the
Davy lamp, and other safety lamps of various kinds. I have here a
candle that was taken out of the Royal Georgei^), it is said, by
Colonel Pasley. It has been sunk in the sea for many years, subject
to the action of salt water. It shows you how well candles may be
preserved; for, though it is cracked about and broken a great deal,

'The Royal George sunk at Spithead on the 29th of August, 1782. Colonel Pasley
commenced operations for the removal of the wreck by the explosion of gunpowder,
in August, 1839. The candle which Professor Faraday exhibited must therefore have
been exposed to the action of salt water for upward of fifty-seven years.

88 FARADAY

yet when lighted it goes on burning regularly, and the tallow
resumes its natural condition as soon as it is fused.

Mr. Field, of Lambeth, has supplied me abundantly with beauti-
ful illustrations of the candle and its materials; I shall therefore now
refer to them. And, first, there is the suet — the fat of the ox —
Russian tallow, I believe, employed in the manufacture of these dips,
which Gay-Lussac, or some one who intrusted him with his knowl-
edge, converted into that beautiful substance, stearin, which you see
lying beside it. A candle, you know, is not now a greasy thing like
an ordinary tallow candle, but a clean thing, and you may almost
scrape off and pulverize the drops which fall from it without soiling
any thing. This is the process he adopted(^) : The fat or tallow is
first boiled with quick-lime, and made into a soap, and then the soap
is decomposed by sulphuric acid, which takes away the lime, and
leaves the fat rearranged as stearic acid, while a quantity of glycerin
is produced at the same time. Glycerin — absolutely a sugar, or a
substance similar to sugar — comes out of the tallow in this chemical
change. The oil is then pressed out of it; and you see here this series
of pressed cakes, showing how beautifully the impurities are carried
out by the oily part as the pressure goes on increasing, and at last you
have left that substance, which is melted, and cast into candles as
here represented. The candle I have in my hand is a stearin candle,
made of stearin from tallow in the way I have told you. Then here
is a sperm candle, which comes from the purified oil of the sper-
maceti whale. Here, also, are yellow beeswax and refined beeswax,
from which candles are made. Here, too, is that curious substance
called parafEne, and some paraffine candles, made of parafEne
obtained from the bogs of Ireland. I have here also a substance
brought from Japan since we have forced an entrance into

2 The fat or tallow consists of a chemical combination of fatty acids with glycerin.
The lime unites with the palmitic, oleic, and stearic acids, and separates the glycerin.
After washing, the insoluble lime soap is decomposed with hot dilute sulphuric acid.
The melted fatty acids thus rise as an oil to the surface, when they are decanted.
They are again washed and cast into thin plates, which, when cold, are placed between
layers of cocoanut matting and submitted to intense hydraulic pressure. In this way
the soft oleic acid is squeezed out, while the hard palmitic and stearic acids remain.
These are farther purified by pressure at a higher temperature and washing in warm
dilute sulphuric acid, when they are ready to be made into candles. These acids
are harder and whiter than the fats from which they were obtained, while at the
same time they are cleaner and more combustible.

CHEMICAL HISTORY OF A CANDLE 89

that out-of-the-way place — a sort of wax which a kind friend has
sent me, and which forms a new material for the manufacture of
candles.

And how are these candles made? I have told you about dips,
and I will show you how moulds are made. Let us imagine any of
these candles to be made of materials which can be cast. "Cast!"
you say. "Why, a candle is a thing that melts, and surely if you can
melt it you can cast it." Not so. It is wonderful, in the progress of
manufacture, and in the consideration of the means best fitted to
produce the required result, how things turn up which one would
not expect beforehand. Candles can not always be cast. A wax
candle can never be cast. It is made by a particular process which I
can illustrate in a minute or two, but I must not spend much time on
it. Wax is a thing which, burning so well, and melting so easily in
a candle, can not be cast. However, let us take a material that can
be cast. Here is a frame, with a number of moulds fastened in it.
The first thing to be done is to put a wick through them. Here is
one — a plaited wick, which does not require snuffing(^) — supported
by a little wire. It goes to the bottom, where it is pegged in — the
little peg holding the cotton tight, and stopping the aperture so that
nothing fluid shall run out. At the upper part there is a little bar
placed across, which stretches the cotton and holds it in the mould.
The tallow is then melted, and the moulds are filled. After a certain
time, when the moulds are cool, the excess of tallow is poured off
at one corner, and then cleaned off altogether, and the ends of the
wick cut away. The candles alone then remain in the mould, and
you have only to upset them, as I am doing, when out they tumble,
for the candles are made in the form of cones, being narrower at
the top than at the bottom; so that, what with their form and their
own shrinking, they only need a little shaking, and out they fall.
In the same way are made these candles of stearin and of paraffine.
It is a curious thing to see how wax candles are made. A lot of
cottons are hung upon frames, as you see here, and covered with
metal tags at the ends to keep the wax from covering the cotton in
those places. These are carried to a heater, where the wax is melted.

' A Htde borax or phosphorus salt is sometimes added in order to make the ash
fusible.

^„ '*" — — -

As you see, the frames can turn round; and, as they turn, a man
takes a vessel of wax and pours it first down one, and then the next,
and the next, and so on. When he has gone once round, if it isi
sufficiently cool, he gives the first a second coat, and so on until they
are all of the required thickness. When they have been thus clothed,
or fed, or made up to that thickness, they are taken off and placed
elsewhere. I have here, by the kindness of Mr. Field, several speci-
mens of these candles. Here is one only half finished. They are then
taken down and well rolled upon a fine stone slab, and the conical
top is moulded by properly shaped tubes, and the bottoms cut off
and trimmed. This is done so beautifully that they can make
candles in this way weighing exactly four or six to the pound, or
any number they please.

We must not, however, take up more time about the mere manu-
facture, but go a little farther into the matter. I have not yet
referred you to luxuries in candles (for there is such a thing as luxury
in candles). See how beautifully these are colored; you see here
mauve, magenta, and all the chemical colors recendy introduced,
applied to candles. You observe, also, different forms employed.
Here is a fluted pillar most beautifully shaped; and I have also here
some candles sent me by Mr. Pearsall, which are ornamented with
designs upon them, so that, as they burn, you have, as it were, a
glowing sun above, and bouquet of flowers beneath. All, however,
that is fine and beautiful is not useful. These fluted candles, pretty
as they are, are bad candles; they are bad because of their external
shape. Nevertheless, I show you these specimens, sent to me from
kind friends on all sides, that you may see what is done and what
may be done in this or that direction; although, as I have said, when
we come to these refinements, we are obliged to sacrifice a little in
utility.

Now as to the light of the candle. We will light one or two, and
set them at work in the performance of their proper functions. You
observe a candle is a very different thing from a lamp. With a lamp
you take a little oil, fill your vessel, put in a little moss or some cotton
prepared by artificial means, and then light the top of the wick.
When the flame runs down the cotton to the oil, it gets extinguished,
but it goes on burning in the part above. Now I have no doubt you
will ask how it is that the oil which will not burn of itself gets up to

CHEMICAL HISTORY OF A CANDLE pi

the top of the cotton, where it will burn. We shall presently examine
that; but there is a much more wonderful thing about the burning of
a candle than this. You have here a solid substance with no vessel to
contain it; and how is it that this solid substance can get up to the
place where the flame is ? How is it that this solid gets there, it not
being a fluid? or, when it is made a fluid, then how is it that it
keeps together? This is a wonderful thing about a candle.

We have here a good deal of wind, which will help us in some of
our illustrations, but tease us in others; for the sake, therefore, of a
little regularity, and to simplify the matter, I shall make a quiet
flame, for who can study a subject when there are difficulties in
the way not belonging to it? Here is a clever invention of some
costermonger or street-stander in the market-place for the shading
of their candles on Saturday nights, when they are selling their
greens, or potatoes, or fish. I have very often admired it. They put a
lamp-glass round the candle, supported on a kind of gallery, which
clasps it, and it can be slipped up and down as required. By the use
of this lamp-glass, employed in the same way, you have a steady
flame, which you can look at, and carefully examine, as I hope you
will do, at home.

You see, then, in the first instance, that a beautiful cup is formed.
As the air comes to the candle, it moves upward by the force of the
current which the heat of the candle produces, and it so cools all the
sides of the wax, tallow, or fuel as to keep the edge much cooler than
the part within; the part within melts by the flame that runs down
the wick as far as it can go before it is extinguished, but the part on
the outside does not melt. If I made a current in one direction, my
cup would be lop-sided, and the fluid would consequently run over;
for the same force of gravity which holds worlds together holds this
fluid in a horizontal position, and if the cup be not horizontal, of
course the fluid will run away in guttering. You see, therefore, that
the cup is formed by this beautifully regular ascending current of
air playing upon all sides, which keeps the exterior of the candle
cool. No fuel would serve for a candle which has not the property of
giving this cup, except such fuel as the Irish bogwood, where the
material itself is like a sponge and holds its own fuel. You see now
why you would have had such a bad result if you were to burn these
beautiful candles that I have shown you, which are irregular, inter-

92 FARADAY

mittent in their shape, and can not, therefore, have that nicely-formed
edge to the cup which is the great beauty in a candle. I hope you
will now see that the perfection of a process — that is, its utility — is
the better point of beauty about it. It is not the best looking thing,
but the best acting thing, which is the most advantageous to us. This
good-looking candle is a bad-burning one. There will be a guttering
round about it because of the irregularity of the stream of air and the
badness of the cup which is formed thereby. You may see some
pretty examples (and I trust you will notice these instances) of the
action of the ascending current when you have a little gutter run
down the side of a candle, making it thicker there than it is else-
where. As the candle goes on burning, that keeps its place and forms
a little pillar sticking up by the side, because, as it rises higher above
the rest of the wax or fuel, the air gets better round it, and it is more
cooled and better able to resist the action of the heat at a little dis-
tance. Now the greatest mistakes and faults with regard to candles,
as in many other things, often bring with them instruction which we
should not receive if they had not occurred. We come here to be
philosophers, and I hope you will always remember that whenever a
result happens, especially if it be new, you should say, "What is the
cause? Why does it occur.''" and you will, in the course of time, find
out the reason.

Then there is another point about these candles which will answer
a question — that is, as to the way in which this fluid gets out of the
cup, up the wick, and into the place of combustion. You know that
the flames on these burning wicks in candles made of beeswax,
stearin, or spermaceti, do not run down to the wax or other matter,
and melt it all away, but keep to their own right place. They are
fenced off from the fluid below, and do not encroach on the cup at
the sides. I can not imagine a more beautiful example than the con-
dition of adjustment under which a candle makes one part subserve
to the other to the very end of its action. A combustible thing like
that, burning away gradually, never being intruded upon by the
flame, is a very beautiful sight, especially when you come to learn
what a vigorous thing flame is — what power it has of destroying the
wax itself when it gets hold of it, and of disturbing its proper form if
it come only too near.

CHEMICAL HISTORY OF A CANDLE 93

But how does the flame get hold of the fuel ? There is a beautiful
point about that — capillary attraction.^) "Capillary attraction!" you
say — "the attraction of hairs." Well, never mind the name; it was
given in old times, before we had a good understanding of what the
real power was. It is by what is called capillary attraction that the
fuel is conveyed to the part where combustion goes on, and is de-
posited there, not in a careless way, but very beautifully in the very
midst of the centre of action, which takes place around it. Now
I am going to give you one or two instances of capillary attraction. It
is that kind of action or attraction which makes two things that do
not dissolve in each other still hold together. When you wash your
hands, you wet them thoroughly; you take a little soap to make the
adhesion better, and you find your hands remain wet. This is by that
kind of attraction of which I am about to speak. And, what is more,
if your hands are not soiled (as they almost always are by the usages
of life), if you put your finger into a little warm water, the water
will creep a little way up the finger, though you may not stop to
examine it. I have here a substance which is rather porous — a column
of salt — and I will pour into the plate
at the bottom, not water, as it appears,
but a saturated solution of salt which
can not absorb more, so that the
action which you see will not be due
to its dissolving any thing. We may
consider the plate to be the candle,
and the salt the wick, and this solu-
tion the melted tallow. (I have
colored the fluid, that you may see the
action better.) You observe that, now
I pour in the fluid, it rises and gradually creeps up the salt higher and
higher (Fig. 55) ; and provided the column does not tumble over, it
will go to the top. If this blue solution were combustible, and we were
to place a wick at the top of the salt, it would burn as it entered into

^ Capillary attraction or repulsion is the cause which determines the ascent or
descent of a fluid in a capillary tube. If a piece of thermometer tubing, open at each
end, be plunged into water, the latter will instantly rise in the tube considerably above
its external level. If, on the other hand, the tube be plunged into mercury, a repulsion
instead of attraction will be exhibited, and the level of the mercury will be lower in
the tube than it is outside.

Fig. 55

94 FARADAY

the wick. It is a most curious thing to see this kind of action taking
place, and to observe how singular some of the circumstances are
about it. When you wash your hands, you take a towel to wipe ofl
the water; and it is by that kind of wetting, or that kind of attraction
which makes the towel become wet with water, that the wick is
made wet with the taliow. I have known some careless boys and girls
(indeed, I have known it happen to careful people as well) who,
having washed their hands and wiped them with a towel, have
thrown the towel over the side of the basin, and before long it has
drawn all the water out of the basin and conveyed it to the floor,
because it happened to be thrown over the side in such a way as to
serve the purpose of a siphon. C") That you may the better see the
way in which the substances act one upon another, I have here a
vessel made of wire gauze filled with water, and you may compare it
in its action to the cotton in one respect, or to a piece of calico in the
other. In fact, wicks are sometimes made of a kind of wire gauze.
You will observe that this vessel is a porous thing; for if I pour a little
water on to the top, it will run out at the bottom. You would be
puzzled for a good while if I asked you what the state of this vessel
is, what is inside it, and why it is there? The vessel is full of water,
and yet you see the water goes in and runs out as if it were empty.
In order to prove this to you, I have only to empty it. The reason is
this: the wire, being once wetted, remains wet; the meshes are so
small that the fluid is attracted so strongly from the one side to
the other, as to remain in the vessel, although it is porous. In like
manner, the particles of melted tallow ascend the cotton and
get to the top; other particles then follow by their mutual attrac-
tion for each other, and as they reach the flame they are gradually
burned.

Here is another application of the same principle. You see this bit
of cane. I have seen boys about the streets, who are very anxious to
appear like men, take a piece of cane, and light it, and smoke it, as
an imitation of a cigar. They are enabled to do so by the permeability

^ The late Duke of Sussex was, we believe, the first to show that a prawn might
be washed upon this principle. If the tail, after pulling off the fan part, be placed in a
tumbler of water, and the head be allowed to hang over the outside, the water will
be sucked up the tail by capillary attraction, and will continue to run out through
the head until the water in the glass has sunk so low that the tail ceases to dip into it.

CHEMICAL HISTORY OF A CANDLE 95

of the cane in one direction, and by its capillarity. If I place this piece
of cane on a plate containing some camphene (which is very much
like paraiSne in its general character), exactly in the same manner as
the blue fluid rose through the salt will this fluid rise through the
piece of cane. There being no pores at the side, the fluid can not go

in that direction, but must pass through
i its length. Already the fluid is at the top
of the cane; now I can light it and make
it serve as a candle. The fluid has risen
by the capillary attraction of the piece
of cane, just as it does through the cotton in the candle.
Now the only reason why the candle does not burn
all down the side of the wick is that the melted tallow
extinguishes the flame. You know that a candle, if
turned upside down, so as to allow the fuel to run
upon the wick, will be put out. The reason is, that the
flame has not had time to make the fuel hot enough
to burn, as it does above, where it is carried in small
quantities into the wick, and has all the effect of the
heat exercised upon it.
There is another condition which you must learn as regards the
candle, without which you would not be able fully to understand
the philosophy of it, and that is the vaporous condition of the fuel.
In order that you may understand that, let me show you a very pretty
but very commonplace experiment. If you blow a candle out cleverly,
you will see the vapor rise from it. You have, I know, often smelt
the vapor of a blown-out candle, and a very bad smell it is; but if
you blow it out cleverly you will be able to see pretty well the vapor
into which this solid matter is transformed. I will blow out one of
these candles in such a way as not to disturb the air around it by the
continuing action of my breath; and now, if I hold a lighted taper
two or three inches from the wick, you will observe a train of fire
going through the air till it reaches the candle (Fig. 56) . I am obliged
to be quick and ready, because if I allow the vapor time to cool, it
becomes condensed into a liquid or solid, or the stream of com-
bustible matter gets disturbed.
Now as to the shape or form of the flame. It concerns us much to

Fig. 56

96 FARADAY

know about the condition which the matter of the candle finally
assumes at the top of the wick, where you have such beauty and
brightness as nothing but combustion or flame can produce. You
have the glittering beauty of gold and silver, and the still higher
lustre of jewels like the ruby and diamond; but none of these rival
the brilliancy and beauty of flame. What diamond can shine like
flame? It owes its lustre at nighttime to the very flame
A shining upon it. The flame shines in darkness, but the light
which the diamond has is as nothing until the flame shines
upon it, when it is brilliant again. The candle alone shines
by itself and for itself, or for those who have arranged the
materials. Now let us look a little at the form of the flame
as you see it under the glass shade. It is steady and equal,
and its general form is that which is represented in the dia-
gram (Fig. 57), varying with atmospheric disturbances, and
also varying according to the size of the candle. It is a
bright oblong, brighter at the top than toward the bottom,
with the wick in the middle, and, besides the wick in the
middle, certain darker parts towards the bottom, where the
ignition is not so.perfect as in the part above. I have a draw-
ing here, sketched many years ago by Hooker, when he
made his investigations. It is the drawing of the flame of a lamp, but
it will apply to the flame of a candle. The cup of the candle is the ves-
sel or lamp; the melted spermaceti is the oil; and the wick is common
to both. Upon that he sets this little flame, and then he represents
what is true, a certain quantity of matter rising about it which you do
not see, and which, if you have not been here before, or are not famil-
iar with the subject, you will not know of. He has here represented
the parts of the surrounding atmosphere that are very essential to the
flame, and that are always present with it. There is a current formed,
which draws the flame out; for the flame which you see is really
drawn out by the current, and drawn upward to a great height, just as
Hooker has here shown you by that prolongation of the current in the
diagram. You may see this by taking a lighted candle, and putting it
in the sun so as to get its shadow thrown on a piece of paper. How re-
markable it is that that thing which is light enough to produce shad-
ows of other objects can be made to throw its own shadow on a piece

Fig. 57

Ili!!!!!!!!

,,l|l,|].!,i!,l„

CHEMICAL HISTORY OF A CANDLE 97

of white paper or card, so that you can actually see streaming round
the flame something which is not part of the flame, but is ascending
and drawing the flame upward. Now I am going to imitate the sun-
hght by applying the voltaic battery to the electric lamp. You now see
our sun and its great luminosity; and by placing a candle
between it and the screen, we get the shadow of the
flame. You observe the shadow of the candle and of the
wick; then there is a darkish part, as represented in the
diagram (Fig. 58), and then a part which is more dis-
tinct. Curiously enough, however, what we see in the
shadow as the darkest part of the flame is, in reality, the
brightest part; and here you see streaming upward the
ascending current of hot air, as shown by Hooker, which
draws out the flame, supplies it with air, and cools the
sides of the cup of melted fuel.

I can give you here a little farther illustration, for the
purpose of showing you how flame goes up or down
according to the current. I have here a flame — it is
not a candle flame — but you can, no doubt, by this
time generalize enough to be able to compare one thing with
another. What I am about to do is to change the ascending current
that takes the flame upward into a descending current. This I can
easily do by the little apparatus you see before me. The flame, as I
have said, is not a candle flame, but it is produced by alcohol, so
that it shall not smoke too much. I will also color the flame with
another substance ("), so that you may trace its course; for, with the
spirit alone, you could hardly see well enough to have the opportunity
of tracing its direction. By lighting this spirit of wine we have then
a flame produced, and you observe that when held in the air it
naturally goes upward. You understand now, easily enough, why
flames go up under ordinary circumstances: it is because of the
draught of air by which the combustion is formed. But now, by
blowing the flame down, you see I am enabled to make it go down-
ward into this little chimney, the direction of the current being
changed (Fig. 59). Before we have concluded this course of lectures

Fig. 58

flame.

The alcohol had chloride of copper dissolved in it: this produces a beautiful green
le.

98 FARADAY

we shall show you a lamp in which the flame goes up and the smoke
goes down, or the flame goes down and the smoke goes up. You see,
then, that we have the power in this way of varying the flame in
different directions.

There are now some other points that I must bring before you.
Many of the flames you see here vary very much in their shape by

the currents of air blowing around them
in different directions; but we can, if we
like, make flames so that they will look
like fixtures, and we can photograph them
-indeed, we have to photograph them —
so that they become fixed to us, if we wish
to find out every thing concerning them.
That, however, is not the only thing I
wish to mention. If I take a flame suffi-
ciently large, it does not keep that homo-
geneous, that uniform condition of shape,
but it breaks out with a power of life
which is quite wonderful. I am about to
use another kind of fuel, but one which is truly and fairly a repre-
sentative of the wax or tallow of a candle. I have here a large ball of
cotton, which will serve as a wick. And, now that I have immersed
it in spirit and applied a light to it, in what way does it differ from
an ordinary candle? Why, it differs very much in one respect, that
we have a vivacity and power about it, a beauty and a life entirely
different from the light presented by a candle. You see those fine
tongues of flame rising up. You have the same general disposition of
the mass of the flame from below upward; but, in addition to that,
you have this remarkable breaking out into tongues which you do
not perceive in the case of a candle. Now, why is this? I must
explain it to you, because, when you understand that perfectly, you
will be able to follow me better in what I have to say hereafter. I
suppose some here will have made for themselves the experiment I
am going to show you. Am I right in supposing that any body here
has played at snapdragon ? I do not know a more beautiful illustra-
tion of the philosophy of flame, as to a certain part of its history, than
the game of snapdragon. First, here is the dish; and let me say, that

CHEMICAL HISTORY OF A CANDLE 99

when you play snapdragon properly you ought to have the dish well
warmed; you ought also to have warm plums, and warm brandy,
which, however, I have not got. When you have put the spirit into
the dish, you have the cup and the fuel; and are not the raisins acting
like the wicks ? I now throw the plums into the dish, and light the
spirit, and you see those beautiful tongues of flame that I refer to.
You have the air creeping in over the edge of the dish forming these

Fig. 60

tongues. Why? Because, through the force of the current and the
irregularity of the action of the flame, it can not flow in one uniform
stream. The air flows in so irregularly that you have what would
otherwise be a single image broken up into a variety of forms, and
each of these little tongues has an independent existence of its own.
Indeed, I might say, you have here a multitude of independent can-
dles. You must not imagine, because you see these tongues all at
once, that the flame is of this particular shape. A flame of that shape
is never so at any one time. Never is a body of flame, like that
which you just saw rising from the ball, of the shape it appears to
you. It consists of a multitude of different shapes, succeeding each
other so fast that the eye is only able to take cognizance of them all
at once. In former times I purposely analyzed a flame of that general
character, and the diagram shows you the different parts of which it
is composed (Fig. 60). They do not occur all at once; it is only be-
cause we see these shapes in such rapid succession that they seem to
us to exist all at one time.

100 FARADAY

It is too bad that we have not got farther than my game of snap-
dragon; but we must not, under any circumstances, keep you beyond
your time. It will be a lesson to me in future to hold you more strictly
to the philosophy of the thing than to take up your time so much
with these illustrations.

LECTURE II

A CANDLE: BRIGHTNESS OF THE FLAME— AIR NECES-
SARY FOR COMBUSTION— PRODUC-
TION OF WATER

WE were occupied the last time we met in considering the
general character and arrangement as regards the fluid
portion of a candle, and the way in which that fluid got
into the place of combustion. You _ see, when we have a candle
burning fairly in a regular, steady atmosphere, it will have a shape
something like the one shown in the diagram, and will look pretty
uniform, although very curious in its character. And now I have
to ask your attention to the means by which we are enabled to ascer-
tain what happens in any particular part of the flame; why it hap-
pens; what it does in happening; and where, after all, the whole
candle goes to; because, as you know very well, a candle being
brought before us and burned, disappears, if burned properly, with-
out the least trace of dirt in the candle stick; and this is a very
curious circumstance. In order, then, to examine this candle care-
fully, I have arranged certain apparatus, the use of which you will see
as I go on. Here is a candle; I am about to put the end of this glass
tube into the middle of the flame — into that part which old Hooker
has represented in the diagram as being rather dark, and which you
can see at any time if you will look at a candle carefully, without
blowing it about. We will examine this dark part first.

Now I take this bent glass tube, and introduce one end into that
part of the flame, and you see at once that something is coming
from the flame, out at the other end of the tube; and if I put a flask
there, and leave it for a little while, you will see that something from
the middle part of the flame is gradually drawn out, and goes
through the tube, and into that flask, and there behaves very differ-
ently from what it does in the open air. It not only escapes from the
end of the tube, but falls down to the bottom of the flask like a

102

FARADAY

heavy substance, as indeed it is (Fig. 6i). We find that this is the
wax of the candle made into a vaporous fluid — not a gas. (You
must learn the diflFerence between a gas and a vapor: a gas remains
permanent; a vapor is something that will condense.) If you blow
out a candle, you perceive a very nasty smell, resulting from the con-
densation of this vapor. That is very different from what you have
outside the flame; and, in order to make that more clear to you, I am
about to produce and set fire to a larger portion of this vapor; for

Fig. 6i

what we have in the small way in a candle, to understand thor-
oughly, we must, as philosophers, produce in a larger way, if needful,
that we may examine the different parts. And now Mr. Anderson
will give me a source of heat, and I am about to show you what that
vapor is. Here is some wax in a glass flask, and I am going to make
it hot, as the inside of that candle flame is hot, and the matter about
the wick is hot. [The lecturer placed some wax in a glass flask, and
heated it over a lamp.] Now I dare say that is hot enough for me.
You see that the wax I put in it has become fluid, and there is a
little smoke coming from it. We shall very soon have the vapor
rising up. I will make it still hotter, and now we get more of it, so
that I can actually pour the vapor out of the flask into that basin, and
set it on fire there. This, then, is exactly the same kind of vapor as

CHEMICAL HISTORY OF A CANDLE IO3

we have in the middle of the candle; and that you may be sure
this is the case, let us try whether we have not got here, in this flask,
a real combustible vapor out o£ the middle of the candle. [Taking
the flask into which the tube from the candle proceeded, and intro-
ducing a lighted taper (Fig. 62).] See how it burns. Now this is the

vapor from the middle of the candle,

|C7 produced by its own heat; and that is

one of the first things you have to
consider with respect to the progress of the wax in the
course of its combustion, and as regards the changes it under-
goes. I will arrange another tube carefully in the flame, and
I should not wonder if we were able, by a little care, to get
that vapor to pass through the tube to the other extremity,
where we will light it, and obtain absolutely the flame of the
candle at a place distant from it. Now, look at that. Is not
that a very pretty experiment ? Talk about laying on gas —
why, we can actually lay on a candle! And you see from
Fig. 62 this that there are clearly two different kinds of action —
one the production of the vapor, and the other the combus-
tion of it — both of which take place in particular parts of the candle.
I shall get no vapor from that part which is already burnt. If I
raise the tube (Fig. 61) to the upper part of the flame, so soon as the
vapor has been swept out what comes away wall be no longer com-
bustible; it is already burned. How burned? Why, burned thus:
In the middle of the flame, where the wick is, there is this com-
bustible vapor; on the outside of the flame is the air which we shall
find necessary for the burning of the candle; between the two, in-
tense chemical action takes place, whereby the air and the fuel act
upon each other, and at the very same time that we obtain light the
vapor inside is destroyed. If you examine where the heat of a candle
is, you will find it very curiously arranged. Suppose I take this can-
dle, and hold a piece of paper close upon the flame, where is the heat
of that flame? Do you not see that it is not in the inside? It is in a
ring, exactly in the place where I told you the chemical action was;
and even in my irregular mode of making the experiment, if there
is not too much disturbance, there will always be a ring. This is a
good experiment for you to make at home. Take a strip of paper.

104 FARADAY

have the air in the room quiet, and put the piece of paper right
across the middle of the flame — (I must not talk while I make the
experiment) — and you will find that it is burnt in two places, and
that it is not burnt, or very little so, in the middle; and when you
have tried the experiment once or twice, so as to make it nicely, you
will be very interested to see where the heat is, and to find that it is
where the air and the fuel come together.

This is most important for us as we proceed with our subject. Air
is absolutely necessary for combustion; and, what is more, I must
have you understand that fresh air is necessary, or else we should be
imperfect in our reasoning and our experiments. Here is a jar of
air; I place it over a candle, and it burns very nicely in it at first,
showing that what I have said about it is true; but there will soon
be a change. See how the flame is drawing upward, presently fading,
and at last going out. And going out, why ? Not because it wants
air merely, for the jar is as full now as it was before; but it wants
pure, fresh air. The jar is full of air, partly changed, partly not
changed; but it does not contain sufficient of the fresh air which is
necessary for the combustion of a candle. These are all points which
we, as young chemists, have to gather up; and if we look a little
more closely into this kind of action, we shall find certain steps of
reasoning extremely interesting. For instance, here is the oil-lamp
I showed you — an excellent lamp for our experiments — the old
Argand lamp. I now make it like a candle [obstructing the passage
of air into the centre of the flame]; there is the cotton; there is the
oil rising up in it, and there is the conical flame. It burns poorly
because there is a partial restraint of air. I have allowed no air to
get to it save around the outside of the flame, and it does not burn
well. I can not admit more air from the outside, because the wick is
large; but if, as Argand did so cleverly, I open a passage to the middle
of the flame, and so let air come in there, you will see how much
more beautifully it burns. If I shut the air off, look how it smokes;
and why ? We have now some very interesting points to study : we
have the case of the combustion of a candle; we have the case of a
candle being put out by the want of air; and we have now the case
of imperfect combustion, and this is to us so interesting that I want
you to understand it as thoroughly as you do the case of a candle

CHEMICAL HISTORY OF A CANDLE IO5

burning in its best possible manner. I will now make a great flame,
because we need the largest possible illustrations. Here is a larger
wick [burning turpentine on a ball of cotton]. All these things are
the same as candles, after all. If we have larger wicks, we must have
a larger supply of air, or we shall have less perfect combustion. Look,
now, at this black substance going up into the atmosphere; there is
a regular stream of it. I have provided means to carry off the im-
perfectly burned part, lest it should annoy you. Look at the soots
that fly off from the flame; see what an imperfect combustion it is,
because it can not get enough air. What, then, is happening.? Why,
certain things which are necessary to the combustion of a candle
are absent, and very bad results are accordingly produced; but we
see what happens to a candle when it is burnt in a pure and proper
state of air. At the time when I showed you this charring by the
ring of flame on the one side of the paper, I might have also shown
you, by turning to the other side, that the burning of a candle pro-
duces the same kind of soot — charcoal, or carbon.

But, before I show that, let me explain to you, as it is quite neces-
sary for our purpose, that, though I take a candle, and give you, as
the general result, its combustion in the form of a flame, we must
see whether combustion is always in this condition, or whether there
are other conditions of flame; and we shall soon discover that there
are, and that they are most important to us. I think, perhaps, the
best illustration of such a point to us, as juveniles, is to show the
result of strong contrast. Here is a little gunpowder. You know that
gunpowder burns with flame; we may fairly call it flame. It con-
tains carbon and other materials, which altogether cause it to burn
with a flame. And here is some pulverized iron, or iron filings. Now
I purpose burning these two things together. I have a little mortar
in which I will mix them. (Before I go into these experiments, let
me hope that none of you, by trying to repeat them for fun's sake,
will do any harm. These things may all be very properly used if
you take care, but without that much mischief will be done.) Well,
then, here is a little gunpowder, which I put at the bottom of that
little wooden vessel, and mix the iron filings up with it, my object
being to make the gunpowder set fire to the filings and burn them
in the air, and thereby show the difference between substances burn-

I06 FARADAY

ing with flame and not with flame. Here is the mixture; and when
I set fire to it you must watch the combustion, and you will see that
it is of two kinds. You will see the gunpowder burning with a flame
and the filings thrown up. You will see them burning, too, but
without the production of flame. They will each burn separately.
[The lecturer then ignited the mixture.] There is the gunpowder,
which burns with a flame, and there are the filings: they burn with
a different kind of combustion. You see, then, these two great
distinctions; and upon these differences depend all the utility and all
the beauty of flame which we use for the purpose of giving out light.
When we use oil, or gas, or candle for the purpose of illumination,
their fitness all depends upon these different kinds of combustion.

There are such curious conditions of flame that it requires some
cleverness and nicety of discrimination to distinguish the kinds of
combustion one from another. For instance, here is a powder which
is very combustible, consisting, as you see, of separate little particles.
It is called lycopodium,C) and each of these particles can produce a
vapor, and produce its own flame; but, to see them burning, you
would imagine it was all one flame. I will now set fire to a quan-
tity, and you will see the effect. We saw a cloud of flame, apparently
in one body; but that rushing noise [referring to the sound produced
by the burning] was a proof that the combustion was not a con-
tinuous or regular one. This is the lightning of the pantomimes, and
a very good imitation. [The experiment was twice repeated by blow-
ing lycopodium from a glass tube through a spirit flame.] This is
not an example of combustion like that of the filings I have been
speaking of, to which we must now return.

Suppose I take a candle and examine that part of it which appears
brightest to our eyes. Why, there I get these black particles, which
already you have seen many times evolved from the flame, and which
I am now about to evolve in a different way. I will take this candle
and clear away the gutterage, which occurs by reason of the currents
of air; and if I now arrange the glass tube so as just to dip into this
luminous part, as in our first experiment, only higher, you see the

' Lycopodium is a yellowish powder found in the fruit of the club moss (Lycopo-
dium clavatum). It is used in fireworks.

CHEMICAL HISTORY OF A CANDLE IO7

result. In place of having the same white vapor that you had before,
you will now have a black vapor. There it goes, as black as ink. It
is certainly very different from the white vapor; and when we put a
light to it we shall find that it does not burn, but that it puts the
light out. Well, these particles, as I said before, are just the smoke
of the candle; and this brings to mind that old employment which
Dean Swift recommended to servants for their amusement, namely,
writing on the ceiling of a room with a candle. But what is that
black substance.? Why, it is the same carbon which exists in the
candle. How comes it out of the candle? It evidently existed in the
candle, or else we should not have had it here. And now I want
you to follow me in this explanation. You would hardly think that
all those substances which fly about London, in the form of soots
and blacks, are the very beauty and life of the flame, and wliich are
burned in it as those iron filings were burned here. Here is a piece
of wire gauze, which will not let the flame go through it; and I
think you will see, almost immediately, that when I bring it low
enough to touch that part of the flame which is otherwise so bright,
it quells and quenches it at once, and allows a volume of smoke
to rise up.

I want you now to follow me in this point — that whenever a sub-
stance burns, as the iron filings burnt in the flame of gunpowder,
without assuming the vaporous state (whether it becomes liquid or
remains solid), it becomes exceedingly luminous. I have here taken
three or four examples apart from the candle on purpose to illus-
trate this point to you, because what I have to say is applicable to
all substances, whether they burn or whether they do not burn — that
they are exceedingly bright if they retain their solid state, and that
it is to this presence of soUd particles in the candle flame that it
owes its brilliancy.

Here is a platinum wire, a body which does not change by heat.
If I heat it in this flame, see how exceedingly luminous it becomes.
I will make the flame dim for the purpose of giving a litde light
only, and yet you will see that the heat which it can give to that
platinum wire, though far less than the heat it has itself, is able to
raise the platinum wire to a far higher state of effulgence. This flame

I08 FARADAY

has carbon in it; but I will take one that has no carbon in it. There
is a material, a kind of fuel — a vapor, or gas, whichever you like to
call it — in that vessel, and it has no solid particles in it; so I take that
because it is an example of flame itself burning without any solid
matter whatever; and if I now put this solid substance in it, you see
what an intense heat it has, and how brightly it causes the solid body
to glow. This is the pipe through which we convey this particular
gas, which we call hydrogen, and which you shall know all about
the next time we meet. And here is a substance called oxygen, by
means of which this hydrogen can burn; and although we produce,
by their mixture, far greater heat (') than you can obtain from the
candle, yet there is very little light. If, however, I take a solid
substance, and put that into it, we produce an intense light. If I take
a piece of lime, a substance which will not burn, and which will not
vaporize by the heat (and because it does not vaporize remains solid,
and remains heated), you will soon observe what happens as to its
glowing. I have here a most intense heat produced by the burning
of hydrogen in contact with the oxygen; but there is as yet very little
light — not for want of heat, but for want of particles which can
retain their solid state; but when I hold this piece of lime in the
flame of the hydrogen as it burns in the oxygen, see how it glows!
This is the glorious lime light, which rivals the voltaic light, and
which is almost equal to sunlight. I have here a piece of carbon or
charcoal, which will burn and give us light exactly in the same man-
ner as if it were burnt as part of a candle. The heat that is in the
flame of a candle decomposes the vapor of the wax, and sets free the
carbon particles; they rise up heated and glowing as this now glows,
and then enter into the air. But the particles, when burnt, never pass
off from a candle in the form of carbon. They go off into the air as
a perfectly invisible substance, about which we shall know hereafter.
Is it not beautiful to think that such a process is going on, and
that such a dirty thing as charcoal can become so incandescent ? You
see it comes to this — that all bright flames contain these solid parti-
cles; all things that burn and produce solid particles, either during
the time they are burning, as in the candle, or immediately after

' Bunsen has calculated that the temperature of the oxyhydrogen blowpipe is 8061°
Centigrade. Hydrogen burning in air has a temperature of 3259° C, and coal gas
in air, 2350° C.

CHEMICAL HISTORY OF A CANDLE I Op

being burnt, as in the case of the gunpowder and iron fiUngs — all
these things give us this glorious and beautiful light.

I will give you a few illustrations. Here is a piece of phosphorus,
which burns with a bright flame. Very well; we may now conclude
that phosphorus will produce, either at the moment that it is burn-
ing or afterwards, these solid particles. Here is the phosphorus
lighted, and I cover it over with this glass
for the purpose of keeping in what is pro-
duced. What is all that smoke ? (Fig. 63.)
That smoke consists of those very particles
which are produced by the combustion of
the phosphorus. Here, again, are two sub-
stances. This is chlorate of potassa, and this
other sulphuret of antimony. I shall mix
these together a little, and then they may

be burnt in many ways. I shall touch them with a drop of sulphuric
acid, for the purpose of giving you an illustration of chemical action,
and they will instantly burn.(^) [The lecturer then ignited the mix-
ture by means of sulphuric acid.] Now, from the appearance of
things, you can judge for yourselves whether they produce solid
matter in burning. I have given you the train of reasoning which
will enable you to say whether they do or do not; for what is this
bright flame but the solid particles passing off.'*

Mr. Anderson has in the furnace a very hot crucible. I am about
to throw into it some zinc filings, and they will burn with a flame
like gunpowder. I make this experiment because you can make it
well at home. Now I want you to see what will be the result of the
combustion of this zinc. Here it is burning — burning beautifully like
a candle, I may say. But what is all that smoke, and what are those
little clouds of wool which will come to you if you can not come to
them, and make themselves sensible to you in the form of the old
philosophic wool, as it was called ? We shall have left in that crucible,
also, a quantity of this woolly matter. But I will take a piece of this

' The following is the action of the sulphuric in inflaming the mixture of sulphuret
of antimony and chlorate of potassa. A portion of the latter is decomposed by the
sulphuric acid into oxide of chlorine, bisulphate of potassa, and perchlorate of potassa.
The oxide of chlorine inflames the sulphuret of antimony, which is a combustible
body, and the whole mass instantly bursts into flame.

no FARADAY

same zinc, and make an experiment a little more closely at home, as
it were. You will have here the same thing happening. Here is the
piece of zinc; there [pointing to a jet of hydrogen] is the furnace, and
we will set to work and try and burn the metal. It glows, you see;
there is the combustion; and there is the white substance into which
it burns. And so, if I take that flame of hydrogen as the representa-
tive of a candle, and show you a substance like zinc burning in the
flame, you will see that it was merely during the action of combus-
tion that this substance glowed — while it was kept hot; and if I take
a flame of hydrogen and put this white substance from the zinc into
it, look how beautifully it glows, and just because it is a solid
substance.

I will now take such a flame as I had a moment since, and set free
from it the particles of carbon. Here is some camphene, which will
burn with a smoke; but if I send these particles of smoke through this
pipe into the hydrogen flame you will see they will burn and become
luminous, because we heat them a second time. There they are.
Those are the particles of carbon reignited a second time. They are
those particles which you can easily see by holding a piece of paper
behind them, and which, while they are in the flame, are ignited by
the heat produced, and, when so ignited, produce this brightness.
When the particles are not separated you get no brightness. The
flame of coal gas owes its brightness to the separation, during com-
bustion, of these particles of carbon, which are equally in that as
in a candle. I can very quickly alter that arrangement. Here, for
instance, is a bright flame of gas. Supposing I add so much air to
the flame as to cause it all to burn before those particles are set free,
I shall not have this brightness; and I can do that in this way: If
I place over the jet this wire-gauze cap, as you see, and then light
the gas over it, it burns with a non-luminous flame, owing to its
having plenty of air mixed with it before it burns; and if I raise the
gauze, you see it does not burn below('°). There is plenty of carbon

'"The "air-burner," which is of such value in the laboratory, owes its advantage
to this principle. It consists of a cylindrical metal chimney, covered at the top with
a piece of rather coarse iron wire gauze. This is supported over an Argand burner in
such a manner that the gas may mix in the chimney with an amount of air sufficient
to burn the carbon and hydrogen simultaneously, so that there may be no separation
of carbon in the flame with consequent deposition of soot. The flame, being unable
to pass through the wire gauze, burns in a steady, nearly invisible manner above.

CHEMICAL HISTORY OF A CANDLE III

in the gas; but, because the atmosphere can get to it, and mix with
it before it burns, you see how pale and blue the flame is. And if I
blow upon a bright gas flame, so as to consume all this carbon before
it gets heated to the glowing point, it will also burn blue. [The lec-
turer illustrated his remarks by blowing on the gas light.] The only
reason why I have not the same bright light when I thus blow upon
the flame is that the carbon meets with sufficient air to burn it be-
fore it gets separated in the flame in a free state. The difference is
solely due to the solid particles not being separated before the gas is
burnt.

You observe that there are certain products as the result of the com-
bustion of a candle, and that of these products one portion may be
considered as charcoal, or soot; that charcoal, when afterward burnt,
produces some other product; and it concerns us very much now to
ascertain what that other product is. We showed that something
was going away; and I want you now to understand how much is
going up into the air; and for that purpose we will have combustion
on a litde larger scale. From that candle ascends heated air, and two
or three experiments will show you the ascending current; but, in
order to give you a notion of the quantity of matter which ascends in
this way, I will make an experiment by which I shall try to imprison
some of the products of this combustion. For this purpose I have here
what boys call a fire-balloon; I use this fire-balloon merely as a sort
of measure of the result of the combustion we are considering; and
I am about to make a flame in such an easy and simple manner as
shall best serve my present purpose. This plate shall be the "cup,"
we will so say, of the candle; this spirit shall be our fuel; and I am
about to place this chimney over it, because it is better for me to do
so than to let things proceed at random. Mr. Anderson will now
light the fuel, and here at the top we shall get the results of the
combustion. What we get at the top of that tube is exactly the
same, generally speaking, as you get from the combustion of a
candle; but we do not get a luminous flame here, because we use a
substance which is feeble in carbon. I am about to put this balloon-
not into action, because that is not my object — but to show you the
effect which results from the action of those products which arise
from the candle, as they arise here from the furnace. [The balloon

112

FARADAY

was held over the chimney (Fig. 64), when it immediately com-
menced to fill.] You see how it is disposed to ascend; but we must
not let it up; because it might come in contact with those upper
gaslights, and that would be very inconvenient. [The upper gas-
lights were turned out at the request of the
lecturer, and the balloon was allowed to
ascend.] Does not that show you what a
large bulk of matter is being evolved .'' Now
there is going through this tube [placing a
large glass tube over a candle] all the prod-
ucts of that candle, and you will presently
see that the tube will become quite opaque.
Suppose I take another candle, and place it
under a jar, and then put a light on the other
side, just to show you what is going on. You
see that the sides of the jar become cloudy,
and the light begins to burn feebly. It is the
products, you see, which make the light so
dim, and this is the same thing which makes
the sides of the jar so opaque. If you go
home, and take a spoon that has been in the
cold air, and hold it over a candle — not so as
to soot it — you will find that it becomes dim
just as that jar is dim. If you can get a silver dish, or something of
that kind, you will make the experiment still better; and now, just
to carry your thoughts forward to the time we shall next meet, let
me tell you that it is water which causes the dimness, and when we
next meet I will show you that we can make it, without difficulty,
assume the form of a liquid.

Fig. 64

LECTURE III

PRODUCTS: WATER FROM THE COMBUSTION-
NATURE OF WATER— A COMPOUND-
HYDROGEN

I DARE say you well remember that when we parted we had
just mentioned the word "products" from the candle; for when
a candle burns we found we were able, by nice adjustment, to
get various products from it. There was one substance which was
not obtained when the candle was burning properly, which was
charcoal or smoke, and there was some other substance that went
upward from the flame which did not appear as smoke, but took
some other form, and made part of that general current which,
ascending from the candle upward, becomes invisible, and escapes.
There were also other products to mention. You remember that
in that rising current having its origin at the candle we found that
one part was condensable against a cold spoon, or against a clean
plate, or any other cold thing, and another part was incondensable.

We will first take the condensable part, and examine it, and,
strange to say, we find that that part of the product is just water —
nothing but water. On the last occasion I spoke of it incidentally,
merely saying that water was produced among the condensable
products of the candle; but to-day I wish to draw your attention to
water, that we may examine it carefully, especially in relation to this
subject, and also with respect to its general existence on the surface
of the globe.

Now, having previously arranged an experiment for the purpose
of condensing water from the products of the candle, my next point
will be to show you this water; and perhaps one of the best means
that I can adopt for showing its presence to so many at once is to
exhibit a very visible action of water, and then to apply that test to
what is collected as a drop at the bottom of that vessel. I have here
a chemical substance, discovered by Sir Humphry Davy, which

"3

114 FARADAY

has a very energetic action upon water, which I shall use as a test
of the presence of water. If I take a little piece of it — it is called
potassium, as coming from potash — if I take a little piece of it, and
throw it into that basin, you see how it shows the presence of water
by lighting up and floating about, burning with a violet flame. I
am now going to take away the candle which has been burning
beneath the vessel containing ice and salt, and you see a drop of
water — a condensed product of the candle — hanging
from the under surface of the dish (Fig. 65) . I will
show you that potassium has the same action upon it
as upon the water in that basin in the experiment we
have just tried. See! it takes fire, and burns in just
the same manner. I will take another drop upon this
glass slab, and when I put the potassium on to it, you
see at once, from its taking fire, that there is water
present. Now that water was produced by the candle.
In the same manner, if I put this spirit lamp under
that jar, you will soon see the latter become damp
from the dew which is deposited upon it — that dew
being the result of combustion; and I have no doubt
you will shortly see, by the drops of water which fall
upon the paper below, that there is a good deal of
water produced from the combustion of the lamp. I will let it remain,
and you can afterward see how much water has been collected. So, if
I take a gas lamp, and put any cooling arrangement over it, I shall
get water — water being likewise produced from the combustion of
gas. Here, in this bottle, is a quantity of water — perfectly pure,
distilled water, produced from the combustion of a gas lamp — in no
point different from the water that you distill from the river, or
ocean, or spring, but exactly the same thing. Water is one individual
thing; it never changes. We can add to it by careful adjustment for
a little while, or we can take it apart and get other things from it;
but water, as water, remains always the same, either in a solid, liquid,
or fluid state. Here again [holding another bottle] is some water
produced by the combustion of an oil lamp. A pint of oil, when
burnt fairly and properly, produces rather more than a pint of
water. Here, again, is some water, produced by a rather long experi-

FiG. 65

CHEMICAL HISTORY OF A CANDLE II5

ment, from a wax candle. And so we can go on with almost all
combustible substances, and find that if they burn with a flame, as
a candle, they produce water. You may make these experiments
yourselves: the head of a poker is a very good thing to try with,
and if it remains cold long enough over the candle, you may get
water condensed in drops on it; or a spoon, or ladle, or any thing
else may be used, provided it be clean, and can carry ofl the heat, and
so condense the water.

And now — to go into the history of this wonderful production of
water from combustibles, and by combustion — I must first of all
tell you that this water may exist in different conditions; and
although you may now be acquainted with all its forms, they still
require us to give a little attention to them for the present; so that
we may perceive how the water, while it goes through its Protean
changes, is entirely and absolutely the same thing, whether it is
produced from a candle, by combustion, or from the rivers or ocean.

First of all, water, when at the coldest, is ice. Now we philosophers
— I hope that I may class you and myself together in this case —
speak of water as water, whether it be in its solid, or liquid, or
gaseous state — we speak of it chemically as water. Water is a thing
compounded of two substances, one of which we have derived from
the candle, and the other we shall find elsewhere. Water may occur
as ice; and you have had most excellent opportunities lately of seeing
this. Ice changes back into water — for we had on our last Sabbath
a strong instance of this change by the sad catastrophe which
occurred in our own house, as well as in the houses of many of our
friends — ice changes back into water when the temperature is raised;
water also changes into steam when it is warmed enough. The
water which we have here before us is in its densest state("); and,
although it changes in weight, in condition, in form, and in many
other qualities, it still is water; and whether we alter it into ice by
cooling, or whether we change it into steam by heat, it increases in
volume — in the one case very strangely and powerfully, and in the
other case very largely and wonderfully. For instance, I will now
take this tin cylinder, and pour a little water into it, and, seeing how
much water I pour in, you may easily estimate for yourselves how

" Water is in its densest state at a temperature of 39.1 ° Fahrenheit.

Il6 FARADAY

high it will rise in the vessel: it will cover the bottom about two
inches. I am now about to convert the water into steam for the
purpose of showing to you the different volumes which water
occupies in its different states of water and steam.

Let us now take the case of water changing into ice: we can effect
that by cooling it in a mixture of salt and pounded ice('^) — and I
shall do so to show you the expansion of water into a thing of
larger bulk when it is so changed. These bottles [holding one] are
made of strong cast iron, very strong and very thick — I suppose they
are the third of an inch in thickness; they are very carefully filled
with water, so as to exclude all air, and then they are screwed down
tight. We shall see that when we freeze the water in these iron
vessels, they will not be able to hold the ice, and the expansion within
them will break them in pieces as these [pointing to some fragments]
are broken, which have been bottles of exactly the same kind. I am
about to put these two bottles into that mixture of ice and salt for
the purpose of showing that when water becomes ice it changes in
volume in this extraordinary way.

In the mean time, look at the change which has taken place in
the water to which we have applied heat; it is losing its fluid state.
You may tell this by two or three circumstances. I have covered the
mouth of this glass flask, in which water is boiling, with a watch-
glass. Do you see what happens? It rattles away like a valve
chattering, because the steam rising from the boiling water sends
the valve up and down, and forces itself out, and so makes it clatter.
You can very easily perceive that the flask is quite full of steam, or
else it would not force its way out. You see, also, that the flask
contains a substance very much larger than the water, for it fills
the whole of the flask over and over again, and there it is blowing
away into the air; and yet you can not observe any great diminution
in the bulk of the water, which shows you that its change of bulk
is very great when it becomes steam.

I have put our iron bottles containing water into this freezing mix-
ture, that you may see what happens. No communication will take
place, you observe, between the water in the bottles and the ice in

'^A mixture of salt and pounded ice reduces the temperature from 32° F. to zero,
the ice at the same time becoming fluid.

CHEMICAL HISTORY OF A CANDLE II 7

the outer vessel. But there will be a conveyance of heat from the
one to the other, and if we are successful — we are making our
experiment in very great haste — I expect you will by-and-by, so soon
as the cold has taken possession of the bottles and their contents,
hear a pop on the occasion of the bursting of the one bottle or the
other, and, when we come to examine the bottles, we shall find their
contents masses of ice, partly inclosed by the covering of iron which
is too small for them, because the ice is larger in bulk than the

Fig. 66

water. You know very well that ice floats upon water; if a boy falls
through a hole into the water, he tries to get on the ice again to
float him up. Why does the ice float? Think of that, and philoso-
phize. Because the ice is larger than the quantity of water which
can produce it, and therefore the ice weighs the lighter and the
water is the heavier.

To return now to the action of heat on water. See what a stream
of vapor is issuing from this tin vessel! You observe, we must have
made it quite full of steam to have it sent out in that great quantity.
And now, as we can convert the water into steam by heat, we con-
vert it back into liquid water by the application of cold. And if we

Il8 FARADAY

take a glass, or any other cold thing, and hold it over this steam,
see how soon it gets damp with water: it will condense it until the
glass is warm — it condenses the water which is now running down
the sides of it. I have here another experiment to show the con-
densation of water from a vaporous state back into a liquid state,
in the same way as the vapor, one of the products of the candle, was
condensed against the bottom of the dish and obtained in the form
of water; and to show you how truly and thoroughly these changes
take place, I will take this tin flask, which is now full of steam,
and close the top. We shall see what takes place when we cause this
water or steam to return back to the fluid state by pouring some cold
water on the outside. [The lecturer poured the cold water over the
vessel, when it immediately collapsed (Fig. 66).] You see what has
happened. If I had closed the stopper, and still kept the heat applied
to it, it would have burst the vessel; yet, when the steam returns to
the state of water, the vessel collapses, there being a vacuum pro-
duced inside by the condensation of the steam. I show you these
experiments for the purpose of pointing out that in all these occur-
rences there is nothing that changes the water into any other thing;
it still remains water; and so the vessel is obliged to give way, and
is crushed inward, as in the other case, by the farther application of
heat, it would have been blown outward.

And what do you think the bulk of that water is when it assumes
the vaporous condition.'' You see that cube [pointing to a cubic

foot]. There, by its side, is a cubic
inch (Fig. 67), exactly the same shape
as the cubic foot, and that bulk of
water [the cubic inch] is sufficient to
expand into that bulk [the cubic foot]
of steam; and, on the contrary, the
application of cold will contract that
large quantity of steam into this
'°' ^ small quantity of water. [One of the

iron bottles burst at that moment.] Ah! There is one of our bottles
burst, and here, you see, is a crack down one side an eighth of an
inch in width. [The other now exploded, sending the freezing
mixture in all directions.] This other Ijottle is also broken; although

CHEMICAL HISTORY OF A CANDLE II 9

the iron was nearly half an inch thick, the ice has burst it asunder.
These changes always take place in water; they do not require to be
always produced by artificial means; we only use them here because
we want to produce a small winter round that little bottle instead
of a long and severe one. But if you go to Canada, or to the North,
you will find the temperature there out of doors will do the same
thing as has been done here by the freezing mixture.

To return to our quiet philosophy. We shall not in future be
deceived, therefore, by any changes that are produced in water.
Water is the same every where, whether produced from the ocean or
from the flame of the candle. Where, then, is this water which we
get from a candle? I must anticipate a little, and tell you. It
evidently comes, as to part of it, from the candle, but is it within the
candle beforehand? No, it is not in the candle; and it is not in the
air around about the candle which is necessary for its combustion.
It is neither in one nor the other, but it comes from their conjoint
action, a part from the candle, a part from the air; and this we
have now to trace, so that we may understand thoroughly what is
the chemical history of a candle when we have it burning on our
table. How shall we get at this? I myself know plenty of ways,
but I want you to get at it from the association in your own minds
of what I have already told you.

I think you can see a little in this way. We had just now the
case of a substance which acted upon the water in the way that
Sir Humphry Davy showed us('^), and which I am now going to
recall to your minds by making again an experiment upon that dish.
It is a thing which we have to handle very carefully; for you see, if
I allow a little splash of water to come upon this mass, it sets fire
to part of it; and if there were free access of air, it would quickly set
fire to the whole. Now this is a metal — a beautiful and bright
metal — which rapidly changes in the air, and, as you know, rapidly
changes in water. I will put a piece on the water, and you see it
burns beautifully, making a floating lamp, using the water in the
place of air. Again, if we take a few iron filings or turnings and

"Potassium, the metallic basis of potash, was discovered by Sir Humphry Davy
in 1807, who succeeded in separating it from potash by means of a powerful voltaic
battery. Its great affinity for oxygen causes it to decompose water with evolution of
hydrogen, which takes fire with the heat produced.

120 FARADAY

put them in water, we find that they Ukewise undergo an alteration.
They do not change so much as this potassium does, but they change
somewhat in the same way; they become rusty, and show an action
upon the water, though in a different degree of intensity to what
this beautiful metal does; but they act upon the water in the same
manner generally as this potassium. I want you to put these different
facts together in your minds. I have another metal here [zinc],
and when we examined it with regard to the solid substance pro-
duced by its combustion, we had an opportunity of seeing that it
burned; and I suppose, if I take a little strip of this zinc and put it
over the candle, you will see something half way, as it were, between
the combustion of potassium on the water and the action of iron —
you see there is a sort of combustion. It has burned, leaving a white
ash or residuum; and here also we find that the metal has a certain
amount of action upon water.

By degrees we have learned how to modify the action of these
different substances, and to make them tell us what we want to
know. And now, first of all, I take iron. It is a common thing in
all chemical reactions, where we get any result of this kind, to find
that it is increased by the action of heat; and if we want to examine
minutely and carefully the action of bodies one upon another, we
often have to refer to the action of heat. You are aware, I believe,
that iron filings burn beautifully in the air; but I am about to show
you an experiment of this kind, because it will impress upon you
what I am going to say about iron in its action on water. If I take a
flame and make it hollow — you know why, because I want to get
air to it and into it, and therefore I make it hollow — and then take
a few iron filings and drop them into the flame, you see how well
they burn. That combustion results from the chemical action which
is going on when we ignite those particles. And so we proceed to
consider these different effects, and ascertain what iron will do when
it meets with water. It will tell us the story so beautifully, so gradu-
ally and regularly, that I think it will please you very much.

I have here a furnace with a pipe going through it like an iron
gun barrel (Fig. 68), and I have stuffed that barrel full of bright
iron turnings, and placed it across the fire to be made red-hot. We
can either send air through the barrel to come in contact with the

CHEMICAL HISTORY OF A CANDLE

121

iron, or we can send steam from this little boiler at the end of the
barrel. Here is a stop-cock which shuts off the steam from the
barrel until we wish to admit it. There is some water in these glass
jars, which I have colored blue, so that you may see what happens.
Now you know very well that any steam I might send through that
barrel, if it went through into the water, would be condensed; for
you have seen that steam can not retain its gaseous form if it be
cooled down; you saw it here [pointing to the tin flask] crushing
itself into a small bulk, and causing the flask holding it to collapse;

Fig. 68

SO that if I were to send steam through that barrel it would be con-
densed, supposing the barrel were cold; it is, therefore, heated to
perform the experiment I am now about to show you. I am going
to send the steam through the barrel in small quantities, and you
shall judge for yourselves, when you see it issue from the other end,
whether it still remains steam. Steam is condensible into water, and
when you lower the temperature of steam you convert it back into
fluid water; but I have lowered the temperature of the gas which
I have collected in this jar by passing it through water after it has
traversed the iron barrel, and still it does not change back into water.
I will take another test and apply to this gas. (I hold the jar in an
inverted position, or my substance would escape.) If I now apply
a light to the mouth of the jar, it ignites with a slight noise. That
tells you that it is not steam; steam puts out a fire: it does not burn;
but you saw that what I had in that jar burnt. We may obtain this

122

FARADAY

substance equally from water produced from the candle flame as
from any other source. When it is obtained by the action of the
iron upon the aqueous vapor, it leaves the iron in a state very
similar to that in which these filings were after they were burnt. It
makes the iron heavier than it was before. So long as the iron
remains in the tube and is heated, and is cooled again without the
access of air or water, it does not change in its weight; but after
having had this current of steam passed over it, it then comes out
heavier than it was before, having taken something out of the
steam, and having allowed something else to pass forth, which we
see here. And now, as we have another jar full, I will show you
something most interesting. It is a combustible gas; and I might at
once take this jar and set fire to the contents, and show you that it
is combustible; but I intend to show you more, if I can. It is also
a very light substance. Steam will condense; this body will rise in
the air, and not condense. Suppose I take another glass jar, empty
of all but air: if I examine it with a taper I shall find that it con-
tains nothing but air. I will now take this jar full of the gas that
I am speaking of, and deal with it as though it were a light body;

I will hold both upside down, and turn
the one up under the other (Fig. 69);
and that which did contain the gas pro-
cured from the steam, what does it con-
tain now? You will find it now only
contains air. But look! Here is the com-
bustible substance [taking the other jar]
which I have poured out of the one jar
^'°- ^9 into the other. It still preserves its qual-

ity, and condition, and independence, and therefore is the more
worthy of our consideration, as belonging to the products of a candle.
Now this substance which we have just prepared by the action
of iron on the steam or water, we can also get by means of those
other things which you have already seen act so well upon the
water. If I take a piece of potassium, and make the necessary
arrangements, it will produce this gas; and if, instead, a piece of
zinc, I find, when I come to examine it very carefully, that the main
reason why this zinc can not act upon the water continuously as

CHEMICAL HISTORY OF A CANDLE 1 23

the other metal does, is because the result of the action of the water
envelops the zinc in a kind of protecting coat. We have learned in
consequence, that if we put into our vessel only the zinc and water,
they, by themselves, do not give rise to much action, and we get no
result. But suppose I proceed to dissolve off this varnish — this
encumbering substance — which I can do by a little acid; the moment
I do this I find the zinc acting upon the water exactly as the iron
did, but at the common temperature. The acid in no way is altered,
except in its combination with the oxide of zinc which is produced.
I have now poured the acid into the glass, and the effect is as though
I were applying heat to cause this boiling up. There is something
coming off from the zinc very abundantly, which is not steam.
There is a jar full of it; and you will find that I have exactly the
same combustible substance remaining in the vessel, when I hold it
upside down, that I produced during the experiment with the iron
barrel. This is what we get from water, the same substance which
is contained in the candle.

Let us now trace distinctly the connection between these two
points. This is hydrogen — a body classed arnong those things which

'

llj^f^

l\ ffe^

^

^^^^^te:'

W

^\

^\

-^r^ ""'*\=^\^^-

— ^

:rr\

\. n^^

\

rr

- ■ — ■ — 'fc .- .' \ — 't-

w

■ — N;^

\

U- - — ^^ ■§

1

■ . \

Fig. 70

in chemistry we call elements, because we can get nothing else out
of them. A candle is not an elementary body, because we can get
carbon out of it; we can get this hydrogen out of it, or at least out of
the water which it supplies. And this gas has been so named
hydrogen, because it is that element which, in association with
another, generates water.* Mr. Anderson having now been able to
get two or three jars of gas, we shall have a few experiments to
make, and I want to show you the best way of making these experi-

*''TSup, "water," and yemau, "1 generate."

124 FARADAY

ments. I am not afraid to show you, for I wish you to make ex-
periments, if you will only make them with care and attention, and
the assent of those around you. As we advance in chemistry we are
obliged to deal with substances which are rather injurious if in
their wrong places; the acids, and heat, and combustible things we
use, might do harm if carelessly employed. If you want to make
hydrogen, you can make it easily from bits of zinc, and sulphuric
or muriatic acid. Here is what in former times was called the
"philosopher's candle." It is a little phial with a cork and a tube or
pipe passing through it. And I am now putting a few little pieces
of zinc into it. This little instrument I am going to apply to a useful
purpose in our demonstrations, for I want to show you that you can
prepare hydrogen, and make some experiments with it as you please,
at your own homes. Let me here tell you why I am so careful to
fill this phial nearly, and yet not quite full. I do it because the
evolved gas, which, as you have seen, is very combustible, is explosive
to a considerable extent when mixed with air, and might lead to
harm if you were to apply a light to the end of that pipe before all
the air had been swept out of the space above the water. I am now
about to pour in the sulphuric acid. I have used very little zinc and
more sulphuric acid and water, because I want to keep it at work
for some time. I therefore take care in this way to modify
the proportions of the ingredients so that I may have a
regular supply — not too quick and not too slow. Supposing
I now take a glass and put it upside down over the end of
the tube, because the hydrogen is light I expect that it will
remain in that vessel a little while. We will now test the
contents of our glass to see if there be hydrogen in it; I
think I am safe in saying we have caught some [applying
a light]. There it is, you see. I will now apply a light to the
top of the tube. There is the hydrogen burning (Fig. 71).
There is our philosophical candle. It is a foolish, feeble sort
of a flame, you may say, but it is so hot that scarcely any
common flame gives out so much heat. It goes on burning
regularly, and I am now about to put that flame to burn under a
certain arrangement, in order that we may examine its results and
make use of the information which we may thereby acquire. Inas-

CHEMICAL HISTORY OF A CANDLE 1 25

much as the candle produces water, and this gas comes out o£ the
water, let us see what this gives us by the same process of combustion
that the candle went through when it burnt in the atmosphere; and
for that purpose I am going to put the lamp under this apparatus
(Fig. 72), in order to condense whatever may arise from the com-

^ bustion within it. In

J,ff/'.('''^\^r "■- '■ ^^"""-^—^ the course of a short

time you will see mois-
ture appearing in the
cylinder, and you will get the water running down the
side, and the water from this hydrogen flame will have
absolutely the same effect upon all our tests, being obtained
by the same general process as in the former case. This
hydrogen is a very beautiful substance. It is so light that
it carries things up; it is far lighter than the atmosphere;
Fio. 72 gjjj J j^^g 5^y J (,^jj show you this by an experiment which,
if you are very clever, some of you may even have skill enough to re-
peat. Here is our generator of hydrogen, and here are some soapsuds.
I have an India-rubber tube connected with the hydrogen generator,
and at the end of the tube is a tobacco pipe. I can thus put the pipe
into the suds and blow bubbles by means of the hydrogen. You ob-
serve how the bubbles fall downward when I blow them with my
warm breath; but notice the difference when I blow them with hy-
drogen. [The lecturer here blew bubbles with hydrogen, which rose
to the roof of the theatre.] It shows you how light this gas must be in
order to carry with it not merely the ordinary soap bubble, but the
larger portion of a drop hanging to the bottom of it. I can show its
lightness in a better way than this; larger bubbles than these may be
so lifted up; indeed, in former times balloons used to be filled with
this gas. Mr. Anderson will fasten this tube on to our generator,
and we shall have a stream of hydrogen here with which we can
charge this balloon made of collodion. I need not even be very
careful to get all the air out, for I know the power of this gas to
carry it up. [Two collodion balloons were inflated and sent up, one
being held by a string.] Here is another larger one, made of thin
membrane, which we will fill and allow to ascend; you will see
they will all remain floating about until the gas escapes.

126 FARADAY

What, then, are the comparative weights of these substances? I
have a table here which will show you the proportion which their
weights bear to each other. I have taken a pint and a cubic foot as
the measures, and have placed opposite to them the respective figures.
A pint measure of this hydrogen weighs three-quarters of our
smallest weight, a grain, and a cubic foot weighs one-twelfth of an
ounce; whereas a pint of water weighs 8,750 grains, and a cubic foot
of water weighs almost 1,000 ounces. You see, therefore, what a vast
difference there is between the weight of a cubic foot of water and
a cubic foot of hydrogen.

Hydrogen gives rise to no substance that can become solid, either
during combustion or afterward as a product of its combustion;
but when it burns it produces water only; and if we take a cold glass
and put it over the flame, it becomes damp, and you have water
produced immediately in appreciable quantity; and nothing is pro-
duced by its combustion but the same water which you have seen
the flame of the candle produce. It is important to remember that
this hydrogen is the only thing in nature which furnishes water as
the sole product of combustion.

And now we must endeavor to find some additional proof of the
general character and composition of water, and for this purpose I
will keep you a little longer, so that at our next meeting we may
be better prepared for the subject. We have the power of arranging
the zinc which you have seen acting upon the water by the assist-
ance of an acid, in such a manner as to cause all the power to be
evolved in the place where we require it. I have behind me a
voltaic pile, and I am just about to show you, at the end of this
lecture, its character and power, that you may see what we shall have
to deal with when next we meet. I hold here the extremities of the
wires which transport the power from behind me, and which I
shall cause to act on the water.

We have previously seen what a power of combustion is possessed
by the potassium, or the zinc, or the iron filings; but none of them
show such energy as this. [The lecturer here made contact between
the two terminal wires of the battery, when a brilliant flash of light
was produced.] This light is, in fact, produced by a forty-zinc
power of burning; it is a power that I can carry about in my hands

CHEMICAL HISTORY OF A CANDLE IT]

through these wires at pleasure, although if I applied it wrongly to
myself, it would destroy me in an instant, for it is a most intense
thing, and the power you see here put forth while you count five
[bringing the poles in contact and exhibiting the electric light] is
equivalent to the power of several thunder-storms, so great is its
force("). And that you may see what intense energy it has, I will
take the ends of the wires which convey the power from the battery,
and with it I dare say I can burn this iron file. Now this is a
chemical power, and one which, when we next meet, I shall apply to
water, and show you what results we are able to produce.

'* Professor Faraday has calculated that there is as much electricity required to de-
compose one grain of water as there is in a very powerful flash of lightning.

LECTURE IV

HYDROGEN IN THE CANDLE— BURNS INTO

WATER— THE OTHER PART OF

WATER— OXYGEN

I SEE you are not tired of the candle yet, or I am sure you
would not be interested in the subject in the way you are. When
our candle was burning we found it produced water exactly
like the water we have around us; and by farther examination of
this water we found in it that curious body, hydrogen — that light
substance of which there is some in this jar. We afterward saw the
burning powers of that hydrogen, and that it produced water. And
I think I introduced to your notice an apparatus which I very
briefly said was an arrangement of chemical force, or power, or
energy, so adjusted as to convey its power to us in these wires; and
I said I should use that force to pull the water to pieces, to see what
else there was in the water besides hydrogen; because, you remember,
when we passed the water through the iron tube, we by no means
got the weight of water back which we put in in the form of steam,
though we had a very large quantity of gas evolved. We have now
to see what is the other substance present. That you may under-
stand the character and use of this instrument, let us make an
experiment or two. Let us put together, first of all, some sub-
stances, knowing what they are, and then see what that instrument
does to them. There is some copper (observe the various changes
which it can undergo), and here is some nitric acid, and you will
find that this, being a strong chemical agent, will act very power-
fully when I add it to the copper. It is now sending forth a beautiful
red vapor; but as we do not want that vapor, Mr. Anderson will
hold it near the chimney for a short time, that we may have the use
and beauty of the experiment without the annoyance. The copper
which I have put into the flask will dissolve it: it will change the
acid and the water into a blue fluid containing copper and other

128

CHEMICAL HISTORY OF A CANDLE 1 29

things, and I purpose then showing you how this voltaic battery
deals with it; and in the mean time we will arrange another kind of
experiment for you to see what power it has. This is a substance
which is to us like water — that is to say, it contains bodies which
we do not know of as yet, as water contains a body which we do
not know as yet. Now this solution of a salt('^) I will put upon
paper, and spread about, and apply the power of the battery to it, and
observe what will happen. Three or four important things will
happen which we shall take advantage of. I place this wetted paper
upon a sheet of tin foil, which is convenient for keeping all clean,
and also for the advantageous application of the power; and this
solution, you see, is not at all affected by being put upon the paper
or tin foil, nor by any thing else I have brought in contact with it
yet, and which, therefore, is free to us to use as regards that instru-
ment. But first let us see that our instrument is in order. Here are
our wires. Let us see whether it is in the state in which it was last
time. We can soon tell. As yet, when I bring them together, we
have no power, because the conveyers — what we call the electrodes —
the passages or ways for the electricity — are stopped; but now Mr.
Anderson by that [referring to a sudden flash at the ends of the
wires] has given me a telegram to say that it is ready. Before I begin
our experiment I will get Mr. Anderson to break contact again at
the battery behind me, and we will put a platinum wire across to
connect the poles, and then if I find I can ignite a pretty good length
of this wire we shall be safe in our experiment. Now you will see
the power. [The connection was established, and the intermediate
wire became red-hot.] There is the power running beautifully
through the wire, which I have made thin on purpose to show you
that we have those powerful forces; and now, having that power, we
will proceed with it to the examination of water.

I have here two pieces of platinum, and if I lay them down upon
this piece of paper [the moistened paper on the tin foil] you will see
no action; and if I take them up there is no change that you can
see, but the arrangement remains just as it was before. But, now,

'^ A solution of acetate of lead submitted to the action of the voltaic current yields
lead at the negative pole, and brown peroxide of lead at the positive pole. A solution
of nitrate of silver, under the same circumstances, yields silver at the negative pole,
and peroxide of silver at the positive pole.

130 FARADAY

see what happens: if I take these two poles and put either one or
the other of them down separately on the platinum plates, they do
nothing for me; both are perfectly without action; but if I let them
both be in contact at the same moment, see what happens [a brown
spot appeared under each pole of the battery]. Look here at the
effect that takes place, and see how I have pulled something apart
from the white — something brown; and I have no doubt, if I were
to arrange it thus, and were to put one of the poles to the tin foil
on the other side of the paper — why, I get such a beautiful action
upon the paper that I am going to see whether I can not write with
it — a telegram, if you please. [The lecturer here traced the word
"juvenile" on the paper with one of the terminal wires.] See there
how beautifully we can get our results!

You see we have here drawn something which we have not
known about before out of this solution. Let us now take that
flask from Mr. Anderson's hands, and see what we can draw out of
that. This, you know, is a liquid which we have just made up from
copper and nitric acid while our other experiments were in hand;
and though I am making this experiment very hastily, and may
bungle a little, yet I prefer to let you see what I do rather than pre-
pare it beforehand.

Now see what happens. These two platinum plates are the two
ends (or I will make them so immediately) of this apparatus; and
I am about to put them in contact with that solution, just as we
did a moment ago on the paper. It does not matter to us whether the
solution be on the paper or whether it be in the jar, so long as we
bring the ends of the apparatus to it. If I put the two platinums
in by themselves they come out as clean and as white as they go in
[inserting them into the fluid without connecting them with the
battery] ; but when we take the power and lay that on [the platinums
were connected with the battery and again dipped into the solution],
this, you see [exhibiting one of the platinums], is at once turned into
copper, as it were; it has become like a plate of copper; and that
[exhibiting the other piece of platinum] has come out quite clean.
If I take this coppered piece and change sides, the copper will leave
the right-hand side and come over to the left side; what was before
the coppered plate comes out clean, and the plate which was clean

CHEMICAL HISTORY OF A CANDLE I3I

comes out coated with copper; and thus you see that the same copper
we put into this solution we can also take out of it by means of this
instrument.

Putting that solution aside, let us now see what effect this instru-
ment will have upon water (Fig. 73). Here are two little platinum
plates which I intend to make the ends of the battery, and this (C)

Fig. 73

is a little vessel so shaped as to enable me to take it to pieces, and
show you its construction. In these two cups (A and B) I pour
mercury, which touches the ends of the wires connected with the
platinum plates. In the vessel (C) I pour some water containing a
little acid (but which is put in only for the purpose of facilitating
the action; it undergoes no change in the process), and connected
with the top of the vessel is a bent glass tube (D), which may
remind you of the pipe which was connected with the gun barrel
in our furnace experiment, and which now passes under the jar
(F). I have now adjusted this apparatus, and we will proceed to
affect the water in some way or other. In the other case I sent the
water through a tube which was made red-hot; I am now going to
pass the electricity through the contents of this vessel. Perhaps I may
boil the water; if I do boil the water, I shall get steam; and you
know that steam condenses when it gets cold, and you will there-
fore see by that whether I do boil the water or not. Perhaps, how-
ever, I shall not boil the water, but produce some other effect. You
shall have the experiment and see. There is one wire which I will

132 FARADAY

put to this side (A), and here is the other wire which I will put to
the other side (B), and you will soon see whether any disturbance
takes place. Here it is seeming to boil up famously; but does it boil?
Let us see whether that which goes out is steam or not. I think you
will soon see the jar (F) will be filled with vapor, if that which rises
from the water is steam. But can it be steam? Why, certainly not;
because there it remains, you see, unchanged. There it is standing
over the water, and it can not therefore be steam, but must be a
permanent gas of some sort. What is it? Is it hydrogen? Is it
anything else? Well, we will examine it. If it is hydrogen it will
burn. [The lecturer then ignited a portion of the gas collected,
which burnt with an explosion.] It is certainly something combus-
tible, but not combustible in the way that hydrogen is. Hydrogen
would not have given you that noise; but the color of that light,
when the thing did burn, was like that of hydrogen; it will, however,
burn without contact with the air. That is why I have chosen this
other form of apparatus, for the purpose of pointing out to you
what are the particular circumstances of this experiment. In place
of an open vessel, I have taken one that is closed (our battery is so
beautifully active that we are even boiling the mercury, and getting
all things right — not wrong, but vigorously right) ; and I am going
to show you that that gas, whatever it may be, can burn without
air, and in that respect differs from a candle, which can not burn
without the air. And our manner of doing this is as follows : I have
here a glass vessel (G) which is fitted with two platinum wires
(I K) through which I can apply electricity; and we can put the
vessel on the air pump and exhaust the air; and when we have taken
the air out we can bring it here and fasten it on to this jar (F),
and let into the vessel that gas which was formed by the action of
the voltaic battery upon the water, and which we have produced by
changing the water into it; for I may go as far as this, and say we
have really, by that experiment, changed the water into that gas.
We have not only altered its condition, but we have changed it
really and truly into that gaseous substance, and all the water is there
which was decomposed by the experiment. As I screw this vessel
(G H) on here (H), and make the tubes well connected, and when
I open the stop-cocks (H H H), if you watch the level of the water

CHEMICAL HISTORY OF A CANDLE I33

(in F), you will see that the gas will rise. I will now close the stop-
cocks, as I have drawn up as much as the vessel can hold, and being
safely conveyed into that chamber, I will pass into it an electric spark
from this Leyden jar (L), when the vessel, which is now quite clear
and bright, will become dim. There will be no sound, for the
vessel is strong enough to confine the explosion. [A spark was then
passed through the jar, when the explosive mixture was ignited.]
Did you see that brilliant light? If I again screw the vessel on to
the jar, and open these stop-cocks, you will see that the gas will rise
a second time. [The stop-cocks were then opened.] Those gases
[referring to the gases first collected in the jar, and which had just
been ignited by the electric spark] have disappeared, as you see;
their place is vacant, and fresh gas has gone in. Water has been
formed from them; and if we repeat our operation [repeating the
last experiment], I shall have another vacancy, as you will see by the
water rising. I always have an empty vessel after the explosion,
because the vapor or gas into which that water has been resolved by
the battery explodes under the influence of the spark, and changes
into water; and by-and-by you will see in this upper vessel some
drops of water trickling down the sides and collecting at the bottom.

We are here dealing with water entirely, without reference to the
atmosphere. The water of the candle had the atmosphere helping to
produce it; but in this way it can be produced independently of the
air. Water, therefore, ought to contain that other substance which
the candle takes from the air, and which, combining with the hydro-
gen, produces water.

Just now you saw that one end of this battery took hold of the
copper, extracting it from the vessel which contained the blue solu-
tion. It was effected by this wire; and surely we may say, if the
battery has such power with a metallic solution which we made and
unmade, may we not find that it is possible to split asunder the com-
ponent parts of the water, and put them into this place and that
place? Suppose I take the poles — the metallic ends of this battery —
and see what will happen with the water in this apparatus (Fig. 74),
where we have separated the two ends far apart. I place one here
(at A), and the other there (at B); and I have Httle shelves with
holes which I can put upon each pole, and so arrange them that

Fig. 74

134 FARADAY

whatever escapes from the two ends of the battery will appear as
separate gases; for you saw that the water did not become vaporous,
but gaseous. The wires are now in perfect and proper connection
with the vessel containing the water, and you see the bubbles rising;
let us collect these bubbles and see what they are. Here is a glass

cylinder (O); I fill it with water and
put it over one end (A) of the pile, and
I will take another (H), and put it
over the other end (B) of the pile. And
so now we have a double apparatus,
with both places delivering gas. Both
these jars will fill with gas. There they
go, that to the right (H) filling very
rapidly; the one to the left (O) filling
not so rapidly; and, though I have al-
lowed some bubbles to escape, yet still
the action is going on pretty regularly;
and were it not that one is rather smaller than the other, you would
see that I should have twice as much in this (H) as I have in that
(O). Both these gases are colorless; they stand over the water with-
out condensing; they are alike in all things — I mean in all apparent
things; and we have here an opportunity of examining these bodies
and ascertaining what they are. Their bulk is large, and we can
easily apply experiments to them. I will take this jar (H) first, and
will ask you to be prepared to recognize hydrogen.

Think of all its qualities — the light gas which stood well in in-
verted vessels, burning with a pale flame at the mouth of the jar,
and see whether this gas does not satisfy all these conditions. If it be
hydrogen it will remain here while I hold this jar inverted. [A light
was then applied, when the hydrogen burnt.] What is there now in
the other jar? You know that the two together made an explosive
mixture. But what can this be which we find as the other constituent
in water, and which must therefore be that substance which made
the hydrogen burn ? We know that the water we put into the vessel
consisted of the two things together. We find one of these is hydro-
gen: what must that other be which was in the water before the
experiment, and which we now have by itself.? I am about to put

CHEMICAL HISTORY OF A CANDLE I35

this lighted splinter of wood into the gas. The gas itself will not
burn, but it will make the splinter of wood burn. [The lecturer
ignited the end of the wood and introduced it into the jar of gas.]
See how it invigorates the combustion of the wood, and how it
makes it burn far better than the air would make it burn; and now
you see by itself that every other substance which is contained in the
water, and which, when the water was formed by the burning of the
candle, must have been taken from the atmosphere. What shall we
call it, A, B, or C? Let us call it O — call it "Oxygen"; it is a very
good, distinct-sounding name. This, then, is the oxygen which was
present in the water, forming so large a part of it.

We shall now begin to understand more clearly our experiments
and researches, because when we have examined these things once
or twice we shall soon see why a candle burns in the air. When we
have in this way analyzed the water — that is to say, separated or
electrolyzed its parts out of it, we get two volumes of hydrogen and
one of the body that burns it. And these two are represented to us
on the following diagram, with their weights also stated; and we
shall find that the oxygen is a very heavy body by comparison with
the hydrogen. It is the other element in water.

I
Hydrogen

8
Oxygen

9

Oxygen .
Hydrogen

Water

88.9
II. I

I had better, perhaps, tell you now how we get this oxygen abund-
antly, having shown you how we can separate it from the water.
Oxygen, as you will immediately imagine, exists in the atmosphere;
for how should the candle burn to produce water without it ? Such
a thing would be absolutely impossible, and chemically impossible,
without oxygen. Can we get it from the air ? Well, there are some
very complicated and difficult processes by which we can get it from
the air; but we have better processes. There is a substance called

136 FARADAY

the black oxide of manganese; it is a very black-looking mineral, but
very useful, and when made red-hot it gives out oxygen. Here is an
iron bottle which has had some of this substance put into it, and
there is a tube fixed to it (Fig. 75), and a iire ready made, and Mr.
Anderson will put that retort into the fire, for it is made of iron, and

can stand the heat. Here is a
salt called chlorate of potassa,
which is now made in large quantities for bleach-
ing, and chemical and medical uses, and for pyro-
technic and other purposes. I will take some and
mix it with some of the oxide of manganese (oxide
of copper, or oxide of iron would do as well) ; and
if I put these together in a retort, far less than a red
heat is sufficient to evolve this oxygen from the mix-
ture. I am not preparing to make much, because we
only want sufBcient for our experiments; only, as
■'''°' ^5 you will see immediately, if I use too small a charge,

the first portion of the gas will be mixed with the air already in the
retort, and I should be obliged to sacrifice the first portion of the gas
because it would be so much diluted with air; the first portion must
therefore be thrown away. You will find in this case that a common
spirit lamp is quite sufficient for me to get the oxygen, and so we shall
have two processes going on for its preparation. See how freely the
gas is coming over from that small portion of the mixture. We will
examine it and see what are its properties. Now in this way we are
producing, as you will observe, a gas just like the one we had in
the experiment with the battery, transparent, undissolved by water,
and presenting the ordinary visible properties of the atmosphere.
(As this first jar contains the air, together with the first portions of
the oxygen set free during the preparation, we wall carry it out of
the way, and be prepared to make our experiments in a regular,
dignified manner.) And inasmuch as that power of making wood,
wax, or other things burn, was so marked in the oxygen we obtained
by means of the voltaic battery from water, we may expect to find
the same property here. We will try it. You see there is the com-
bustion of a lighted taper in air, and here is its combustion in this
gas [lowering the taper into the jar]. See how brightly and how

CHEMICAL HISTORY OF A CANDLE I37

beautifully it burns! You can also see more than this: you will per-
ceive it is a heavy gas, while the hydrogen would go up like a
balloon, or even faster than a balloon, when not encumbered with
the weight of the envelope. You may easily see that although we
obtained from water twice as much in volume of the hydrogen as of
oxygen, it does not follow that we have twice as much in weight,
because one is heavy and the other a very light gas. We have means
of weighing gases or air; but, without stopping to explain that, let
me just tell you what their respective weights are. The weight _
of a pint of hydrogen is three-quarters of a grain; the weight of ^
the same quantity of oxygen is nearly twelve grains. This is a
very great difference. The weight of a cubic foot of hydrogen
is one-twelfth of an ounce; and the weight of a cubic foot of
oxygen is one ounce and a third. And so on we might come
to masses of matter which may be weighed in the balance, and
which we can take account of as to hundred-weights and as to
tons, as you will see almost immediately.

Now, as regards this very property of oxygen supporting
combustion, which we may compare to air, I will take a piece
of candle to show it you in a rough way — and the result will be
rough. There is our candle burning in the air: how will it burn
in oxygen ? I have here a jar of this gas, and I am about to put it over
the candle for you to compare the action of this gas with that of the
air. Why, look at it; it looks something like the light you saw at the
poles of the voltaic battery. Think how vigorous that action must be.
And yet, during all that action, nothing more is produced than what
is produced by the burning of the candle in air. We have the same
production of water, and the same phenomena exactly, when we use
this gas instead of air, as we have when the candle is burnt in air.

But now we have got a knowledge of this new substance, we can
look at it a little more distinctly, in order to satisfy ourselves that
we have got a good general understanding of this part of the product
of a candle. It is wonderful how great the supporting powers of this
substance are as regards combustion. For instance, here is a lamp
which, simple though it be, is the original, I may say, of a great
variety of lamps which are constructed for divers purposes — for
lighthouses, microscopic illuminations, and other uses; and if it was

Fig. 77

138 FARADAY

proposed to make it burn very brightly, you would say, "I£ a candle
burnt better in oxygen, will not a lamp do the same?" Why, it will
do so. Mr. Anderson will give me a tube coming from our oxygen
reservoir, and I am about to apply it to this flame, which I will
previously make burn badly on purpose. There comes the oxygen:
what a combustion that makes! But if I shut it off, what becomes
of the lamp? [The flow of oxygen was stopped, and the lamp
relapsed to its former dimness.] It is wonderful how,
by means of oxygen, we get combustion accelerated. But
it does not affect merely the combustion of hydrogen, or
carbon, or the candle, but it exalts all combustions of the
common kind. We will take one which relates to iron, for
instance, as you have already seen iron burn a little in the
atmosphere. Here is a jar of oxygen, and this is a piece
of iron wire; but if it were a bar as thick as my wrist,
it would burn the same. I first attach a little piece of
wood to the iron; I then set the wood on fire, and let them both
down together into the jar (Fig. 77). The wood is now alight, and
there it burns as wood should burn in oxygen; but it will soon com-
municate its combustion to the iron. The iron is now burning
brilliantly, and will continue so for a long time. As long as we
supply oxygen, so long can we carry on the combustion
of the iron, until the latter is consumed.

We will now put that on one side, and take some
other substance; but we must limit our experiments, for
we have not time to spare for all the illustrations you
would have a right to if we had more time. We will
take a piece of sulphur: you know how sulphur burns in
the air; well, we put it into the oxygen, and you will see
that whatever can burn in air can burn with a far
greater intensity in oxygen, leading you to think that
perhaps the atmosphere itself owes all its power of com-
bustion to this gas. The sulphur is now burning very quietly in the
oxygen; but you can not for a moment mistake the very high and
increased action which takes place when it is so burnt, instead of
being burnt merely in common air.

I am now about to show you the combustion of another sub-

FlG. 78

CHEMICAL HISTORY OF A CANDLE 1 39

Stance — phosphorus. I can do it better for you here than you can do
it at home. This is a very combustible substance; and if it be so
combustible in air, what might you expect it would be in oxygen?
I am about to show it to you not in its fullest intensity, for if I did
so we should almost blow the apparatus up; I may even now crack
the jar, though I do not want to break things carelessly. You see
how it burns in the air. But what a glorious light it gives out when
I introduce it into oxygen! [Introducing the lighted phosphorus
into the jar of oxygen.] There you see the solid particles going off
which cause that combustion to be so brilliantly luminous.

Thus far we have tested this power of oxygen, and the high com-
bustion it produces, by means of other substances. We must now,
for a little while longer, look at it as respects the hydrogen. You
know, when we allowed the oxygen and the hydrogen derived from
the water to mix and burn together, we had a little explosion. You
remember also that when I burnt the oxygen and the hydrogen in a
jet together, we got very little light, but great heat; I am now about
to set fire to oxygen and hydrogen mixed in the proportion in
which they occur in water. Here is a vessel containing one volume of
oxygen and two volumes of hydrogen. This mixture is exactly of
the same nature as the gas we just now obtained from the voltaic
battery; it would be far too much to burn at once; I have therefore
arranged to blow soap bubbles with it and burn those bubbles, that
we may see by a general experiment or two how this oxygen sup-
ports the combustion of the hydrogen. First of all, we will see
whether we can blow a bubble. Well, there goes the gas [causing
it to issue through a tobacco-pipe into some soapsuds]. Here I have
a bubble. I am receiving them on my hand, and you will perhaps
think I am acting oddly in this experiment, but it is to show you
that we must not always trust to noise and sounds, but rather to
real facts. [Exploding a bubble on the palm of his hand.] I am
afraid to fire a bubble from the end of the pipe, because the explo-
sion would pass up into the jar and blow it to pieces. This oxygen,
then, will unite with the hydrogen, as you see by the phenomena,
and hear by the sound, with the utmost readiness of action, and all
its powers are then taken up in its neutralization of the qualities of
the hydrogen.

140 FARADAY

So now I think you will perceive the whole history of water with
reference to oxygen and the air from what we have before said.
Why does a piece of potassium decompose water? Because it finds
oxygen in the water. What is set free when I put it in the water, as
I am about to do again? It sets free hydrogen, and the hydrogen
burns; but the potassium itself combines with oxygen; and this
piece of potassium, in taking the water apart — the water, you may
say, derived from the combustion of the candle — takes away the
oxygen which the candle took from the air, and so sets the hydrogen
free; and even if I take a piece of ice, and put a piece of potassium
upon it, the beautiful affinities by which the oxygen and the hydro-
gen are related are such that the ice will absolutely set fire to the
potassium. I show this to you to-day, in order to enlarge your ideas
of these things, and that you may see how greatly results are modified
by circumstances. There is the potassium on the ice, producing a sort
of volcanic action.

It will be my place when next we meet, having pointed out these
anomalous actions, to show you that none of these extra and strange
effects are met with by us — that none of these strange and injurious
actions take place when we are burning, not merely a candle, but
gas in our streets, or fuel in our fireplaces, so long as we confine
ourselves within the laws that Nature has made for our guidance.

LECTURE V

OXYGEN PRESENT IN THE AIR— NATURE OF THE
ATMOSPHERE— ITS PROPERTIES— OTHER PROD-
UCTS FROM THE CANDLE— CARBONIC
ACID— ITS PROPERTIES

WE have now seen that we can produce hydrogen and
oxygen from the water that we obtained from the candle.
Hydrogen, you know, comes from the candle, and oxygen,
you believe, comes from the air. But then you have a right to ask me,
"How is it that the air and the oxygen do not equally well bum'
the candle?" If you remember what happened when I put a jar of
oxygen over a piece of candle, you recollect there was a very dif-
ferent kind of combustion to that which took place in the air. Now,
why is this? It is a very important question, and one I shall endeavor
to make you understand; it relates most intimately to the nature of
the atmosphere, and is most important to us.

We have several tests for oxygen besides the mere burning of
bodies; you have seen a candle burnt in oxygen or in the air; you
have seen phosphorus burnt in the air or in oxygen, and you have
seen iron filings burnt in oxygen. But we have other tests besides
these, and I am about to refer to one or two of them for the purpose
of carrying your conviction and your experience farther. Here we
have a vessel of oxygen. I will show its presence to you: if I take a
litde spark and put it into that oxygen, you know by the experience
you gained the last time we met what will happen — ^if I put that
spark into the jar, it will tell you whether we have oxygen here or
not. Yes! We have proved it by combustion; and now here is an-
other test for oxygen, which is a very curious and useful one. I have
here two jars full of gas, with a plate between them to prevent their
mixing; I take the plate away, and the gases are creeping one into
the other. "What happens?" say you; "they together produce no
such combustion as was seen in the case of the candle." But see how

141

142 FARADAY

the presence of oxygen is told by its association with this other sub-
stance.('°) What a beautifully colored gas I have obtained in this
way, showing me the presence of the oxygen! In the same way we
can try this experiment by mixing common air with this test-gas.
Here is a jar containing air — such air as the candle would burn in —
and here is a jar or bottle containing the test-gas. I let them come
together over water, and you see the result: the contents of the test-
bottle are flowing into the jar of air, and you see I obtain exactly
the same kind of action as before, and that shows me that there is
oxygen in the air — the very same substance that has been already
obtained by us from the water produced by the candle. But then,
beyond that, how is it that the candle does not burn in air as well
as in oxygen ? We will come to that point at once. I have here two
jars; they are filled to the same height with gas, and the appearance
to the eye is alike in both, and I really do not know at present which
of these jars contains oxygen and which contains air, although I
know they have previously been filled with these gases. But here
is our test-gas, and I am going to work with the two jars, in order to
examine whether there is any difference between them in the quality
of reddening this gas. I am now going to turn this test-gas into one
of the jars, and observe what happens. There is reddening, you see;
there is, then, oxygen present. We will now test the other jar;
but you see this is not so distinctly red as the first, and, farther,
this curious thing happens: if I take these two gases and shake them
well together with water, we shall absorb the red gas; and then, if I
put in more of this test-gas and shake again, we shall absorb more;
and I can go on as long as there be any oxygen present to produce
that effect. If I let in air, it will not matter; but the moment I intro-
duce water, the red gas disappears; and I may go on in this way, put-
ting in more and more of the test-gas, until I come to something left
behind which will not redden any longer by the use of that particular
body that rendered the air and the oxygen red. Why is that? You
see in a moment it is because there is, besides oxygen, something else
present which is left behind. I will let a little more air into the jar,
and if it turns red you will know that some of that reddening gas

'^ The gas which is thus employed as a test for the presence of oxygen is the
binoxide of nitrogen, or nitrous oxide. It is a colorless gas, which, when brought in
contact with oxygen, unites with it, forming hyponitric acid, the red gas referred to.

CHEMICAL HISTORY OF A CANDLE 1 43

is still present, and that, consequently, it was not for the want of
this producing body that that air was left behind.

Now you will begin to understand what I am about to say. You
saw that when I burnt phosphorus in a jar, as the smoke produced
by the phosphorus and the oxygen of the air condensed, it left a good
deal of gas unburnt, just as this red gas left something untouched;
there was, in fact, this gas left behind, which the phosphorus can not
touch, which the reddening gas can not touch, and this something is
not oxygen, and yet is part of the atmosphere.

So that is one way of opening out air into the two things of which
it is composed — oxygen, which burns our candles, our phosphorus,
or any thing else, and this other substance — nitrogen — which will not
burn them. This other part of the air is by far the larger proportion,
and it is a very curious body when we come to examine it; it is re-
markably curious, and yet you say, perhaps, that it is very uninterest-
ing. It is uninteresting in some respects because of this, that it shows
no brilliant effects of combustion. If I test it with a taper as I do
oxygen and hydrogen, it does not burn like hydrogen, nor does it
make the taper burn like oxygen. Try it in any way I will, it does
neither the one thing nor the other; it will not take fire; it will not let
the taper burn; it puts out the combustion of every thing. There is
nothing that will burn in it in common circumstances. It has no
smell; it is not sour; it does not dissolve in water; it is neither an
acid nor an alkali; it is as indifferent to all our organs as it is possible
for a thing to be. And you might say, "It is nothing; it is not worth
chemical attention; what does it do in the air.?" Ah! then come our
beautiful and fine results shown us by an observant philosophy. Sup-
pose, in place of having nitrogen, or nitrogen and oxygen, we had
pure oxygen as our atmosphere; what would become of us? You
know very well that a piece of iron lit in a jar of oxygen goes on burn-
ing to the end. When you see a fire in an iron grate, imagine where
the grate would go to if the whole of the atmosphere were oxygen.
The grate would burn up more powerfully than the coals; for the
iron of the grate itself is even more combustible than the coals which
we burn in it. A fire put into the middle of a locomotive would be a
fire in a magazine of fuel, if the atmosphere were oxygen. The ni-
trogen lowers it down and makes it moderate and useful for us,

144 FARADAY

and then, with all that, it takes away with it the fumes that you have
seen produced from the candle, disperses them throughout the whole
of the atmosphere, and carries them away to places where they are
wanted to perform a great and glorious purpose of good to man, for
the sustenance of vegetation, and thus does a most wonderful work,
although you say, on examining it, "Why, it is a perfectly indifferent
thing." This nitrogen in its ordinary state is an inactive element; no
action short of the most intense electric force, and then in the most
infinitely small degree, can cause the nitrogen to combine directly
with the other element of the atmosphere, or with other things round
about it; it is a perfectly indifferent, and therefore to say, a safe sub-
stance.

But, before I take you to that result, I must tell you about the atmos-
phere itself. I have written on this diagram the composition of one
hundred parts of atmospheric air:

Bulk. Weight.

Oxygen 20 23.3

Nitrogen 80 77.7

It is a true analysis of the atmosphere so far as regards the quantity
of oxygen and the quantity of nitrogen present. By our analysis, we
find that 5 pints of the atmosphere contain only i pint of oxygen,
and 4 pints, or 4 parts, of nitrogen by bulk. That is our analysis
of the atmosphere. It requires all that quantity of nitrogen to reduce
the oxygen down, so as to be able to supply the candle properly with
fuel, so as to supply us with an atmosphere which our lungs can
healthily and safely breathe; for it is just as important to make the
oxygen right for us to breathe, as it is to make the atmosphere right
for the burning of the fire and the candle.

But now for this atmosphere. First of all, let me tell you the weight
of these gases. A pint of nitrogen weighs 10 4-10 grains, or a cubic
foot weighs i 1-6 ounces. That is the weight of the nitrogen. The
oxygen is heavier: a pint of it weighs 11 9-10 grs., and a cubic foot
weighs I 3-4 oz. A pint of air weighs about 10 7-10 grs., and a cubic
foot I 1-5 oz.

You have asked me several times, and I am very glad you have,
"How do you weigh gases.?" I will show you: it is very simple, and

CHEMICAL HISTORY OF A CANDLE 1 45

easily done. Here is a balance, and here a copper bottle, made as light
as we can consistent with due strength, turned very nicely in the
lathe, and made perfectly air-tight, with a stop-cock, which we can
open and shut, which at present is open, and, therefore, allows the
bottle to be full of air. I have here a nicely-adjusted bal-
ance in which I think the bottle, in its present condition,
will be balanced by the weight on the other side. And
here is a pump by which we can force the air into this
bottle, and with it we will force in a certain number of
volumes of air as measured by the pump. [Twenty meas-
ures were pumped in.] We will shut that in and put it
in the balance. See how it sinks; it is much heavier than
it was. By what? By the air that we have forced into it
by the pump. There is not a greater bull^^ of air, but there
is the same bulk of heavier air, because we have forced
in air upon it. And that you may have a fair notion in
your mind as to how much this air measures, here is a
jar full of water. We will open that copper vessel into
this jar, and let the air return to its former -state. All I
have to do now is to screw them tightly together, and to
turn the taps, when there, you see, is the bulk of the
twenty pumps of air which I forced into the bottle; and p,^ j^
to make sure that we have been quite correct in what we
have been doing, we will take the bottle again to the balance, and, if it
is now counterpoised by the original weight, we shall be quite sure we
have made our experiment correctly. It is balanced; so, you see, we
can find out the weight of the extra volumes of air forced in in that
way, and by that means we are able to ascertain that a cubic foot of
air weighs i 1-5 oz. But that small experiment will by no means
convey to your mind the whole literal truth of this matter. It is won-
derful how it accumulates when you come to larger volumes. This
bulk of air [a cubic foot] weighs i 1-5 oz. What do you think of the
contents of that box above there which I have had made for the pur-
pose? The air which is within that box weighs one pound — a full
pound; and I have calculated the weight of the air in this room: you
would hardly imagine it, but it is above a ton. So rapidly do the
weights rise up, and so important is the presence of the atmosphere,

146 FARADAY

and of the oxygen and the nitrogen in it, and the use it performs in
conveying things to and fro from place to place, and carrying bad
vapors to places where they will do good instead of harm.

Having given you that little illustration with respect to the weight
of the air, let me show you certain consequences of it. You have a
right to them, because you would not un-
derstand so much without it. Do you
remember this kind of experiment? Have
you ever seen it? Suppose I take a pump
somewhat similar to the one I had a little
while ago to force air into the bottle, and
suppose I place it in such a manner that
by certain arrangements I can apply my
hand to it. My hand moves about in the
air so easily that it seems to feel nothing,
and I can hardly get velocity enough by
any motion of my own in the atmosphere
to make sure that there is much resistance
to it. But when I put my hand here [on
the air-pump receiver, which was afterward
exhausted] you see what happens. Why is
my hand fastened to this place, and why
am I able to pull this pump about? And
see! how is it that I can hardly get my hand
away? Why is this? It is the weight of the air— the weight of the
air that is above. I have another experiment here, which I think will
explain to you more about it. When the air is pumped from under-
neath the bladder which is stretched over this glass, you will see
the effect in another shape: the top is quite flat at present, but I will
make a very little motion with the pump, and now look at it; see
how it has gone down, see how it is bent in; you will see the bladder
go in more and more, until, at last, I expect it will be driven in and
broken by the force of the atmosphere pressing upon it. [The bladder
at last broke with a loud report.] Now that was done entirely by the
weight of the air pressing on it, and you can easily understand how
that is. The particles that are piled up in the atmosphere stand upon
each other, as these five cubes do; you can easily conceive that four of

Fig. 80

CHEMICAL HISTORY OF A CANDLE 1 47

these five cubes are resting upon the bottom one, and if I take that
away the others will all sink down. So it is with the atmosphere; the
air that is above is sustained by the air that is beneath, and when the
air is pumped away from beneath them, the change occurs which you

Fig. 81

saw when I placed my hand on the air-pump, and which you saw in
the case of the bladder, and which you shall see better here. I have tied
over this jar a piece of sheet India-rubber, and I am now about to take
away the air from the inside of the jar; and if you will watch the
India-rubber — which acts as a partition between the air below and the
air above — you will see, when I pump, how the pressure shows itself.
See where it is going to: I can actually put
my hand into the jar; and yet this result is
only caused by the great and powerful action
of the air above. How beautifully it shows
this curious circumstance!

Here is something that you can have a pull
at when I have finished to-day. It is a little
apparatus of two hollow brass hemispheres,
closely fitted together, and having connected
with it a pipe and a cock, through which we
can exhaust the air from the inside; and
although the two halves are so easily taken
apart, while the air is left within, yet you will
see, when we exhaust it by-and-by, no power of any two of you will
be able to pull them apart. Every square inch of surface that is
contained in the area of that vessel sustains fifteen pounds by weight,
or nearly so, when the air is taken out, and you may try your strength

Fig. 82

148 FARADAY

presently in seeing whether you can overcome that pressure of the
atmosphere.

Here is another very pretty thing — the boys' sucker, only refined
by the philosopher. We young ones have a perfect right to take toys,
and make them into philosophy, inasmuch as nowadays we are turn-
ing philosophy into toys. Here is a sucker, only it is made of India-
rubber. If I clap it upon the table, you see at once it holds. Why does
it hold? I can slip it about, and yet if I try to pull it up, it seems as
if it would pull the table with it. I can easily make it slip about
from place to place, but only when I bring it to the edge of the table
can I get it off. It is only kept down by the pressure of the at-
mosphere above; we have a couple of them, and if you take these
two and press them together, you will see how firmly they stick.
And, indeed, we may use them as they are proposed to be used, to
stick against windows, or against walls, where they will adhere for an
evening, and serve to hang any thing on that you want. I think, how-
ever, that you boys ought to be shown experiments that you can
make at home; and so here is a very pretty experiment in illustration
of the pressure of the atmosphere. Here is a tumbler of water.
Suppose I were to ask you to turn that tumbler upside down so that
the water should not fall out, and yet not be kept in by your hand,
but merely by using the pressure of the atmosphere; could you do
that? Take a wine-glass, either quite full or half full of water, and
put a flat card on the top; turn it upside down, and then see what
becomes of the card and of the water. The air can not get in because
the water, by its capillary attraction round the edge, keeps it out.

I think this will give you a correct notion of what you may call the
materiality of the air; and when I tell you that that box holds a
pound of it, and this room more than a ton, you will begin to think
that air is something very serious. I will make another experiment to
convince you of this positive resistance. There is that beautiful ex-
periment of the popgun, made so well and so easily, you know, out
of a quill, or a tube, or any thing of that kind, — where we take a slice
of potato, for instance, or an apple, and take the tube and cut out
a pellet, as I have now done, and push it to one end. I have made
that end tight; and now I take another piece and put it in: it will
confine the air that is within the tube perfectly and completely for

CHEMICAL HISTORY OF A CANDLE 1 49

our purpose; and I shall now find it absolutely impossible by any
force of mine to drive that little pellet close up to the other. It
can not be done; I may press the air to a certain extent, but if I
go on pressing, long before it comes to the second, the confined air
will drive the front one out with a force something like that of
gunpowder; for gunpowder is in part dependent upon the same
action that you see here exemplified.

I saw the other day an experiment which pleased me much, as I
thought it would serve our purpose here. (I ought to have held my
tongue for four or five minutes before beginning this experiment,
because it depends upon my lungs for success.) By the proper ap-
plication of air, I expect to be able to drive this egg out of one cup
into the other by the force of my breath; but if I fail it is in a good
cause, and I do not promise success, because I have been talking more
than I ought to do to make the experiment succeed.

[The lecturer here tried the experiment, and succeeded in blowing
the egg from one egg cup to the other.]

You see that the air which I blow goes downward between the egg
and the cup, and makes a blast under the egg, and is thus able to lift
a heavy thing; for a full egg is a very heavy thing for air to lift.
If you want to make the experiment, you had better boil the egg
quite hard first, and then you may very safely try to blow it from
one cup to the other, with a little care.

I have now kept you long enough upon this property of the weight
of the air, but there is another thing I should like to mention. You
saw the way in which, in this popgun, I was able to drive the second
piece of potato half or two-thirds of an inch before the first piece
started, by virtue of the elasticity of the air, just as I pressed into the
copper bottle the particles of air by means of the pump. Now this
depends upon a wonderful property in the air, namely, its elasticity,
and I should like to give you a good illustration of this. If I take any
thing that confines the air properly, as this membrane, which also
is able to contract and expand so as to give us a measure of the
elasticity of the air, and confine in this bladder a certain portion of
air; and then, if we take the atmosphere off from the outside of it,
just as in these cases we put the pressure on — ^if we take the pressure
off, you will see how it will then go on expanding and expanding,

150 FARADAY

larger and larger, until it will fill the whole of this bell jar, showing
you that wonderful property of the air, its elasticity, its compressi-
bility, and expansibility, to an exceedingly large extent, and which is
very essential for the purposes and services it performs in the
economy of creation.

We will now turn to another very important part of our subject,
remembering that we have examined the candle in its burning, and
have found that it gives rise to various products. We have the prod-
ucts, you know, of soot, of water, and of something else, which you
have not yet examined. We have collected the water, but have
allowed the other things to go into the air. Let us now examine
some of these other products.

Here is an experiment which I think will help you in part in this
way. We will put our candle there, and place over it a chimney.

thus. I think my candle will go on burning, because the air passage
is open at the bottom and the top. In the first place, you see the
moisture appearing — that you know about. It is water produced
from the candle by the action of the air upon its hydrogen. But,
besides that, something is going out at the top : it is not moisture — it is
not water — it is not condensible; and yet, after all, it has very singu-
lar properties. You will find that the air coming out of the top of

CHEMICAL HISTORY OF A CANDLE I5I

our chimney is nearly sufficient to blow the light out I am holding
to it; and if I put the light fairly opposed to the current, it will blow
it quite out. You will say, that is as it should be, and I am supposing
that you think it ought to do so, because the nitrogen does not sup-
port combustion, and ought to put the candle out, since the candle
will not burn in nitrogen. But is there nothing else there than nitro-
gen? I must now anticipate — ^that is to say, I must use my own
knowledge to supply you with the means that we adopt for the pur-
pose of ascertaining these things, and examining such gases as these.
I will take an empty bottle — here is one — and if I hold it over this
chimney, I shall get the combustion of the candle below sending its
results into the bottle above; and we shall soon find that this bottle
contains, not merely an air that is bad as regards the combustion of
a taper put into it, but having other properties.

Let me take a little quick-lime and pour some common water on
to it — the commonest water will do. I will stir it a moment, then
pour it upon a piece of filtering paper in a funnel, and we shall
very quickly have a clear water proceeding to the bottle below, as I
have here. I have plenty of this water in another bottle, but never-
theless I should like to use the lime-water that was prepared before
you, so that you may see what its uses are. If I take some of this
beautiful clear lime-water, and pour it into this jar which has col-
lected the air from the candle, you will see a change coming about.
Do you see that the water has become quite milky? Observe, that
will not happen wdth air merely. Here is a bottle filled with air; and
if I put a little lime-water into it, neither the oxygen nor the nitro-
gen, nor any thing else that is in that quantity of air, will make any
change in the lime-water. It remains perfectly clear, and no shaking
of that quantity of lime-water with that quantity of air in its com-
mon state will cause any change; but if I take this bottle with the
lime-water and hold it so as to get the general products of the
candle in contact with it, in a very short time we shall have it milky.
There is the chalk, consisting of the lime which we used in making
the lime-water, combined with something that came from the candle
— that other product which we are in search of, and which I want
to tell you about today. This is a substance made visible to us by its
action, which is not the action of the lime-water either upon the

152 . FARADAY

oxygen or upon the nitrogen, nor upon the water itself, but it is some-
thing new to us from the candle. And then we find this white pow-
der, produced by the lime-water and the vapor from the candle, ap-
pears to us very much like whitening or chalk, and when examined
it does prove to be exactly the same substance as whitening or chalk.
So we are led, or have been led, to observe upon the various circum-
stances of this experiment, and to trace this production of chalk
to its various causes, to give us the true knowledge of the nature
of this combustion of the candle — to find that this substance issuing
from the candle is exactly the same as that substance which would
issue from a retort if I were to put some chalk into it with a little
moisture and make it red-hot; you would then find that exactly the
same substance would issue from it as from the candle.

But we have a better means of getting this substance, and in
greater quantity, so as to ascertain what its general characters are.
We find this substance in very great abundance in a multitude of
cases where you would least expect it. All limestones contain a great
deal of this gas which issues from the candle, and which we call
carbonic acid. All chalks, all shells, all corals, contain a great quan-
tity of this curious air. We find it fixed in these stones, for which
reason Dr. Black called it "fixed air" — finding it in these fixed things
like marble and chalk — he called it fixed air because it lost its
quality of air, and assumed the condition of a solid body. We can
easily get this air from marble. Here is a jar containing a little
muriatic acid, and here is a taper which, if I put it into that jar, will
show only the presence of common air. There is, you see, pure air
down to the bottom; the jar is full of it. Here is a substance — marble
("), a very beautiful and superior marble — and if I put these pieces
of marble into the jar, a great boiling apparently goes on. That,
however, is not steam; it is a gas that is rising up; and if I now
search the jar by a candle, I shall have exactly the same effect pro-
duced upon the taper as I had from the air which issued from the
end of the chimney over the burning candle. It is exactly the same
action, and caused by the very same substance that issued from the
candle; and in this way we can get carbonic acid in great abundance

^''Marble is a compound of carbonic acid and lime. The muriatic acid, being the
stronger of the two, takes the place of the carbonic acid, which escapes as a gas,
the residue forming muriate of lime or chloride of calcium.

CHEMICAL HISTORY OF A CANDLE I53

— we have already nearly filled the jar. We also find that this gas is
not merely contained in marble. Here is a vessel in which I have
put some common whitening — chalk which has been washed in
water and deprived of its coarser particles, and so supplied to the
plasterer as whitening — here is a large jar containing this whitening
and water; and I have here some strong sulphuric acid, which is the
acid you might have to use if you were to make these experiments
(only, in using this acid with limestone, the body that is produced
is an insoluble substance, whereas the muriatic acid produces a
soluble substance that does not so much thicken the water). And
you will seek out a reason why I take this kind of apparatus for
the purpose of showing this experiment. I do it because you may
repeat in a small way what I am about to do in a large one. You
will have here just the same kind of action; and I am evolving in
this large jar carbonic acid exactly the same in its nature and
properties as the gas which we obtained from the combustion of the
candle in the atmosphere. And, no matter how different the two
methods by which we prepare this carbonic acid, you will see, when
we get to the end of our subject, that it. is all exactly the same,
whether prepared in the one way or in the other.

We will now proceed to the next experiment with regard to this
gas. What is its nature ? Here is one of the vessels full, and we will
try it as we have done so many other gases — by combustion. You
see it is not combustible, nor does it support combustion. Neither, as
we know, does it dissolve much in water, because we collect it over
water very easily. Then you know that it has an effect, and becomes
white in contact with lime-water; and when it does become white
in that way, it becomes one of the constituents to make carbonate
of lime or limestone.

The next thing I must show you is that it really does dissolve a
little in water, and therefore that it is unlike oxygen and hydrogen
in that respect. I have here an apparatus by which we can produce
this solution. In the lower part of this apparatus is marble and acid,
and in the upper part cold water. The valves are so arranged that
the gas can get from one to the other. I will set it in action now,
and you can see the gas bubbling up through the water, as it has been
doing all night long, and by this time we shall find that we have

Cubic Foot.

^\

oz.

li

,J

i^

»f

I*

ll»5

1>

154 FARADAY

this substance dissolved in the water. If I take a glass and draw off
some of the water, I find that it tastes a little acid to the mouth; it
is impregnated with carbonic acid; and if I now apply a little lime-
water to it, that will give us a test of its presence. This water will
make the lime-water turbid and white, which is proof of the pres-
ence of carbonic acid.

Then it is a very weighty gas; it is heavier than the atmosphere.
I have put their respective weights at the lower part of this table,
along with, for comparison, the weights of the other gases we have
been examining:

Pint.

Hydrogen | grs.

Oxygen iit'it „

Nitrogen loA „

Air........ lofff „

Carbonic acid 16 J „

A pint of it weighs 16 1-3 grains, and a cubic foot weighs 1 9-10
ounce, almost two ounces. You can see by many experiments that
this is a heavy gas. Suppose I take a glass containing nothing else
but air, and from this vessel containing the carbonic acid I attempt
to pour a little of this gas into that glass —
I wonder whether any has gone in or not. I
can not tell by the appearance, but I can in
this way [introducing the taper]. Yes, there
it is, you see; and if I were to examine it by
lime-water, I should find it by that test also.
I will take this little bucket, and put it down
into the well of carbonic acid — indeed, we
too often have real wells of carbonic acid —
and now, if there is any carbonic acid, I must
have got to it by this time, and it will be in
this bucket, which we will examine with a
taper. There it is, you see; it is full of
carbonic acid.

There is another experiment by which I will show you its weight.
I have here a jar suspended at one end of a balance — it is now
equipoised; but when I pour this carbonic acid into the jar on the
one side which now contains air, you will see it sink down at once

Fig. 84

CHEMICAL HISTORY OF A CANDLE 1 55

because of the carbonic acid that I pour into it. And now, if I exam-
ine this jar with the Hghted taper, I shall find that the carbonic
acid has fallen into it, and it no longer has any power of supporting
the combustion. If I blow a soap bubble, which of course will be
filled with air, and let it fall into this jar of carbonic acid, it will
float. But I shall first of all take one of these little balloons filled with
air. I am not quite sure where the carbonic acid is; we will just try

Fig. 85

the depth, and see whereabouts is its level. There, you see, we
have this bladder floating on the carbonic acid; and if I evolve some
more of the carbonic acid, the bladder will be lifted up higher.
There it goes; the jar is nearly full, and now I will see whether I
can blow a soap bubble on that and float it in the same way. [The
lecturer here blew a soap bubble and allowed it to fall into the
jar of carbonic acid, when it floated in it midway.] It is floating,
as the balloon floated, by virtue of the greater weight of the carbonic
acid than of the air. And now, having so far given you the history
of the carbonic acid, as to its sources in the candle, as to its physical
properties and weight, when we next meet I shall show you of what
it is composed, and where it gets its elements from.

LECTURE VI

CARBON OR CHARCOAL— COAL GAS— RESPIRATION

AND ITS ANALOGY TO THE BURNING OF

A CANDLE— CONCLUSION

A LADY who honors me by her presence at these lectures has
conferred a still farther obligation by sending me these two
L. candles, which are from Japan, and, I presume, are made of
that substance to which I referred in a former lecture. You see that
they are even far more highly ornamented than the French candles,
and, I suppose, are candles of luxury, judging from their appearance.
They have a remarkable peculiarity about them, — namely, a hollow
wick, — that beautiful peculiarity which Argand introduced into the
lamp and made so valuable. To those who receive such presents from
the East, I may just say that this and such hke materials gradually
undergo a change which gives them on the surface a dull and dead
appearance; but they may easily be restored to their original beauty
if the surface be rubbed with a clean cloth or silk handkerchief, so
as to polish the little rugosity or roughness: this will restore the
beauty of the colors. I have so rubbed one of these candles, and you
see the difference between it and the other which has not been
polished, but which may be restored by the same process. Observe,
also, that these moulded candles from Japan are made more conical
than the moulded candles in this part of the world.

I told you, when we last met, a good deal about carbonic acid. We
found by the lime-water test that when the vapor from the top of
the candle or lamp was received into bottles and tested by this solu-
tion of lime-water (the composition of which I explained to you, and
which you can make for yourselves), we had that white opacity
which was in fact calcareous matter, like shells and corals, and many
of the rocks and minerals in the earth. But I have not yet told you
fully and clearly the chemical history of this substance, carbonic
acid, as we have it from the candle, and I must now resume that

156

CHEMICAL HISTORY OF A CANDLE 1 57

subject. We have seen the products, and the nature of them, as they
issue from the candle. We have traced the water to its elements, and
now we have to see where are the elements of the carbonic acid
supplied by the candle. A few experiments will show this. You
remember that when a candle burns badly it produces smoke; but
if it is burning well there is no smoke. And you know that the
brightness of the candle is due to this smoke, which becomes ignited.
Here is an experiment to prove this: so long as the smoke remains in
the flame of the candle and becomes ignited, it gives a beautiful
light, and never appears to us in the form of black particles. I will
light some fuel which is extravagant in its burning. This will serve
our purpose — a little turpentine on a sponge. You see the smoke
rising from it, and floating into the air in large quantities; and re-
member now, the carbonic acid that we have from the candle is
from such smoke as that. To make that evident to you, I will intro-
duce this turpentine burning on the sponge into a flask where I have
plenty of oxygen, the rich part of the atmosphere, and you now see
that the smoke is all consumed. This is the first part of our experi-
ment; and now, what follows? The carbon which you saw flying
off from the turpentine flame in the air is now entirely burned in
this oxygen, and we shall find that it will, by this rough and tem-
porary experiment, give us exactly the same conclusion and result
as we had from the combustion of the candle. The reason why I
make the experiment in this manner is solely that I may cause
the steps of our demonstration to be so simple that you can never
for a moment lose the train of reasoning, if you only pay attention.
All the carbon which is burned in oxygen, or air, comes out as
carbonic acid, while those particles which are not so burned show
you the second substance in the carbonic acid, — namely, the carbon —
that body which made the flame so bright while there was plenty of
air, but which was thrown off in excess when there was not oxygen
enough to burn it.

I have also to show you a little more distinctly the history of car-
bon and oxygen in their union to make carbonic acid. You are now
better able to understand this than before, and I have prepared three
or four experiments by way of illustration. This jar is filled with
oxygen, and here is some carbon which has been placed in a crucible

158 FARADAY

for the purpose of being made red-hot. I keep my jar dry, and ven-
ture to give you a result imperfect in some degree, in order that I
may make the experiment brighter. I am about to put the oxygen
and the carbon together. That this is carbon (common charcoal
pulverized) you will see by the way in which it burns in the air
[letting some of the red-hot charcoal fall out of the crucible]. I am
now about to burn it in oxygen gas, and look at the difference. It
may appear to you at a distance as if it were burning with a flame;
but it is not so. Every little piece of charcoal is burning as a spark,
and while it so burns it is producing carbonic acid. I specially want
these two or three experiments to point out what I shall dwell upon
more distinctly by-and-by — that carbon burns in this way, and not
as a flame.

Instead of taking many particles of carbon to burn, I will take
a rather large piece, which will enable you to see the form and size,
and to trace the effects very decidedly. Here is the jar of oxygen,
and here is the piece of charcoal, to which I have fastened a little
piece of wood, which I can set fire to, and so commence the com-
bustion, which I could not conveniently do without. You now see
the charcoal burning, but not as a flame (or if there be a flame it is
the smallest possible one, which I know the cause of, namely, the
formation of a little carbonic oxide close upon the surface of the
carbon). It goes on burning, you see, slowly producing carbonic
acid by the union of this carbon or charcoal (they are equivalent
terms) with the oxygen. I have here another piece of charcoal, a
piece of bark, which has the quality of being blown to pieces — ex-
ploding — as it burns. By the effect of the heat we shall reduce the
lump of carbon into particles that will fly off; still every particle,
equally with the whole mass, burns in this peculiar way — it burns as
a coal and not like a flame. You observe a multitude of little com-
bustions going on, but no flame. I do not know a finer experiment
than this to show that carbon burns with a spark.

Here, then, is carbonic acid formed from its elements. It is pro-
duced at once; and if we examined it by lime-water you will see
that we have the same substance which I have previously described
to you. By putting together 6 parts of carbon by weight (whether
it comes from the flame of a candle or from powdered charcoal)

CHEMICAL HISTORY OF A CANDLE 1 59

and 16 parts of oxygen by weight, we have 22 parts of carbonic acid;
and, as we saw last time, the 22 parts of carbonic acid combined with
28 parts of Hme, produced common carbonate of lime. If you were
to examine an oyster-shell and weigh the component parts, you
would find that every 50 parts would give 6 of carbon and 16
of oxygen combined with 28 of lime. However, I do not want to
trouble you with these minutiae; it is only the general philosophy
of the matter that we can now go into. See how finely the carbon
is dissolving away [pointing to the lump of charcoal burning quietly
in the jar of oxygen]. You may say that the charcoal is actually dis-
solving in the air round about; and if that were perfectly pure char-
coal, which we can easily prepare, there would be no residue what-
ever. When we have a perfectly cleansed and purified piece of car-
bon, there is no ash left. The carbon burns as a solid dense body,
that heat alone can not change as to its solidity, and yet it passes
away into vapor that never condenses into solid or liquid under
ordinary circumstances; and what is more curious still is the fact that
the oxygen does not change in its bulk by the solution of the carbon
in it. Just as the bulk is at first, so it is at last, only it has become
carbonic acid.

There is another experiment which I must give you before you
are fully acquainted with the general nature of carbonic acid. Being
a compound body, consisting of carbon and oxygen, carbonic acid
is a body that we ought to be able to take asunder. And so we can.
As we did with water, so we can with carbonic acid — take the two
parts asunder. The simplest and quickest way is to act upon the
carbonic acid by a substance that can attract the oxygen from it, and
leave the carbon behind. You recollect that I took potassium and
put it upon water or ice, and you saw that it could take the oxygen
from the hydrogen. Now, suppose we do something of the same
kind here with this carbonic acid. You know carbonic acid to be
a heavy gas. I will not test it with lime-water, as that will interfere
with our subsequent experiments, but I think the heaviness of the
gas and the power of extinguishing flame will be sufficient for our
purpose. I introduce a flame into the gas, and you will see whether it
will be put out. You see the light is extinguished. Indeed, the gas
may, perhaps, put out phosphorus, which you know has a pretty

l6o FARADAY

Strong combdstion. Here is a piece of phosphorus heated to a high
degree. I introduce it into gas, and you observe the Hght is put out,
but it will take fire again in the air, because there it re-enters into
combustion. Now let me take a piece of potassium, a substance
which even at common temperatures can act upon carbonic acid,
though not sufficiently for our present purpose, because it soon gets
covered with a protecting coat; but if we warm it up to the burning
point in air, as we have a fair right to do, and as we have done with
phosphorus, you will see that it can burn in carbonic acid; and if it
burns, it will burn by taking oxygen, so that you will see what is left
behind. I am going, then, to burn this potassium in the carbonic
acid, as a proof of the existence of oxygen in the carbonic acid. [In
the preliminary process of heating the potassium exploded.] Some-
times we get an awkward piece of potassium that explodes, or some-
thing like it, when it burns. I will take another piece, and now that
it is heated I introduce it into the jar, and you perceive that it burns
in the carbonic acid — not so well as in the air, because the carbonic
acid contains the oxygen combined; but it does burn, and takes away
the oxygen. If I now put this potassium into water, I find that
besides the potash formed (which you need not trouble about) there
is a quantity of carbon produced. I have here made the experiment
in a very rough way, but I assure you that if I were to make it
carefully, devoting a day to it instead of five minutes, we should get
all the proper amount of charcoal left in the spoon, or in the place
where the potassium was burned, so that there could be no doubt
as to the result. Here, then, is the carbon obtained from the car-
bonic acid, as a common black substance; so that you have the entire
proof of the nature of carbonic acid as consisting of carbon and oxy-
gen. And now I may tell you, that whenever carbon burns under
common circumstances it produces carbonic acid.

Suppose I take this piece of wood, and put it into a bottle with
lime-water. I might shake that lime-water up with wood and the
atmosphere as long as I pleased, it would still remain clear as
you see it; but suppose I burn the piece of wood in the air of that
bottle. You, of course, know I get water. Do I get carbonic acid?
[The experiment was performed.] There it is, you see — that is to
say, the carbonate of lime, which results from carbonic acid, and

CHEMICAL HISTORY OF A CANDLE l6l

that carbonic acid must be formed from the carbon which comes
from the wood, from the candle, or any other thing. Indeed, you
have yourselves frequently tried a very pretty experiment, by which
you may see the carbon in wood. If you take a piece of wood, and
partly burn it, and then blow it out, you have carbon left. There
are things that do not show carbon in this way. A candle does not
so show it, but it contains carbon. Here also is a jar of coal gas,
which produces carbonic acid abundantly; you do not see the carbon,
but we can soon show it to you. I will light it, and as long as there
is any gas in the cylinder it will go on burning. You see no carbon,
but you see a flame, and because that is bright it will lead you to guess
that there is carbon in the flame. But I will show it to you by
another process. I have some of the same gas in another vessel, mixed
with a body that will burn the hydrogen of the gas, but will not burn
the carbon. I will light them with a burning taper, and you perceive
the hydrogen is consumed, but not the carbon, which is left behind as
a dense black smoke. I hope that by these three or four experiments
you will learn to see when carbon is present, and understand what
are the products of combustion when gas or other bodies are thor-
oughly burned in the air.

Before we leave the subject of carbon, let us make a few experi-
ments and remarks upon its wonderful condition as respects ordinary
combustion. I have shown you that the carbon, in burning, burns
only as a solid body, and yet you perceive that, after it is burned,
it ceases to be a solid. There are very few fuels that act like this. It
is, in fact, only that great source of fuel, the carbonaceous series, the
coals, charcoals, and woods, that can do it. I do not know that there
is any other elementary substance besides carbon that burns with
these conditions; and if it had not been so, what would happen to us.''
Suppose all fuel had been like iron, which, when it burns, burns into
a solid substance. We could not then have such a combustion as you
have in this fireplace. Here also is another kind of fuel which burns
very well — as well as, if not better, than carbon — so well, indeed, as
to take fire of itself when it is in the air, as you see [breaking a tube
full of lead pyrophorus]. This substance is lead, and you see how
wonderfully combustible it is. It is very much divided, and is like a
heap of coals in the fireplace; the air can get to its surface and inside.

I 62 FARADAY

and so it burns. But why does it not burn in that way now, when it
is lying in a mass ? [emptying the contents of the tube in a heap on
to a plate of iron.] Simply because the air can not get to it. Though
it can produce a great heat, the great heat which we want in our
furnaces and under our boilers, still that which is produced can not
get away from the portion which remains unburned underneath, and
that portion, therefore, is prevented from coming in contact with
the atmosphere, and can not be consumed. How different is that
from carbon! Carbon burns just in the same way as this lead does,
and so gives an intense fire in the furnace, or wherever you choose
to burn it; but then the body produced by its combustion passes
away, and the remaining carbon is left clear. I showed you how
carbon went on dissolving in the oxygen, leaving no ash, whereas
here [pointing to the heap of pyrophorus] we have actually more ash
than fuel, for it is heavier by the amount of the oxygen which has
united with it. Thus you see the difference between carbon and lead
or iron — if we choose iron, which gives so wonderful a result in our
applications of this fuel, either as light or heat. If, when the carbon
burnt, the product went off as a solid body, you would have had the
room filled with an opaque substance, as in the case of the phos-
phorus; but when carbon burns, every thing passes up into the
atmosphere. It is in a fixed, almost unchangeable condition before
the combustion; but afterward it is in the form of gas, which it is
very difficult (though we have succeeded) to produce in a solid or
liquid state.

Now I must take you to a very interesting part of our subject —
to the relation between the combustion of a candle and that living
kind of combustion which goes on within us. In every one of us
there is a living process of combustion going on very similar to that
of a candle, and I must try to make that plain to you. For it is not
merely true in a poetical sense — ^the relation of the life of man to a
taper; and if you will follow, I think I can make this clear. In order
to make the relation very plain, I have devised a little apparatus
which we can soon build up before you. Here is a board, and a
groove cut in it, and I can close the groove at the top part by a
little cover; I can then continue the groove as a channel by a glass
tube at each end, there being a free passage through the whole.

CHEMICAL HISTORY OF A CANDLE 1 63

Suppose I take a taper or candle (we can now be liberal in our use
of the word "candle," since we understand what it means), and place
it in one of the tubes; it will go on, you see, burning very well.
You observe that the air which feeds the flame passes down the tube
at one end, then goes along the horizontal tube, and ascends the tube
at the other end in which the taper is placed. If I stop the aperture
through which the air enters, I stop combustion, as you perceive.
I stop the supply of air, and consequently the candle goes out. But
now what will you think of this fact? In a former experiment I

Fig. 86

showed you the air going from one burning candle to a second
candle. If I took the air proceeding from another candle, and sent
it down by a complicated arrangement into this tube, I should put
this burning candle out. But what will you say when I tell you that
my breath will put out that candle ? I do not mean by blowing at all,
but simply that the nature of my breath is such that a candle can
not burn in it. I will now hold my mouth over the aperture, and
without blowing the flame in any way, let no air enter the tube but
what comes from my mouth. You see the result. I did not blow
the candle out. I merely let the air which I expired pass into the
aperture, and the result was that the light went out for want of
oxygen, and for no other reason. Something or other — namely, my
lungs — had taken away the oxygen from the air and there was no
more to supply the combustion of the candle. It is, I think, very
pretty to see the time it takes before the bad air which I throw into
this part of the apparatus has reached the candle. The candle at first
goes on burning, but so soon as the air has had time to reach it, it

164 FARADAY

goes out. And now I will show you another experiment, because this
is an important part of our philosophy. Here is a jar which contains
fresh air, as you can see by the circumstance of a candle or gas light
burning it. I make it close for a little time, and by means of a pipe
I get my mouth over it so that I can inhale the air. By putting it
over water, in the way that you see, I am able to draw up this air
(supposing the cork to be quite tight), take it
into my lungs, and throw it back into the jar:
we can then examine it, and see the result. You
observe, I first take up the air, and then throw
it back, as is evident from the ascent and de-
scent of the water; and now, by putting a taper
into the air, you will see the state in which it is,
by the light being extinguished. Even one in-
spiration, you see, has completely spoiled this
air, so that it is no use my trying to breathe it
a second time. Now you understand the ground
of the impropriety of many of the arrangements
among the houses of the poorer classes, by
which the air is breathed over and over again
for the want of a supply, by means of proper
ventilation, sufficient to produce a good result.
You see how bad the air becomes by a single breathing, so that
you can easily understand how essential fresh air is to us.

To pursue this a little farther, let us see what will happen with
lime-water. Here is a globe which contains a little lime-water, and
it is so arranged as regards the pipes, as to give access to the air
within, so that we can ascertain the effect of respired or unrespired
air upon it. Of course I can either draw in air (through A), and
so make the air that feeds my lungs go through the lime-water, or
I can force the air out of my lungs through the tube (B), which
goes to the bottom, and so show its effect upon the lime-water. You
will observe that however long I draw the external air into the lime-
water and then through it to my lungs, I shall produce no effect
upon the water — it will not make the lime-water turbid; but if I
throw the air from my lungs through the lime-water several times
in succession, you see how white and milky the water is getting,

Fig. 87

CHEMICAL HISTORY OF A CANDLE 1 65

showing the effect which expired air has had upon it; and now you
begin to know that the atmosphere which we have spoiled by respira-
tion is spoiled by carbonic acid, for you see it here
in contact with the lime-water.

I have here two bottles, one containing lime-
water and the other common water, and tubes
which pass into the bottles and connect them. The
apparatus is very rough, but it is useful notwith-
standing. If I take these two bottles, inhaling here
and exhaling there, the arrangement of the tubes
will prevent the air going backward. The air
coming in will go to my mouth and lungs, and
in going out will pass through the lime-water,
so that I can go on breathing, and making an
experiment very refined in its nature and very good in its re-
sults. You will observe that the good air has done nothing to the
lime-water; in the other case, nothing has come to the lime-water
but my respiration, and you see the difference in the two cases.

Let us now go a little farther. What
is all this process going on within us
which we can not do without, either
day or night, which is so provided for
by the Author of all things that He has
arranged that it shall be independent
of all will.'' If we restrain our respira-
tion, as we can to a certain extent, we
should destroy ourselves. When we
are asleep the organs of respiration and
the parts that are associated with them
still go on with their action, so neces-
sary is this process of respiration to us,
this contact of the air with the lungs. I
must tell you, in the briefest possible
manner, what this process is. We consume food; the food goes
through that strange set of vessels and organs within us, and is
brought into various parts of the system, into the digestive parts
especially; and alternately the portion which is so changed is

Fig. 89

I 66 FARADAY

carried through our lungs by one set of vessels, while the air that
we inhale and exhale is drawn into and thrown out of the lungs
by another set of vessels, so that the air and the food come close
together, separated only by an exceedingly thin surface: the air
can thus act upon the blood by this process, producing precisely
the same results in kind as we have seen in the case of the candle.
The candle combines with parts of the air, forming carbonic acid,
and evolves heat; so in the lungs there is this curious, wonderful
change taking place. The air entering, combines with the carbon
(not carbon in a free state, but, as in this case, placed ready for
action at the moment), and makes carbonic acid and is so thrown out
into the atmosphere, and thus this singular result takes place; we
may thus look upon the food as fuel. Let me take that piece of
sugar, which will serve my purpose. It is a compound of carbon,
hydrogen, and oxygen, similar to a candle, as containing the same
elements, though not in the same proportion, the proportions being
as shown in this table:

SUGAK.

Carbon 72

Hydrogen " I 00

Oxygen 88 M'

This is, indeed, a very curious thing, which you can well remem-
ber, for the oxygen and hydrogen are in exactly the proportions
which form water, so that sugar may be said to be compounded of
72 parts of carbon and 99 parts of water; and it is the carbon in
the sugar that combines with the oxygen carried in by the air in
the process of respiration — so making us like candles — producing
these actions, warmth, and far more wonderful results besides, for
the sustenance of the system, by a most beautiful and simple process.
To make this still more striking, I will take a little sugar; or, to
hasten the experiment, I will use some sirup, which contains about
three-fourths of sugar and a little water. If I put a little oil of vitriol
on it, it takes away the water, and leaves the carbon in a black mass.
[The lecturer mixed the two together.] You see how the carbon is
coming out, and before long we shall have a solid mass of charcoal,
all of which has come out of sugar. Sugar, as you know, is food, and
here we have absolutely a solid lump of carbon where you would not

CHEMICAL HISTORY OF A CANDLE 1 67

have expected it. And if I make arrangements so as to oxidize the
carbon of sugar we shall have a much more striking result. Here is
sugar, and I have here an oxidizer — a quicker one than the at-
mosphere; and so we shall oxidize this fuel by a process different
from respiration in its form, though not different in its kind. It is
the combustion of the carbon by the contact of oxygen which the
body has supplied to it. If I set this into action at once, you will see
combustion produced. Just what occurs in my lungs — taking in oxy-
gen from another source, namely, the atmosphere — takes place here
by a more rapid process.

You will be astonished when I tell you what this curious play of
carbon amounts to. A candle will burn some four, five, six, or seven
hours. What, then, must be the daily amount of carbon going up
into the air in the way of carbonic acid! What a quantity of carbon
must go from each of us in respiration! What a wonderful change
of carbon must take place under these circumstances of combustion
or respiration! A man in twenty-four hours converts as much as
seven ounces of carbon into carbonic acid; a milch cow will convert
seventy ounces, and a horse seventy-nine ounces, solely by the act
of respiration. That is, the horse in twenty-four hours burns seventy-
nine ounces of charcoal, or carbon, in his organs of respiration to
supply his natural warmth in that time. All the warm-blooded
animals get their warmth in this way, by the conversion of carbon,
not in a free state, but in a state of combination. And what an
extraordinary notion this gives us of the alterations going on in
our atmosphere. As much as 5,000,000 pounds, or 548 tons, of
carbonic acid is formed by respiration in London alone in twenty-
four hours. And where does all this go? Up into the air. If the
carbon had been like the lead which I showed you, or the iron which,
in burning, produces a solid substance, what would happen? Com-
bustion could not go on. As charcoal burns it becomes a vapor and
passes off into the atmosphere, which is the great vehicle, the great
carrier for conveying it away to other places. Then what becomes
of it? Wonderful is it to find that the change produced by respira-
tion, which seems so injurious to us (for we can not breathe air twice
over), is the very life and support of plants and vegetables that grow
upon the surface of the earth. It is the same also under the surface,

I 68 FARADAY

in the great bodies of water; for fishes and other animals respire
upon the same principle, though not exactly by contact with the
open air.

Such fish as I have here [pointing to a globe of goldfish] respire
by the oxygen which is dissolved from the air by the water, and form
carbonic acid, and they all move about to produce the one great
work of making the animal and vegetable kingdoms subservient to
each other. And all the plants growing upon the surface of the
earth, like that which I have brought here to serve as an illustration,
absorb carbon; these leaves are taking up their carbon from the at-
mosphere to which we have given it in the form of carbonic acid,
and they are growing and prospering. Give them a pure air like
ours, and they could not live in it; give them carbon with other
matters, and they live and rejoice. This piece of wood gets all its
carbon, as the trees and plants get theirs, from the atmosphere,
which, as we have seen, carries away what is bad for us and at the
same time good for them — ^what is disease to the one being health
to the other. So are we made dependent not merely upon our fellow-
creatures, but upon our fellow-existers, all Nature being tied to-
gether by the laws that make one part conduce to the good of
another.

There is another little point which I must mention before we draw
to a close— a point which concerns the whole of these operations,
and most curious and beautiful it is to see it clustering upon and
associated with the bodies that concern us — oxygen, hydrogen, and
carbon, in different states of their existence. I showed you just now
some powdered lead, which I set burning ('*) ; and you saw that the
moment the fuel was brought to the air, it acted, even before it
got out of the bottle — the moment the air crept in, it acted. Now
there is a case of chemical affinity by which all our operations pro-
ceed. When we breathe, the same operation is going on within us.
When we burn a candle, the attraction of the different parts one to
the other is going on. Here it is going on in this case of the lead, and
it is a beautiful instance of chemical affinity. If the products of com-

" Lead pyrophorus is made by heating dry tartrate of lead in a glass tube (closed at
one end, and drawn out to a fine point at the other) until no more vapors are evolved.
The open end of the tube is then to be sealed before the blowpipe. When the tube
is broken and the contents shaken out into the air, they burn with a red flash.

CHEMICAL HISTORY OF A CANDLE 1 69

bustion rose off from the surface, the lead would take fire, and go
on burning to the end; but you remember that we have this dif-
ference between charcoal and lead — that, while the lead can start
into action at once if there be access of air to it, the carbon will
remain days, weeks, months, or years. The manuscripts of Her-
culaneum were written with carbonaceous ink, and there they have
been for 1,800 years or more, not having been at all changed by the
atmosphere, though coming in contact with it under various circum-
stances. Now, what is the circumstance which makes the lead and
carbon differ in this respect.'' It is a striking thing to see that the
matter which is appointed to serve the purpose of fuel waits in its
action; it does not start off burning, like the lead and many other
things that I could show you, but which I have not encumbered the
table with; but it waits for action. This waiting is a curious and
wonderful thing. Candles — those Japanese candles, for instance — do
not start into action at once like the lead or iron (for iron finely
divided does the same thing as lead), but there they wait for years,
perhaps for ages, without undergoing any alteration. I have here a
supply of coal gas. The jet is giving forth the gas, but you see it
does not take fire — it comes out into the air, but it waits till it is
hot enough before it burns. If I make it hot enough, it takes fire.
If I blow it out, the gas that is issuing forth waits till the light is
applied to it again. It is curious to see how different substances wait
—how some will wait till the temperature is raised a little, and others
till it is raised a good deal. I have here a httle gunpowder and some
guncotton; even these things differ in the conditions under which
they will burn. The gunpowder is composed of carbon and other
substances, making it highly combustible; and the guncotton is
another combustible preparation. They are both waiting, but they
will start into activity at different degrees of heat, or under different
conditions. By applying a heated wire to them, we shall see which,
will start first [touching the guncotton with the hot iron]. You see
the guncotton has gone off, but not even the hottest part of the
wire is now hot enough to fire the gunpowder. How beautifully
that shows you the difference in the degree in which bodies act in
this way! In the one case the substance will wait any time until
the associated bodies are made active by heat; but in the other, as

170 FARADAY

in the process of respiration, it waits no time. In the lungs, as soon
as the air enters, it unites with the carbon; even in the lowest tem-
perature which the body can bear short of being frozen, the action
begins at once, producing the carbonic acid of respiration; and so
all things go on fitly and properly. Thus you see the analogy between
respiration and combustion is rendered still more beautiful and
striking. Indeed, all I can say to you at the end of these lectures
(for we must come to an end at one time or other) is to express a
wish that you may, in your generation, be fit to compare to a candle;
that you may, like it, shine as lights to those about you; that, in all
your actions, you may justify the beauty of the taper by making
your deeds honorable and effectual in the discharge of your duty
to your fellow-men.

ON THE CONSERVATION OF FORCE
ICE AND GLACIERS

BY
HERMANN VON HELMHOLTZ

TRANSLATED BY
EDMUND ATKINSON

INTRODUCTORY NOTE

Hermann Ludwig Ferdinand von Helmholtz was born at Potsdam,
near Berlin, on August 31, 1821. His father was a man of high culture,
a teacher in the gymnasium, whose influence ensured to his son the
foundations of a broad general education. His mother was a descendant
from William Penn, the English Quaker.

Helmholtz early showed mathematical ability, and wished to devote
his life to the study of physics; but practical considerations led him to
take up medicine, and he became a surgeon in the Prussian army. He
began the publication of original contributions to science in 1842, and for
fifty-two years, till his death in 1894, he continued to produce in an
unbroken stream. He held a succession of academic positions, teaching
physiology at Konigsberg, Bonn, and Heidelberg, and for the last twenty-
three years of his life filling the chair of physics at Berlin.

The tides of his professorships, however, give a very inadequate idea
of his range. His contributions to science cover medicine, physiology,
optics, acoustics, mathematics, mechanics, and electricity. His interests
in science and art came together in his work on esthetics, and he had a
lively appreciation of painting, poetry, and music.

The practice of popular lecturing on scientific subjects was almost
unknown in Germany when Helmholtz began, and he did much to give
it dignity and to set a standard. His own lectures, as the reader of the
following papers will perceive, are masterpieces of their kind. "The
matter," says a biographer, "is discussed by a master, who brings to bear
upon it all his wealth of learning and research, while there is the ever-
enduring interest that attaches to an exposition by one who is giving
forth from his own treasury." It is fortunate for the layman when a
scientist and thinker of the first order has the skill and the inclination to
share with the outside world the rich harvest of his brilliant and laborious
research.

ON THE CONSERVATION
OF FORCE

INTRODUCTION TO A SERIES OF LECTURES

DELIVERED AT CARLSRUHE IN THE

WINTER OF 1862-1863

AS I have undertaken to deliver here a series of lectures, I think
/ \ the best way in which I can discharge that duty will be
A. jL, to bring before you, by means of a suitable example, some
view of the special character of those sciences to the study of which
I have devoted myself. The natural sciences, partly in consequence
of their practical applications, and partly from their intellectual
influence on the last four centuries, have so profoundly, and with
such increasing rapidity, transformed all the relations of the life of
civilised nations; they have given these nations such increase of
riches, of enjoyment of life, of the preservation of health, of means
of industrial and of social intercourse, and even such increase of
political power, that every educated man who tries to understand the
forces at work in the world in which he is living, even if he does
not wish to enter upon the study of a special science, must have
some interest in that peculiar kind of mental labour, which works
and acts in the sciences in question.

On a former occasion I have already discussed the characteristic
differences which exist between the natural and the mental sciences
as regards the kind of scientific work. I then endeavoured to show
that it is more especially in the thorough conformity with law which
natural phenomena and natural products exhibit, and in the com-
parative ease with which laws can be stated, that this difference
exists. Not that I wish by any means to deny, that the mental life
of individuals and peoples is also in conformity with law, as is the
object of philosophical, philological, historical, moral, and social
sciences to establish. But in mental life, the influences are so inter-

173

174 HELMHOLTZ

woven, that any definite sequence can but seldom be demonstrated
In Nature the converse is the case. It has been possible to discover
the law of the origin and progress of many enormously extended
series of natural phenomena with such accuracy and completeness
that we can predict their future occurrence with the greatest cer-
tainty; or in cases in which we have power over the conditions under
which they occur, we can direct them just according to our will.
The greatest of all instances of what the human mind can effect by
means of a well-recognised law of natural phenomena is that
afforded by modern astronomy. The one simple law of gravitation
regulates the motions of the heavenly bodies not only of our own
planetary system, but also of the far more distant double stars; from
which, even the ray of light, the quickest of all messengers, needs
years to reach our eye; and, just on account of this simple con-
formity with law, the motions of the bodies in question can be
accurately predicted and determined both for the past and for future
years and centuries to a fraction of a minute.

On this exact conformity with law depends also the certainty with
which we know how to tame the impetuous force of steam, and to
make it the obedient servant of our wants. On this conformity
depends, moreover, the intellectual fascination which chains the
physicist to his subjects. It is an interest of quite a different kind to
that which mental and moral sciences afford. In the latter it is man
in the various phases of his intellectual activity who chains us.
Every great deed of which history tells us, every mighty passion
which art can represent, every picture of manners, of civic arrange-
ments, of the culture of peoples of distant lands or of remote times,
seizes and interests us, even if there is no exact scientific connection
among them. We continually find points of contact and comparison
in our conceptions and feelings; we get to know the hidden capac-
ities and desires of the mind, which in the ordinary peaceful course
of civilised life remain unawakened.

It is not to be denied that, in the natural sciences, this kind of
interest is wanting. Each individual fact, taken by itself, can indeed
arouse our curiosity or our astonishment, or be useful to us in its
practical applications. But intellectual satisfaction we obtain only
from a connection of the whole, just from its conformity with law.

ON THE CONSERVATION OF FORCE 1 75

Reason we call that faculty innate in us of discovering laws and
applying them with thought. For the unfolding of the peculiar
forces of pure reason in their entire certainty and in their entire
bearing, there is no more suitable arena than inquiry into Nature in
the wider sense, the mathematics included. And it is not only the
pleasure at the successful activity of one of our most essential mental
powers; and the victorious subjections to the power of our thought
and will of an external world, partly unfamiliar, and partly hostile,
which is the reward of this labour; but there is a kind, I might almost
say, of artistic satisfaction, when we are able to survey the enormous
wealth of Nature as a regularly-ordered whole — a kosmos, an image
of the logical thought of our own mind.

The last decades of scientific development have led us to the
recognition of a new universal law of all natural phenomena, which,
from its extraordinarily extended range, and from the connection
which it constitutes between natural phenomena of all kinds, even
of the remotest times and the most distant places, is especially fitted
to give us an idea of what I have described as the character of the
natural sciences, which I have chosen as the subject of this lecture.

This law is the Law of the Conservation of Force, a term the
meaning of which I must first explain. It is not absolutely new; for
individual domains of natural phenomena it was enunciated by
Newton and Daniel Bernoulli; and Rumford and Humphry Davy
have recognised distinct features of its presence in the laws of heat.

The possibility that it was of universal application was first stated
by Dr. Julius Robert Mayer, a Schwabian physician (now living in
Heilbronn), in the year 1842, while almost simultaneously with, and
independently of him, James Prescot Joule, an English manufacturer,
made a series of important and difficult experiments on the rela-
tion of heat to mechanical force, which supplied the chief points in
which the comparison of the new theory with experience was still
wanting.

The law in question asserts, that the quantity of force which can
be brought into action in the whole of Nature is unchangeable, and
can neither be increased nor diminished. My first object will be to
explain to you what is understood by quantity of force; or, as the
same idea is more popularly expressed with reference to its technical

176 HELMHOLTZ

application, what we call amount of wor\ in the mechanical sense
of the word.

The idea of work for machines, or natural processes, is taken from
comparison with the working power of man; and we can therefore
best illustrate from human labour the most important features of
the question with which we are concerned. In speaking of the work
of machines and of natural forces we must, of course, in this com-
parison eliminate anything in which activity of intelligence comes
into play. The latter is also capable of the hard and intense work of
thinking, which tries a man just as muscular exertion does. But
whatever of the actions of intelligence is met with in the work of
machines, of course is due to the mind of the constructor and cannot
be assigned to the instrument at work.

Now, the external work of man is of the most varied kind as
regards the force or ease, the form and rapidity, of the motions used
on it, and the kind of work produced. But both the arm of the black-
smith who delivers his powerful blows with the heavy hammer, and
that of the violinist who produces the most delicate variations in
sound, and the hand of the lace-maker who works with threads so
fine that they are on the- verge of the invisible, all these acquire the
force which moves them in the same manner and by the same
organs, namely, the muscles of the arms. An arm the muscles of
which are lamed is incapable of doing any work; the moving force
of the muscle must be at work in it, and these must obey the nerves,
which bring to them orders from the brain. That member is then
capable of the greatest variety of motions; it can compel the most
varied instruments to execute the most diverse tasks.

Just so it is with machines: they are used for the most diversified
arrangements. We produce by their agency an infinite variety of
movements, with the most various degrees of force and rapidity,
from powerful steam hammers and rolling mills, where gigantic
masses of iron are cut and shaped like butter, to spinning and
weaving frames, the work of which rivals that of the spider. Modern
mechanism has the richest choice of means of transferring the motion
of one set of rolling wheels to another with greater or less velocity;
of changing the rotating motion of wheels into the up-and-down
motion of the piston rod, of the shuttle, of falling hammers and

ON THE CONSERVATION OF FORCE 1 77

Stamps; or, conversely, of changing the latter into the former; or it
can, on the other hand, change movements of uniform into those of
varying velocity, and so forth. Hence this extraordinarily rich
utility of machines for so extremely varied branches of industry.
But one thing is common to all these differences; they all need a
moving force, which sets and keeps them in motion, just as the works
of the human hand all need the moving force of the muscles.

Now, the work of the smith requires a far greater and more
intense exertion of the muscles than that of the violin player; and
there are in machines corresponding differences in the power and
duration of the moving force required. These differences, which
correspond to the different degree of exertion of the muscles in
human labour, are alone what we have to think of when we speak of
the amount of tvorI{ of a machine. We have nothing to do here
with the manifold character of the actions and arrangements which
the machines produce; we are only concerned with an expenditure
of force.

This very expression which we use so fluently, 'expenditure of
force,' which indicates that the force applied has been expended and
lost, leads us to a further characteristic analogy between the effects
of the human arm and those of machines. The greater the exer-
tion, and the longer it lasts, the more is the. arm tired, and the more
is the store of its moving force for the time exhausted. We shall see
that this peculiarity of becoming exhausted by work is also met with
in the moving forces of inorganic nature; indeed, that this capacity
of the human arm of being tired is only one of the consequences of
the law with which we are now concerned. When fatigue sets in,
recovery is needed, and this can only be effected by rest and nourish-
ment. We shall find that also in the inorganic moving forces, when
their capacity for work is spent, there is a possibility of reproduction,
although in general other means must be used to this end than in
the case of the human arm.

From the feeling of exertion and fatigue in our muscles, we can
form a general idea of what we understand by amount of work;
but we must endeavour, instead of the indefinite estimate afforded
by this comparison, to form a clear and precise idea of the standard
by which we have to measure the amount of work. This we can do

178 HELMHOLTZ

better by the simplest inorganic moving forces than by the actions
of our muscles, which are a very complicated apparatus, acting in
an extremely intricate manner.

Let us now consider that moving force which we know best, and
which is simplest — gravity. It acts, for example, as such in those
clocks which are driven by a weight. This weight, fastened to a
string, which is wound round a pulley connected with the first
toothed wheel of the clock, cannot obey the pull of gravity without
setting the whole clockwork in motion. Now I must beg you to
pay special attention to the following points: the weight cannot
put the clock in motion without itself sinking; did the weight not
move, it could not move the clock, and its motion can only be such
a one as obeys the action of gravity. Hence, if the clock is to go, the
weight must continually sink lower and lower, and must at length
sink so far that the string which supports it is run out. The clock
then stops. The usual effect of its weight is for the present exhausted.
Its gravity is not lost or diminished; it is attracted by the earth as
before, but the capacity of this gravity to produce the motion of the
clockwork is lost. It can only keep the weight at rest in the lowest
point of its path, it cannot farther put it in motion.

But we can wind up the clock by the power of the arm, by which
the weight is again raised. When this has been done, it has regained
its former capacity, and can again set the clock in motion.

We learn from this that a raised weight possesses a moving force,
but that it must necessarily sink if this force is to act; that by sink-
ing, this moving force is exhausted, but by using another extraneous
moving force — that of the arm — its activity can be restored.

The work which the weight has to perform in driving the clock
is not indeed great. It has continually to overcome the small resist-
ances which the friction of the axles and teeth, as well as the
resistance of the air, oppose to the motion of the wheels, and it has
to furnish the force for the small impulses and sounds which the
pendulum produces at each oscillation. If the weight is detached
from the clock, the pendulum swings for a while before coming
to a rest, but its motion becomes each moment feebler, and ultimately
ceases entirely, being gradually used up by the small hindrances I
have mentioned. Hence, to keep the clock going, there must be a

ON THE CONSERVATION OF FORCE 1 79

moving force, which, though small, must be continually at work.
Such a one is the weight.

We get, moreover, from this example, a measure for the amount of
work. Let us assume that a clock is driven by a weight of a pound,
which falls five feet in twenty-four hours. If we fix ten such clocks,
each with a weight of one pound, then ten clocks will be driven
twenty-four hours; hence, as each has to overcome the same resist-
ances in the same time as the others, ten times as much work is
performed for ten pounds fall through five feet. Hence, we con-
clude that the height of the fall being the same, the work increases
directly as the weight.

Now, if we increase the length of the string so that the weight
runs down ten feet, the clock will go two days instead of one; and,
with double the height of fall, the weight will overcome on the
second day the same resistances as on the first, and will therefore
do twice as much work as when it can only run down five feet. The
weight being the same, the work increases as the height of fall.
Hence, we may take the product of the weight into the height of
fall as a measure of work, at any rate, in the present case. The appli-
cadon of this measure is, in fact, not limited to the individual case,
but the universal standard adopted in manufactures for measuring
magnitude of work is a foot pound — that is, the amount of work
which a pound raised through a foot can produce.'

We may apply this measure of work to all kinds of machines,
for we should be able to set them all in motion by means of a weight
sufficient to turn a pulley. We could thus always express the magni-
tude of any driving force, for any given machine, by the magnitude
and height of fall of such a weight as would be necessary to keep
the machine going with its arrangements until it had performed a
certain work. Hence it is that the measurement of work by foot
pounds is universally applicable. The use of such a weight as a
driving force would not indeed be practically advantageous in those
cases in which we were compelled to raise it by the power of our
own arm; it would in that case be simpler to work the machine by
the direct action of the arm. In the clock we use a weight so that

' This is the technical measure of work; to convert it into scientific measure it must
be multiplied by the intensity of gravity.

l8o HELMHOLTZ

we need not stand the whole day at the clockwork, as we should
have to do to move it directly. By winding up the clock we accumu-
late a store of working capacity in it, which is sufficient for the
expenditure of the next twenty-four hours.

The case is somewhat different when Nature herself raises the
weight, which then works for us. She does not do this with solid

Fig. 90

bodies, at least not with such regularity as to be utilised; but she
does it abundantly with water, which, being raised to the tops of
mountains by meteorological processes, returns in streams from them.
The gravity of water we use as moving force, the most direct appli-
cation being in what are called overshot wheels, one of which is
represented in Fig. 90. Along the circumference of such a wheel are
a series of buckets, which act as receptacles for the water, and, on
the side turned to the observer, have the tops uppermost; on the

ON THE CONSERVATION OF FORCE l8l

opposite side the tops of the buckets are upside-down. The water
flows at M into the buckets of the front of the wheel, and at F,
where the mouth begins to incHne downwards, it flows out. The
buckets on the circumference are filled on the side turned to the
observer, and empty on the other side. Thus the former are weighted
by the water contained in them, the latter not; the weight of the
water acts continuously on only one side of the wheel, draws this
down, and thereby turns the wheel; the other side of the wheel
offers no resistance, for it contains no water. It is thus the weight
of the falling water which turns the wheel, and furnishes the motive
power. But you will at once see that the mass of water which turns
the wheel must necessarily fall in order to do so, and that though,
when it has reached the bottom, it has lost none of its gravity, it is
no longer in a position to drive the wheel, if it is not restored to its
original position, either by the power of the human arm or by means
of some other natural force. If it can flow from the mill-stream to
still lower levels, it may be used to work other wheels. But when
it has reached its lowest level, the sea, the last remainder of the
moving force is used up, which is due to gravity — that is, to the
attraction of the earth, and it cannot act by its weight until it has
been again raised to a high level. As this is actually effected by
meteorological processes, you will at once observe that these are to
be considered as sources of moving force.

Water power was the first inorganic force which man learnt to
use instead of his own labour or of that of domestic animals.
According to Strabo, it was known to King Mithridates of Pontus,
who was also otherwise celebrated for his knowledge of Nature;
near his palace there was a water wheel. Its use was first introduced
among the Romans in the time of the first Emperors. Even now we
find water mills in all mountains, valleys, or wherever there are
rapidly-flowing regularly-filled brooks and streams. We find water
power used for all purposes which can possibly be effected by ma-
chines. It drives mills which grind corn, sawmills, hammers and
oil presses, spinning frames and looms, and so forth. It is the cheap-
est of all motive powers, it flows spontaneously from the inexhaust-
ible stores of Nature; but it is restricted to a particular place, and
only in mountainous countries is it present in any quantity; in level

1 82 HELMHOLTZ

countries extensive reservoirs are necessary for damming the rivers
to produce any amount of water power.

Before passing to the discussion of other motive forces I must
answer an objection which may readily suggest itself. We all know
that there are numerous machines, systems of pulleys, levers and
cranes, by the aid of which heavy burdens may be
lifted by a comparatively small expenditure of
force. We have all of us often seen one or two
workmen hoist heavy masses of stones to great
heights, which they would be quite unable to do
directly; in like manner, one or two men, by
means of a crane, can transfer the largest and
heaviest chests from a ship to the quay.
Now, it may be asked. If a large, heavy
weight had been used for driving a machine,
would it not be very easy, by means of a
crane or a system of pulleys, to raise it anew,
so that it could again be used as a motor, and thus
acquire motive power, without being compelled to use
a corresponding exertion in raising the weight?

The answer to this is, that all these machines, in that
degree in which for the moment they facilitate the exer-
tion, also prolong it, so that by their help no motive
power is ultimately gained. Let us assume that four
labourers have to raise a load of four hundredweight
by means of a rope passing over a single pulley. Every
time the rope is pulled down through four feet, the
load is also raised through four feet. But now, for the
sake of comparison, let us suppose the same load hung
to a block of four pulleys, as represented in Fig. 91. A
single labourer would now be able to raise the load by
the same exertion of force as each one of the four put
forth. But when he pulls the rope through four feet, the
load only rises one foot, for the length through which
he pulls the rope, at a, is uniformly distributed in the
block over four ropes, so that each of these is only shortened by a
foot. To raise the load, therefore, to the same height, the one man

Fig. 91

ON THE CONSERVATION OF FORCE 1 83

must necessarily work four times as long as the four together did.
But the total expenditure of work is the same, whether four labour-
ers work for a quarter of an hour or one works for an hour.

If, instead of human labour, we introduce the work of a weight,
and hang to the block a load of 400, and at a, where otherwise the
labourer works, a weight of 100 pounds, the block is then in equi-
librium, and, without any appreciable exertion of the arm, may be
set in motion. The weight of 100 pounds sinks, that of 400 rises.

Fig. 92

Without any measurable expenditure of force, the heavy weight has
been raised by the sinking of the smaller one. But observe that the
smaller weight will have sunk through four times the distance that
the greater one has risen. But a fall of 100 pounds through four
feet is just as much 400 foot-pounds as a fall of 400 pounds through
one foot.

The action of levers in all their various modifications is precisely
similar. Let a b, Fig. 92, be a simple lever, supported at c, the arm
c b being four times as long as the other arm a c. Let a weight of
one pound be hung at b, and a weight of four pounds at a, the lever
is then in equilibrium, and the least pressure of the finger is sufficient,
without any appreciable exertion of force, to place it in the position
fl' b^, in which the heavy weight of four pounds has been raised,
while the one-pound weight has sunk. But here, also, you will ob-
serve no work has been gained, for while the heavy weight has been

184 HELMHOLTZ

raised through one inch, the lighter one has fallen through four
inches; and four pounds through one inch is, as work, equivalent to
the product of one pound through four inches.

Most other fixed parts of machines may be regarded as modified
and compound levers; a toothed-wheel, for instance as a series of
levers, the ends of which are represented by the individual teeth,
and one after the other of which is put in activity in the degree in

Fig. 93

which the tooth in question seizes or is seized by the adjacent pinion.
Take, for instance, the crabwinch, represented in Fig. 93. Suppose
the pinion on the axis of the barrel of the winch has twelve teeth,
and the toothed-wheel, H H, seventy-two teeth, that is, six times as
many as the former. The winch must now be turned round six
times before the toothed-wheel, H, and the barrel, D, have made one
turn, and before the rope which raises the load has been lifted by a
length equal to the circumference of the barrel. The workman thus
requires six times the time, though to be sure only one-sixth of the
exertion, which he would have to use if the handle were directly
applied to the barrel, D. In all these machines, and parts of ma-

ON THE CONSERVATION OF FORCE 1 85

chines, we find it confirmed that in proportion as the velocity of the
motion increases its power diminishes, and that when the power
increases the velocity diminishes, but that the amount of work is
never thereby increased.

In the overshot mill wheel, described above, water acts by its
weight. But there is another form of mill wheels, what is called the

Fig. 94

undershot wheel, in which it only acts by its impact, as represented
in Fig. 94. These are used where the height from which the water
comes is not great enough to flow on the upper part of the wheel.
The lower part of undershot wheels dips in the flowing water which
strikes against their float-boards and carries them along. Such
wheels are used in swift-flowing streams which have a scarcely per-
ceptible fall, as, for instance, on the Rhine. In the immediate neigh-
borhood of such a wheel, the water need not necessarily have a great
fall if it only strikes with considerable velocity. It is the velocity of
the water, exerting an impact against the float-boards, which acts
in this case, and which produces the motive power.

Windmills, which are used in the great plains of Holland and
North Germany to supply the want of falling water, afford another

l86 HELMHOLTZ

instance of the action of velocity. The sails are driven by air in
motion — by wind. Air at rest could just as little drive a windmill
as water at rest a water wheel. The driving force depends here on
the velocity of moving masses.

A bullet resting in the hand is the most harmless thing in the
world; by its gravity it can exert no great effect; but when fired and
endowed with great velocity it drives through all obstacles with the
most tremendous force.

If I lay the head of a hammer gently on a nail, neither its small
weight nor the pressure of my arm is quite sufficient to drive the
nail into the wood; but if I swing the hammer and allow it to fall
with great velocity, it acquires a new force, which can overcome far
greater hindrances.

These examples teach us that the velocity of a moving mass can
act as motive force. In mechanics, velocity in so far as it is motive
force, and can produce work, is called vis viva. The name is not
well chosen; it is too apt to suggest to us the force of living beings.
Also in this case you will see, from the instances of the hammer and
of the bullet, that velocity is lost, as such, when it produces working
power. In the case of the water mill, or of the windmill, a more
careful investigation of the moving masses of water and air is
necessary to prove that part of their velocity has been lost by the
work which they have performed.

The relation of velocity to working power is most simply and
clearly seen in a simple pendulum, such as can be constructed by any
weight which we suspend to a cord. Let M, Fig. 95, be such a weight,
of a spherical form; A B, a horizontal line drawn through the centre
of the sphere; P the point at which the cord is fastened. If now I
draw the weight M on one side towards A, it moves in the arc M a,
the end of which, a, is somewhat higher than the point A in the
horizontal line. The weight is thereby raised to the height A a.
Hence my arm must exert a certain force to bring the weight to a.
Gravity resists this motion, and endeavours to bring back the weight
to M, the lowest point which it can reach.

Now, if after I have brought the weight to a I let it go, it obeys
this force of gravity and returns to M, arrives there with a certain
velocity, and no longer remains quietly hanging at M as it did

ON THE CONSERVATION OF FORCE 1 87

before, but swings beyond M towards b, where its motion stops as
soon as it has traversed on the side of B an arc equal in length to
that on the side of A, and after it has risen to a distance B b above
the horizontal line, which is equal to the height A a, to which my
arm had previously raised it. In b the pendulum returns, swings the
same way back through M towards a, and so on, until its oscillations

are gradually diminished, and ultimately annulled by the resistance
of the air and by friction.

You see here that the reason why the weight, when it comes from
a to M, and does not stop there, but ascends to b, in opposition to
the action of gravity, is only to be sought in its velocity. The velocity
which it has acquired in moving from the height A a is capable of
again raising it to an equal height, B b. The velocity of the moving
mass, M, is thus capable of raising this mass; that is to say, in the
language of mechanics, of performing work. This would also be
the case if we had imparted such a velocity to the suspended weight
by a blow.

From this we learn further how to measure the working power of
velocity — or, what is the same thing, the vis viva of the moving

1 88 HELMHOLTZ

mass. It is equal to the work, expressed in foot-pounds, which the
same mass can exert after its velocity has been used to raise it,
under the most favourable circumstances, to as great a height as
possible.^ This does not depend on the direction of the velocity; for
if we swing a weight attached to a thread in a circle, we can even
change a downward motion into an upward one.

The motion of the pendulum shows us very distinctly how the
forms of working power hitherto considered — that of a raised weight
and that of a moving mass — may merge into one another. In the
points a and b, Fig. 95, the mass has no velocity; at the point M it
has fallen as far as possible, but possesses velocity. As the weight
goes from a torn the work of the raised weight is changed into vis
viva; as the weight goes further from m to b the vis viva is changed
into the work of a raised weight. Thus the work which the arm
originally imparted to the pendulum is not lost in these oscillations
provided we may leave out of consideration the influence of the
resistance of the air and of friction. Neither does it increase, but it
continually changes the form of its manifestation.

Let us now pass to other mechanical forces, those of elastic
bodies. Instead of the weights which drive our clocks, we find in
timepieces and in watches, steel springs which are coiled in winding
up the clock, and are uncoiled by the working of the clock. To
coil up the spring we consume the force of the arm; this has to
overcome the resisting elastic force of the spring as we wind it up,
just as in the clock we have to overcome the force of gravity which
the weight exerts. The coiled spring can, however, perform work;
it gradually expends this acquired capability in driving the clock-
work.

If I stretch a crossbow and afterwards let it go, the stretched string
moves the arrow; it imparts to it force in the form of velocity. To
stretch the cord my arm must work for a few seconds; this work is
imparted to the arrow at the moment it is shot off. Thus the cross-
bow concentrates into an extremely short time the entire work
which the arm had communicated in the operation of stretching; the

^ The measure of vis viva in theoretical mechanics is half the product of the weight
into the square of the velocity. To reduce it to the technical measure of the work
we must divide it by the intensity of gravity; that is, by the velocity at the end of
the first second of a freely falling body.

ON THE CONSERVATION OF FORCE 1 89

clock, on the contrary, spreads it over one or several days. In both
cases no work is produced which my arm did not originally impart
to the instrument; it is only expended more conveniently.

The case is somewhat different if by any other natural process I
can place an elastic body in a state of tension without having to
exert my arm. This is possible and is most easily observed in the
case of gases.

If, for instance, I discharge a firearm loaded with gunpowder,
the greater part of the mass of the powder is converted into gases
at a very high temperature, which have a powerful tendency to
expand, and can only be retained in the narrow space in which they
are formed, by the exercise of the most powerful pressure. In
expanding with enormous force they propel the bullet, and impart
to it a great velocity, which we have already seen is a form of work.

In this case, then, I have gained work which my arm has not
performed. Something, however, has been lost — the gunpowder,
that is to say, whose constituents have changed into other chemical
compounds, from which they cannot, without further ado, be
restored to their original condition. Here, then, a chemical change
has taken place, under the influence of which work has been
gained.

Elastic forces are produced in gases by the aid of heat, on a far
greater scale.

Let us take, as the most simple instance, atmospheric air. In
Fig. 96 an apparatus is represented such as Regnault used for meas-
uring the expansive force of heated gases. If no great accuracy is
required in the measurement, the apparatus may be arranged more
simply. At C is a glass globe filled with dry air, which is placed
in a metal vessel, in which it can be heated by steam. It is connected
with the U-shaped tube, S s, which contains a liquid, and the limbs
of which communicate with each other when the stopcock R is
closed. If the liquid is in equilibrium in the tube S / when the globe
is cold, it rises in the leg s, and ultimately overflows when the globe
is heated. If, on the contrary, when the globe is heated, equilibrium
be restored by allowing some of the liquid to flow out at R, as the
globe cools it will be drawn up towards n. In both cases liquid is
raised, and work thereby produced.

190 HELMHOLTZ

The same experiment is continuously repeated on the largest
scale in steam engines, though, in order to keep up a continual dis-
engagement o£ compressed gases from the boiler, the air in the globe
in Fig. 96, which would soon reach the maximum of its expansion,
is replaced by water, which is gradually changed into steam by the
application of heat. But steam, so long as it remains as such, is an

Fig. 96

elastic gas which endeavours to expand exactly like atmospheric air.
And instead of the column of liquid which was raised in our last
experiment, the machine is caused to drive a solid piston which
imparts its motion to other parts of the machine. Fig. 97 represents
a front view of the working parts of a high-pressure engine, and
Fig. 98 a section. The boiler in which steam is generated is not
represented; the steam passes through the tube z z, Fig. 98, to the
cylinder A A, in which moves a tightly fitting piston C. The parts
between the tube z z and the cylinder A A, that is the slide valve in
the valve-chest K K, and the two tubes d and e allow the steam to

ON THE CONSERVATION OF FORCE

Fig. 97

192 HELMHOLTZ

pass first below and then above the piston, while at the same time
the steam has free exit from the other half of the cylinder. When
the steam passes under the piston, it forces it upward; when the
piston has reached the top of its course the position of the valve in
K K changes, and the steam passes above the piston and forces it
down again. The piston rod acts by means of the connecting rod
P, on the crank Q of the flywheel X and sets this in motion. By
means of the rod s, the motion of the rod regulates the opening and
closing of the valve. But we need not here enter into those mechani-
cal arrangements, however ingeniously they have been devised. We
are only interested in the manner in which heat produces elastic
vapour, and how this vapour, in its endeavour to expand, is
compelled to move the solid parts of the machine, and furnish
work.

You all know how powerful and varied are the effects of which
steam engines are capable; with them has really begun the great
development of industry which has characterised our century before
all others. Its most essential superiority over motive powers formerly
known is that it is not restricted to a particular place. The store of
coal and the small quantity of water which are the sources of its
power can be brought everywhere, and steam engines can even be
made movable, as is the case with steam ships and locomotives. By
means of these machines we can develop motive power to almost an
indefinite extent at any place on the earth's surface, in deep mines
and even on the middle of the ocean; while water and windmills
are bound to special parts of the surface of the land. The locomotive
transports travellers and goods over the land in numbers and with a
speed which must have seemed an incredible fable to our fore-
fathers, who looked upon the mailcoach with its six passengers in
the inside, and its ten miles an hour, as an enormous progress.
Steam engines traverse the ocean independently of the direction of
the wind, and, successfully resisting storms which would drive
sailing vessels far away, reach their goal at the appointed time. The
advantages which the concourse of numerous and variously skilled
workmen in all branches offers in large towns where wind and water
power are wanting, can be utilised, for steam engines find place
everywhere, and supply the necessary crude force; thus the more

ON THE CONSERVATION OF FORCE

193

Fig. 98

194 HELMHOLTZ

intelligent human force may be spared for better purposes; and,
indeed, wherever the nature of the ground or the neighbourhood
of suitable lines of communication present a favourable opportunity
for the development of industry, the motive power is also present
in the form of steam engines.

We see, then, that heat can produce mechanical power; but in the
cases which we have discussed we have seen that the quantity of
force which can be produced by a given measure of a physical
process is always accurately defined, and that the further capacity
for work of the natural forces is either diminished or exhausted by
the work which has been performed. How is it now with Heat in
this respect?

This question was of decisive importance in the endeavour to
extend the law of the Conservation of Force to all natural processes.
In the answer lay the chief difference between the older and newer
views in these respects. Hence it is that many physicists designate
that view of Nature corresponding to the law of the conservation of
force with the name of Mechanical Theory of Heat.

The older view of the nature of heat was that it is a substance,
very fine and imponderable indeed, but indestructible, and unchange-
able in quantity, which is an essential fundamental property of all
matter. And, in fact, in a large number of natural processes, the
quantity of heat which can be demonstrated by the thermometer is
unchangeable.

By conduction and radiation, it can indeed pass from hotter to
colder bodies; but the quantity of heat which the former lose can
be shown by the thermometer to have reappeared in the latter. Many
processes, too, were known, especially in the passage of bodies from
the solid to the liquid and gaseous states, in which heat disappeared
— at any rate, as regards the thermometer. But when the gaseous
body was restored to the liquid, and the liquid to the solid state,
exactly the same quantity of heat reappeared which formerly seemed
to have been lost. Heat was said to have become latent. On this
view, liquid water differed from solid ice in containing a certain
quantity of heat bound, which, just because it was bound, could not
pass to the thermometer, and therefore was not indicated by it.
Aqueous vapour contains a far greater quantity of heat thus bound.

ON THE CONSERVATION OF FORCE 1 95

But if the vapour be precipitated, and the Uquid water restored to
the state of ice, exactly the same amount of heat is Uberated as had
become latent in the melting of the ice and in the vaporisation of
the water.

Finally, heat is sometimes produced and sometimes disappears in
chemical processes. But even here it might be assumed that the
various chemical elements and chemical compounds contain certain
constant quantities of latent heat, which, when they change their
composition, are sometimes liberated and sometimes must be sup-
plied from external sources. Accurate experiments have shown that
the quantity of heat which is developed by a chemical process — for
instance, in burning a pound of pure carbon into carbonic acid — is
perfectly constant, whether the combustion is slow or rapid, whether
it takes place all at once or by intermediate stages. This also agreed
very well with the assumption, which was the basis of the theory of
heat, that heat is a substance entirely unchangeable in quantity. The
natural processes which have here been briefly mentioned, were the
subject of extensive experimental and mathematical investigations,
especially of the great French physicists in the last decade of the
former, and the first decade of the present, century; and a rich and
accurately-worked chapter of physics had been developed, in which
everything agreed excellently with the hypothesis — that heat is a
substance. On the other hand, the invariability in the quantity of
heat in all these processes could at that time be explained in no other
manner than that heat is a substance.

But one relation of heat — namely, that to mechanical work — had
not been accurately investigated. A French engineer, Sadi Carnot,
son of the celebrated War Minister of the Revolution, had indeed
endeavoured to deduce the work which heat performs, by assuming
that the hypothetical caloric endeavoured to expand like a gas; and
from this assumption he deduced in fact a remarkable law as to the
capacity of heat for work, which even now, though with an essential
alteration introduced by Clausius, is among the bases of the modern
mechanical theory of heat, and the practical conclusions from which,
so far as they could at that time be compared with experiments, have
held good.

But it was already known that whenever two bodies in motion

196 HELMHOLTZ

rubbed against each other, heat was developed anew, and it could
not be said whence it came.

The fact is universally recognised; the axle of a carriage which is
badly greased and where the friction is great, becomes hot — so hot,
indeed, that it may take fire; machine wheels with iron axles going
at a great rate may become so hot that they weld to their sockets.

A powerful degree of friction is not, indeed, necessary to disengage
an appreciable degree of heat; thus, a lucifer match, which by rub-
bing is so heated that the phosphoric mass ignites, teaches this fact.
Nay, it is enough to rub the dry hands together to feel the heat
produced by friction, and which is far greater than the heating which
takes place when the hands lie gently on each other. Uncivilised
people use the friction of two pieces of wood to kindle a fire. With
this view, a sharp spindle of hard wood is made to revolve rapidly
on a base of soft wood in the manner represented in Fig. 99.

So long as it was only a question of the friction of solids, in which
particles from the surface become detached and compressed, it might

ON THE CONSERVATION OF FORCE 1 97

be supposed that some changes in structure of the bodies rubbed
might here Hberate latent heat, which would thus appear as heat of
friction.

But heat can also be produced by the friction of liquids, in which
there could be no question of changes in structure, or of the libera-
tion of latent heat. The first decisive experiment of this kind was
made by Sir Humphry Davy in the commencement of the present
century. In a cooled space he made two pieces of ice rub against
each other, and thereby caused them to melt. The latent heat which
the newly formed water must have here assimilated could not have
been conducted to it by the cold ice, or have been produced by a
change of structure; it could have come from no other cause than
from friction, and must have been created by friction.

Heat can also be produced by the impact of imperfectly elastic
bodies as well as by friction. This is the case, for instance, when we
produce fire by striking flint against steel, or when an iron bar is
worked for some time by powerful blows of the hammer.

If we inquire into the mechanical effects of friction and of inelastic
impact, we find at once that these are the processes by which all
terrestrial movements are brought to rest. A moving body whose
motion was not retarded by any resisting force would continue to
move to all eternity. The motions of the planets are an instance of
this. This is apparently never the case with the motion of the
terrestrial bodies, for they are always in contact with other bodies
which are at rest, and rub against them. We can, indeed, very much
diminish their friction, but never completely annul it. A wheel
which turns about a well-worked axle, once set in motion, continues
it for a long time; and the longer, the more truly and smoother the
axle is made to turn, the better it is greased, and the less the pressure
it has to support. Yet the tns viva of the motion which we have
imparted to such a wheel when we started it, is gradually lost in con-
sequence of friction. It disappears, and if we do not carefully con-
sider the matter, it seems as if the vis viva which the wheel had
possessed had been simply destroyed without any substitute.

A bullet which is rolled on a smooth horizontal surface continues
to roll until its velocity is destroyed by friction on the path, caused
by the very minute impacts on its little roughnesses.

198 HELMHOLTZ

A pendulum which has been put in vibration can continue to
oscillate for hours if the suspension is good, without being driven by
a weight; but by the friction against the surrounding air, and by
that at its place of suspension, it ultimately comes to rest.

A stone which has fallen from a height has acquired a certain
velocity on reaching the earth; this we know is the equivalent of a
mechanical work; so long as this velocity continues as such, we can
direct it upwards by means of suitable arrangements, and thus
utilise it to raise the stone again. Ultimately the stone strikes against
the earth and comes to rest; the impact has destroyed its velocity, and
therewith apparently also the mechanical work which this velocity
could have affected.

If we review the results of all these instances, which each of you
could easily add to from your own daily experience, we shall see
that friction and inelastic impact are processes in which mechanical
work is destroyed, and heat produced in its place.

The experiments of Joule, which have been already mentioned,
lead us a step further. He has measured in foot pounds the amount
of work which is destroyed by the friction of solids and by the
friction of liquids; and, on the other hand, he has determined the
quantity of heat which is thereby produced, and has established a
definite relation between the two. His experiments show that when
heat is produced by the consumption of work, a definite quantity of
work is required to produce that amount of heat which is known to
physicists as the unit of heat; the heat, that is to say, which is neces-
sary to raise one gramme of water through one degree centigrade.
The quantity of work necessary for this is, according to Joule's best
experiments, equal to the work which a gramme would perform in
falling through a height of 425 metres.

In order to show how closely concordant are his numbers, I will
adduce the results of a few series of experiments which he obtained
after introducing the latest improvements in his methods.

I. A series of experiments in which water was heated by friction in
a brass vessel. In the interior of this vessel a vertical axle provided
with sixteen paddles was rotated, the eddies thus produced being
broken by a series of projecting barriers, in which parts were cut out
large enough for the paddles to pass through. The value of the
equivalent was 424.9 metres.

ON THE CONSERVATION OF FORCE 1 99

2. Two similar experiments, in which mercury in an iron vessel
was substituted for water in a brass one, gave 425 and 426.3 metres.

3. Two series of experiments, in which a conical ring rubbed
against another, both surrounded by mercury, gave 426.7 and 425.6
metres.

Exactly the same relations between heat and work were also found
in the reverse process — that is, when work was produced by heat.
In order to execute this process under physical conditions that could
be controlled as perfectly as possible, permanent gases and not
vapours were used, although the latter are, in practice, more con-
venient for producing large quantities of work, as in the case of the
steam engine. A gas which is allowed to expand with moderate
velocity becomes cooled. Joule was the first to show the reason of
this cooling. For the gas has, in expanding, to overcome the resist-
ance which the pressure of the atmosphere and the slowly yielding
side of the vessel oppose to it: or, if it cannot of itself overcome this
resistance, it supports the arm of the observer which does it. Gas
thus performs work, and this work is produced at the cost of its
heat. Hence the cooling. If, on the contrary, the gas is suddenly
allowed to issue into a perfectly exhausted space where it finds no
resistance, it does not become cool, as Joule has shown; or if indi-
vidual parts of it become cool, others become warm; and, after the
temperature has become equalised, this is exactly as much as before
the sudden expansion of the gaseous mass.

How much heat the various gases disengage when they are com-
pressed, and how much work is necessary for their compression; or,
conversely, how much heat disappears when they expand under a
pressure equal to their own counterpressure, was partly known from
the older physical experiments, and has partly been determined by
the recent experiments of Regnault by extremely perfect methods.
Calculations with the best data of this kind give us the value of the
thermal equivalent from experiments: —

With atmospheric air 426.0 metres

" oxygen 425.7

" nitrogen 431.3 "

" hydrogen . . 425.3 "

Comparing these numbers with those which determine the equiv-
alence of heat and mechanical work in friction, as close an agree-

200 HELMHOLTZ

ment is seen as can at all be expected from numbers which have
been obtained by such varied investigations of different observers.

Thus then: a certain quantity of heat may be changed into a
definite quantity of work; this quantity of work can also be retrans-
formed into heat, and, indeed, into exactly the same quantity of heat
as that from which it originated; in a mechanical point of view, they
are exactly equivalent. Heat is a new form in which a quantity of
work may appear.

These facts no longer permit us to regard heat as a substance, for
its quantity is not unchangeable. It can be produced anew from the
vis viva of motion destroyed; it can be destroyed, and then produces
motion. We must rather conclude from this that heat itself is a
motion, an internal invisible motion of the smallest elementary
particles of bodies. If, therefore, motion seems lost in friction and
impact, it is not actually lost, but only passes from the great visible
masses to their smallest particles; while in steam engines the internal
motion of the heated gaseous particles is transferred to the piston
of the machine, accumulated in it, and combined in a resultant
whole.

But what is the nature of this internal motion can only be asserted
with any degree of probability in the case of gases. Their particles
probably cross one another in rectilinear paths in all directions, until,
striking another particle, or against the side of the vessel, they are
reflected in another direction. A gas would thus be analogous to a
swarm of gnats, consisting, however, of particles infinitely small and
infinitely more closely packed. This hypothesis, which has been
developed by Kronig, Clausius, and Maxwell, very well accounts
for all the phenomena of gases.

What appeared to the earlier physicists to be the constant quantity
of heat is nothing more than the whole motive power of the motion
of heat, which remains constant so long as it is not transformed into
other forms of work, or results afresh from them.

We turn now to another kind of natural forces which can produce
work — I mean the chemical. We have to-day already come across
them. They are the ultimate cause of the work which gunpowder
and the steam engine produce; for the heat which is consumed in
the latter, for example, originates in the combustion of carbon — that

ON THE CONSERVATION OF FORCE 201

is to say, in a chemical process. The burning of coal is the chemical
union of carbon with the oxygen of the air, taking place under the
influence of the chemical affinity of the two substances.

We may regard this force as an attractive force between the two,
which, however, only acts through them with extraordinary power,
if the smallest particles of the two substances are in closest proximity
to each other. In combustion this force acts; the carbon and oxygen
atoms strike against each other and adhere firmly, inasmuch as they
form a new compound — carbonic acid — a gas known to all of you
as that which ascends from all fermenting and fermented liquids —
from beer and champagne. Now this attraction between the atoms
of carbon and of oxygen performs work just as much as that which
the earth in the form of gravity exerts upon a raised weight. When
the weight falls to the ground, it produces an agitation, which is
partly transmitted to the vicinity as sound waves, and partly remains
as the motion of heat. The same result we must expect from chemi-
cal action. When carbon and oxygen atoms have rushed against each
other, the newly-formed particles of carbonic acid must be in the
most violent molecular motion — that is, in the motion of heat. And
this is so. A pound of carbon burned with oxygen to form carbonic
acid, gives as much heat as is necessary to raise 80.9 pounds of water
from the freezing to the boiling point; and just as the same amount
of work is produced when a weight falls, whether it falls slowly or
fast, so also the same quantity of heat is produced by the combustion
of carbon, whether this is slow or rapid, whether it takes place all
at once, or by successive stages.

When the carbon is burned, we obtain in its stead, and in that of
the oxygen, the gaseous product of combustion — carbonic acid. Im-
mediately after combustion it is incandescent. When it has after-
wards imparted heat to the vicinity, we have in the carbonic acid
the entire quantity of carbon and the entire quantity of oxygen, and
also the force of affinity quite as strong as before. But the action of
the latter is now limited to holding the atoms of carbon and oxygen
firmly united; they can no longer produce either heat or work any
more than a fallen weight can do work if it has not been again raised
by some extraneous force. When the carbon has been burnt we take
no further trouble to retain the carbonic acid; it can do no more

202 HELMHOLTZ

service, we endeavour to get it out of the chimneys of our houses as
fast as we can.

Is it possible, then, to tear asunder the particles of carbonic acid,
and give to them once more the capacity of work which they had
before they were combined, just as we can restore the potentiality
of a weight by raising it from the ground? It is indeed possible. We
shall afterwards see how it occurs in the life of plants; it can also be
effected by inorganic processes, though in roundabout ways, the
explanation of which would lead us too far from our present course.

This can, however, be easily and directly shown for another
element, hydrogen, which can be burnt just hke carbon. Hydrogen
with carbon is a constituent of all combustible vegetable substances,
among others, it is also an essential constituent of the gas which is
used for lighting our streets and rooms; in the free state it is also a
gas, the lightest of all, and burns when ignited with a feebly lu-
minous blue flame. In this combustion — that is, in the chemical com-
bination of hydrogen with oxygen, a very considerable quantity of
heat is produced; for a given weight of hydrogen, four times as much
heat as in the combustion of the same weight of carbon. The product
of combustion is water, which, therefore, is not of itself further
combustible, for the hydrogen in it is completely saturated with
oxygen. The force of affinity, therefore, of hydrogen for oxygen,
like that of carbon for oxygen, performs work in combustion, which
appears in the form of heat. In the water which has been formed
during combustion, the force of affinity is exerted between the ele-
ments as before, but its capacity for work is lost. Hence the two
elements must be again separated, their atoms torn apart, if new
effects are to be produced from them.

This we can do by the aid of currents of electricity. In the appara-
tus depicted in Fig. loo, we have two glass vessels filled with acidu-
lated water a and a^, which are separated in the middle by a porous
plate moistened with water. In both sides are fitted platinum wires,
\, which are attached to platinum plates, i and /j. As soon as a
galvanic current is transmitted through the water by the platinum
wires, \, you see bubbles of gas ascend from the plates i and i^. These
bubbles are the two elements of water, hydrogen on the one hand,
and oxygen on the other. The gases emerge through the tubes g and

ON THE CONSERVATION OF FORCE 203

gi. If we wait until the upper part of the vessels and the tubes have
been filled with it, we can inflame hydrogen at one side; it burns
with a blue flame. If I bring a glimmering spill near the mouth
of the other tube, it bursts into flame, just as happens with oxygen
gas, in which the processes of combustion are far more intense than

Fig. 100

in atmospheric air, where the oxygen mixed with nitrogen is only
one-fifth of the whole volume.

If I hold a glass Hask filled with water over the hydrogen flame,
the water, newly formed in combustion, condenses upon it.

If a platinum wire be held in the almost non-luminous flame, you
see how intensely it is ignited; in a plentiful current of a mixture
of the gases, hydrogen and oxygen, which have been liberated in the
above experiment, the almost infusible platinum might even be
melted. The hydrogen which has here been liberated from the water
by the electrical current has regained the capacity of producing large
quantities of heat by a fresh combination with oxygen; its affinity
for oxygen has regained for it its capacity for work.

We here become acquainted with a new source of work, the
electric current which decomposes water. This current is itself pro-
duced by a galvanic battery, Fig. ioi. Each of the four vessels con-
tains nitric acid, in which there is a hollow cylinder of very compact
carbon. In the middle of the carbon cylinder is a cylindrical porous
vessel of white clay, which contains dilute sulphuric acid; in this

204 HELMHOLTZ

dips a zinc cylinder. Each zinc cylinder is connected by a metal ring
with the carbon cylinder of the next vessel, the last zinc cylinder, n,
is connected with one platinum plate, and the first carbon cylinder, p,
with the other platinum plate of the apparatus for the decomposition
of water.

If now the conducting circuit of this galvanic apparatus is com-
pleted, and the decomposition of water begins, a chemical process

Fig. ioi

takes place simultaneously in the cells of the voltaic battery. Zinc
takes oxygen from the surrounding water and undergoes a slow
combustion. The product of combustion thereby produced, oxide
of zinc, unites further with sulphuric acid, for which it has a power-
ful affinity, and sulphate of zinc, a saline kind of substance, dissolves
in the liquid. The oxygen, moreover, which is withdrawn from it
is taken by the water from the nitric acid surrounding the cylinder
of carbon, which is very rich in it, and readily gives it up. Thus, in
the galvanic battery, zinc burns to sulphate of zinc at the cost of the
oxygen of nitric acid.

Thus, while one product of combustion, water, is again separated,
a new combustion is taking place — that of zinc. While we there
reproduce chemical affinity which is capable of work, it is here lost.
The electrical current is, as it were, only the carrier which transfers

ON THE CONSERVATION OF FORCE 205

the chemical force of the zinc uniting with oxygen and acid to water
in the decomposing cell, and uses it for overcoming the chemical
force of hydrogen and oxygen.

Fig. 102

In this case, we can restore work which has been lost, but only by
using another force, that of oxidising zinc.

Here we have overcome chemical forces by chemical forces,
through the instrumentality of the electrical current. But we can

206 HELMHOLTZ

attain the same object by mechanical forces, if we produce the elec-
trical current by a magneto-electrical machine, Fig. 102. If we turn
the handle, the anker R R', on which is coiled copper-wire, rotates in
front of the poles of the horseshoe magnet, and in these coils electri-
cal currents are produced, which can be led from the points a and b.
If the ends of these wires are connected with the apparatus for
decomposing water, we obtain hydrogen and oxygen, though in far
smaller quantity than by the aid of the battery which we used
before. But this process is interesting, for the mechanical force of
the arm which turns the wheel produces the work which is required
for separating the combined chemical elements. Just as the steam
engine changes chemical into mechanical force the magneto-electrical
machine transforms mechanical force into chemical.

The application of electrical currents opens out a large number
of relations between the various natural forces. We have decomposed
water into its elements by such currents, and should be able to decom-
pose a large number of other chemical compounds. On the other
hand, in ordinary galvanic batteries electrical currents are produced
by chemical forces.

In all conductors through which electrical currents pass they pro-
duce heat; I stretch a thin platinum wire between the ends n and p
of the galvanic battery, Fig. ioi; it becomes ignited and melts. On
the other hand, electrical currents are produced by heat in what are
called thermo-electric elements.

Iron which is brought near a spiral of copper wire, traversed by an
electrical current, becomes magnetic, and then attracts other pieces
of iron, or a suitably placed steel magnet. We thus obtain mechani-
cal actions which meet with extended applications in the electrical
telegraph, for instance. Fig. 103 represents a Morse's telegraph in
one-third of the natural size. The essential part is a horseshoe shaped
iron core, which stands in the copper spirals b b. Just over the top
of this is a small steel magnet c c, which is attracted the moment an
electrical current, arriving by the telegraph wire, traverses the spirals
b b. The magnet c c\s rigidly fixed in the lever d d, at the other end
of which is a style; this makes a mark on a paper band, drawn by a
clock-work, as often and as long as c c is attracted by the magnetic
action of the electrical current. Conversely, by reversing the mag-

ON THE CONSERVATION OF FORCE 20/

netism in the iron core of the spirals b b, we should obtain in them
an electrical current just as we have obtained such currents in the
magneto-electrical machine, Fig. 102; in the spirals of that machine
there is an iron core which, by being approached to the poles of the
large horseshoe magnet, is sometimes magnetised in one and some-
times in the other direction.

I will not accumulate examples of such relations; in subsequent
lectures we shall come across them. Let us review these examples

Fig. 103

once more, and recognise in them the law which is common to all.

A raised weight can produce work, but in doing so it must neces-
sarily sink from its height, and, when it has fallen as deep as it can
fall, its gravity remains as before, but it can no longer do work.

A stretched spring can do work, but in so doing it becomes loose.
The velocity of a moving mass can do work, but in doing so it
comes to rest. Heat can perform work; it is destroyed in the opera-
tion. Chemical forces can perform work, but they exhaust them-
selves in the effort.

Electrical currents can perform work, but to keep them up we
must consume either chemical or mechanical forces, or heat.

208 HELMHOLTZ

We may express this generally. It is a universal character of all
}{nown natural forces that their capacity for wor\ is exhausted in
the degree in u/hich they actually perforin wor\^.

We have seen, further, that when a weight fell without perform-
ing any work, it either acquired velocity or produced heat. We
might also drive a magneto-electrical machine by a falling weight;
it would then furnish electrical currents.

We have seen that chemical forces, when they come into play,
produce either heat or electrical currents or mechanical work.

We have seen that heat may be changed into work; there are
apparatus (thermo-electric batteries) in which electrical currents are
produced by it. Heat can directly separate chemical compounds;
thus, when we burn Umestone, it separates carbonic acid from lime.

Thus, whenever the capacity for work of one natural force is
destroyed, it is transformed into another kind of activity. Even
within the circuit of inorganic natural forces, we can transform each
of them into an active condition by the aid of any other natural force
which is capable of work. The connections between the various
natural forces which modern physics has revealed, are so extraor-
dinarily numerous that several entirely different methods may be
discovered for each of these problems.

I have stated how we are accustomed to measure mechanical work,
and how the equivalent in work of heat may be found. The equiv-
alent in work of chemical processes is again measured by the heat
which they produce. By similar relations, the equivalent in work
of the other natural forces may be expressed in terms of mechanical
work.

If, now, a certain quantity of mechanical work is lost, there is
obtained, as experiments made with the object of determining this
point show, an equivalent quantity of heat, or, instead of this, of
chemical force; and, conversely, when heat is lost, we gain an equiv-
alent quantity of chemical or mechanical force; and, again, when
chemical force disappears, an equivalent of heat or work; so that in
all these interchanges between various inorganic natural forces work-
ing force may indeed disappear in one form, but then it reappears
in exactly equivalent quantity in some other form; it is thus neither
increased nor diminished, but always remains in exactly the same

ON THE CONSERVATION OF FORCE 209

quantity. We shall subsequently see that the same law holds good
also for processes in organic nature, so far as the facts have been
tested.

It follows thence that the total quantity of all the forces capable of
work tn the whole universe remains eternal and unchanged through-
out all their changes. All change in nature amounts to this, that
force can change its form and locality without its quantity being
changed. The universe possesses, once for all, a store of force which
is not altered by any change of phenomena, can neither be increased
nor diminished, and which maintains any change which takes place
on it.

You see how, starting from considerations based on the immediate
practical interests of technical work, we have been led up to a uni-
versal natural law, which, as far as all previous experience extends,
rules and embraces all natural processes; which is no longer restricted
to the practical objects of human utility, but expresses a perfectly
general and particularly characteristic property of all natural forces,
and which, as regards generality, is to be placed by the side of the
laws of the unalterability of mass, and the unalterability of the
chemical elements.

At the same time, it also decides a great practical question which
has been much discussed in the last two centuries, to the decision
of which an infinity of experiments has been made and an infinity
of apparatus constructed — that is, the question of the possibility of
a perpetual motion. By this was understood a machine which was
to work continuously without the aid of any external driving force.
The solution of this problem promised enormous gains. Such a
machine would have had all the advantages of steam without requir-
ing the expenditure of fuel. Work is wealth. A machine which
could produce work from nothing was as good as one which made
gold. This problem had thus for a long time occupied the place of
gold making, and had confused many a pondering brain. That a
perpetual motion could not be produced by the aid of the then
known mechanical forces could be demonstrated in the last century
by the aid of the mathematical mechanics which had at that time
been developed. But to show also that it is not possible even if heat,
chemical forces, electricity, and magnetism were made to co-operate,

210 HELMHOLTZ

could not be done without a knowledge of our law in all its general-
ity. The possibility of a perpetual motion was first finally negatived
by the law of the conservation of force, and this law might also be
expressed in the practical form that no perpetual motion is possible,
that force cannot be produced from nothing; something must be
consumed.

You will only be ultimately able to estimate the importance and
the scope of our law when you have before your eyes a series of its
applications to individual processes in nature.

What I have to-day mentioned as to the origin of the moving
forces which are at our disposal, directs us to something beyond the
narrow confines of our laboratories and our manufactories, to the
great operations at work in the life of the earth and of the universe.
The force of falling water can only flow down from the hills when
rain and snow bring it to them. To furnish these, we must have
aqueous vapour in the atmosphere, which can only be effected by the
aid of heat, and this heat comes from the sun. The steam engine
needs the fuel which the vegetable life yields, whether it be the still
active life of the surrounding vegetation, or the extinct life which has
produced the immense coal deposits in the depths of the earth. The
forces of man and animals must be restored by nourishment; all
nourishment comes ultimately from the vegetable kingdom, and
leads us back to the same source.

You see then that when we inquire into the origin of the moving
forces which we take into our service, we are thrown back upon
the meteorological processes in the earth's atmosphere, on the life
of plants in general, and on the sun.

ICE AND GLACIERS

A LECTURE DELIVERED AT FRANKFORT-ON-

MAIN, AND AT HEIDELBERG, IN

FEBRUARY, 1865

THE world of ice and of eternal snow, as unfolded to us on
the summits of the neighbouring Alpine chain, so stern, so
solitary, so dangerous, it may be, has yet its own peculiar
charm. Not only does it enchain the attention of the natural phi-
losopher, who finds in it the most wonderful disclosures as to the
present and past history of the globe, but every summer it entices
thousands of travellers of all conditions, who find there mental and
bodily recreation. While some content themselves with admiring
from afar the dazzling adornment which the pure, luminous masses
of snowy peaks, interposed between the deeper blue of the sky and
the succulent green of the meadows, lend to the landscape, others
more boldly penetrate into the strange world, willingly subjecting
themselves to the most extreme degrees of exertion and danger, if
only they may fill themselves with the aspect of its sublimity.

I will not attempt what has so often been attempted in vain — to
depict in words the beauty and magnificence of nature, whose aspect
dehghts the Alpine traveller. I may well presume that it is known
to most of you from your own observation; or, it is to be hoped, will
be so. But I imagine that the delight and interest in the magnificence
of those scenes will make you the more inclined to lend a willing ear
to the remarkable results of modern investigations on the more
prominent phenomena of the glacial world. There we see that
minute peculiarities of ice, the mere mention of which might at other
times be regarded as a scientific subtlety, are the causes of the most
important changes in glaciers; shapeless masses of rock begin to
relate their histories to the attentive observer, histories which often

212 HELMHOLTZ

stretch far beyond the past of the human race into the obscurity of
the primeval world; a peaceful, uniform, and beneficent sway of
enormous natural forces, where at first sight only desert wastes are
seen, either extended indefinitely in cheerless, desolate solitudes, or
full of wild, threatening confusion — an arena of destructive forces.
And thus I think I may promise that the study of the connection
of those phenomena of which I can now only give you a very short
outline, will not only afford you some prosaic instruction, but will
make your pleasure in the magnificent scenes of the high mountains
more vivid, your interest deeper, and your admiration more exalted.

Let me first of all recall to your remembrance the chief features
of the external appearance of the snow fields and of the glaciers; and
let me mention the accurate measurements which have contributed
to supplement observation, before I pass to discuss the casual con-
nection of those processes.

The higher we ascend the mountains the colder it becomes. Our
atmosphere is like a warm covering spread over the earth; it is well-
nigh entirely transparent for the luminous darting rays of the sun,
and allows them to pass almost without appreciable change. But it
is not equally penetrable by obscure heat rays, which, proceeding
from heated terrestrial bodies, struggle to diffuse themselves into
space. These are absorbed by atmospheric air, especially when it is
moist; the mass of air is itself heated thereby, and only radiates
slowly into space the heat which has been gained. The expenditure
of heat is thus retarded as compared with the supply, and a certain
store of heat is retained along the whole surface of the earth. But
on high mountains the protective coating of the atmosphere is far
thinner — the radiated heat of the ground can escape thence more
freely into space; there, accordingly, the accumulated store of heat
and the temperature are far smaller than at lower levels.

To this must be added another property of air which acts in the
same direction. In a mass of air which expands, part of its store of
heat disappears; it becomes cooler, if it cannot acquire fresh heat
from without. Conversely, by renewed compression of the air, the
same quantity of heat is reproduced which had disappeared during
expansion. Thus if, for instance, south winds drive the warm air of
the Mediterranean towards the north, and compel it to ascend along

ICE AND GLACIERS 213

the great mountain wall of the Alps, where the air, in consequence
of the diminished pressure, expands by about half its volume, it
thereby becomes very greatly cooled — for a mean height of 11,000
feet, by from 18° to 30° C, according as it is moist or dry — and it
thereby deposits the greater part of its moisture as rain or snow. If
the same wind, passing over to the north side of the mountains as
Fohn-wind, reaches the valleys and plains, it again becomes con-
densed, and is again heated. Thus the same current of air which is
warm in the plains, both on this side of the chain and on the other,
is bitterly cold on the heights, and can there deposit snow, while in
the plain we find it insupportably hot.

The lower temperature at greater heights, which is due to both
these causes, is, as we know, very marked on the lower mountain
chains of our neighbourhood. In central Europe it amounts to about
1° C. for an ascent of 480 feet; in winter it is less — 1° for about 720
feet of ascent. In the Alps the differences of temperature at great
heights are accordingly far more considerable, so that upon the
higher parts of their peaks and slopes the snow which has fallen in
winter no longer melts in summer. This line, above which snow
covers the ground throughout the entire year, is well known as the
snow line; on the northern side of the Alps it is about 8,000 feet
high, on the southern side about 8,800 feet. Above the snow line it
may on sunny days be very warm ; the unrestrained radiation of the
sun, increased by the light reflected from the snow, often becomes
utterly unbearable, so that the tourist of sedentary habits, apart
from the dazzling of his eyes, which he must protect by dark spec-
tacles or by a veil, usually gets severely sunburnt in the face and
hands, the result of which is an inflammatory swelling of the skin
and great blisters on the surface. More pleasant testimonies to the
power of the sunshine are the vivid colours and the powerful odour
of the small Alpine flowers which bloom in the sheltered rocky clefts
among the snow fields. Notwithstanding the powerful radiation of
the sun the temperature of the air above the snow fields only rises
to 5°, or at most 8°; this, however, is sufficient to melt a tolerable
amount of the superficial layers of snow. But the warm hours and
days are too short to overpower the great masses of snow which have
fallen during colder times. Hence the height of the snow line does

214 HELMHOLTZ

not depend merely on the temperature of the mountain slope, but
also essentially on the amount of the yearly snowfall. It is lower,
for instance, on the moist and warm south slope of the Himalayas
than on the far colder but also far drier north slope of the same
mountain. Corresponding to the moist climate of western Europe,
the snowfall upon the Alps is very great, and hence the number and
extent of their glaciers are comparatively considerable, so that few
mountains of the earth can be compared with them in this respect.
Such a development of the glacial world is, as far as we know, met
with only on the Himalayas, favoured by the greater height; in
Greenland and in Northern Norway, owing to the colder climate;
in a few islands in Iceland; and in New Zealand, from the more
abundant moisture.

Places above the snow line are thus characterised by the fact that
the snow which in the course of the year falls on its surface does not
quite melt away in summer, but remains to some extent. This snow,
which one summer has left, is protected from the further action of
the sun's heat by the fresh quantities that fall upon it during the
next autumn, winter, and spring. Of this new snow also next sum-
mer leaves some remains, and thus year by year fresh layers of snow
are accumulated one above the other. In those places where such an
accumulation of snow ends in a steep precipice, and its inner struc-
ture is thereby exposed, the regularly stratified yearly layers are
easily recognised.

But it is clear that this accumulation of layer upon layer cannot go
on indefinitely, for otherwise the height of the snow peak would
continually increase year by year. But the more the snow is accumu-
lated the steeper are the slopes, and the greater the weight which
presses upon the lower and older layers and tries to displace them.
Ultimately a state must be reached in which the snow slopes are too
steep to allow fresh snow to rest upon them, and in which the
burden which presses the lower layers downwards is so great that
these can no longer retain their position on the sides of the moun-
tain. Thus, part of the snow which had originally fallen on the
higher regions of the mountain above the snow line, and had there
been protected from melting, is compelled to leave its original posi-
tion and seek a new one, which it of course finds only below the

ICE AND GLACIERS 21 5

snow line on the lower slopes of the mountain, and especially in the
valleys, where, however, being exposed to the influence of a warmer
air, it ultimately melts and flows away as water. The descent of
masses of snow from their original positions sometimes happens
suddenly in avalanches, but it is usually very gradual in the form of
glaciers.

Thus we must discriminate between two distinct parts of the ice
fields; that is, first, the snow which originally fell — called firn in
Switzerland — above the snow line, covering the slopes of the peaks
as far as it can hang on to them, and filling up the upper wdde
kettle-shaped ends of the valleys forming widely extending fields of
snow or firnmeere. Secondly, the glaciers, called in the Tyrol firner,
which as prolongations of the snow fields often extend to a distance
of from 4,000 to 5,000 feet below the snow line, and in which the
loose snow of the snow fields is again found changed into trans-
parent solid ice. Hence the name glacier, which is derived from the
Latin, glacies; French, glace, glacier.

The outward appearance of glaciers is very characteristically
described by comparing them, with Goethe, to currents of ice. They
generally stretch from the snow fields along the depth of the valleys,
filling them throughout their entire breadth, and often to a con-
siderable height. They thus follow all the curvatures, windings, con-
tractions, and enlargements of the valley. Two glaciers frequently
meet the valleys of which unite. The two glacial currents then join
in one common principal current, filling up the valley common to
them both. In some places these ice currents present a tolerably level
and coherent surface, but they are usually traversed by crevasses, and
both over the surface and through the crevasses countless small and
large water rills ripple, which carry off the water formed by the
melting of the ice. United, and forming a stream, they burst, through
a vaulted and clear blue gateway of ice, out at the lower end of the
larger glacier.

On the surface of the ice there is a large quantity of blocks of
stone, and of rocky debris, which at the lower end of the glacier are
heaped up and form immense walls; these are called the lateral and
terminal moraine of the glacier. Other heaps of rock, the central
moraine, stretch along the surface of the glacier in the direction of its

2l6 HELMHOLTZ

length, forming long regular dark lines. These always start from
the places where two glacier streams coincide and unite. The central
moraines are in such places to be regarded as the continuations of
the united lateral moraines of the two glaciers.

The formation of the central moraine is well represented in the
view below given of the Unteraar Glacier (Fig. 104). In the back-
ground are seen the two glacier currents emerging from different

Fig. 104

valleys; on the right from the Shreckhorn, and on the left from the
Finsteraarhorn. From the place where they unite the rocky wall
occupying the middle of the picture descends, constituting the cen-
tral moraine. On the left are seen individual large masses of rock
resting on pillars of ice, which are known as glacier tables.

To exemplify these circumstances still further, I lay before you in
Fig. 105 a map of the Mer de Glace of Chamouni, copied from that
of Forbes.

The Mer de Glace in size is well known as the largest glacier in
Switzerland, although in length it is exceeded by the Aletsch Glacier.

ICE AND GLACIERS 217

It is formed from the snow fields that cover the heights directly
north of Mont Blanc, several of which, as the Grande Jorasse, the
Aiguille Verte (a, Figs. 105 and 106), the Aiguille du Geant (b),
Aiguille du Midi (c), and the Aiguille du Dru (d), are only 2,000
to 3,000 feet below that king of the European mountains. The snow
fields which lie on the slopes and in the basins between these moun-
tains collect in three principal currents, the Glacier du Geant, Glacier
de Lechaud, and Glacier du Talefre, which, ultimately united as
represented in the map, form the Mer de Glace; this stretches as an
ice current 2,600 to 3,000 feet in breadth down into the valley of
Chamouni, where a powerful stream, the Arveyron, bursts from its
lower end at k, and plunges into the Arve. The lowest precipice of
the Mer de Glace, which is visible from the valley of Chamouni, and
forms a large cascade of ice, is commonly called Glacier des Bois,
from a small village which lies below.

Most of the visitors at Chamouni only set foot on the lowest part
of the Mer de Glace from the inn at the Montanvert, and when they
are free from giddiness cross the glacier at this place to the little
house on the opposite side, the Chapeau (n). Although, as the map
shows, only a comparatively very small portion of the glacier is thus
seen and crossed, this way shows sufficiently the magnificent scenes,
and also the difficulties of a glacier excursion. Bolder wanderers
march upwards along the glacier to the Jardin, a rocky cliff clothed
with some vegetation, which divides the glacial current of the Glacier
du Talefre into two branches; and bolder still they ascend yet higher,
to the Col du Geant (11,000 feet above the sea), and down the Italian
side to the valley of Aosta.

The surface of the Mer de Glace shows four of the rocky walls
which we have designated as medial moraines. The first, nearest the
side of the glacier, is formed where the two arms of the Glacier du
Talefre unite at the lower end of the Jardin; the second proceeds
from the union of the glacier in question with the Glacier de
Lechaud; the third, from the union of the last with the Glacier du
Geant; and the fourth, finally, from the top of the rock ledge which
stretches from the Aiguille du Geant towards the cascade (g) of
the Glacier du Geant.

To give you an idea of the slope and the fall of the glacier, I have

2l8 HELMHOLTZ

given in Fig. io6 a longitudinal section of it according to the levels
and measurements taken by Forbes, with the view of the right bank
of the glacier. The letters stand for the same objects as in Fig. 105;
p is the Aiguille de Lechaud, q the Aiguille Noire, r the Mont Tacul,
f is the Col du Geant, the lowest point in the high wall of rock that
surrounds the upper end of the snow fields which feed the Mer de
Glace. The base line corresponds to a length of a little more than

^^n</f

'eJorn

#!

^"l^ii.*

#«'^^S*"^fM

Fig. 105

nine miles : on the right the heights above the sea are given in feet.
The drawing shows very distinctly how small in most places is the
fall of the glacier. Only an approximate estimate could be made of
the depth, for hitherto nothing certain has been made out in refer-
ence to it. But that it is very deep is obvious from the following
individual and accidental observations.

At the end of a vertical rock wall of the Tacul, the edge of the
Glacier du Geant is pushed forth, forming an ice wall 140 feet in
height. This would give the depth of one of the upper arms of the
glacier at the edge. In the middle and after the union of the three

ICE AND GLACIERS 219

glaciers the depth must be far greater. Somewhat below the junction
Tyndall and Hirst sounded a moulin, that is, a cavity through which
the surface glacier waters escape, to a depth of 160 feet; the guides
alleged that they had sounded a similar aperture to a depth of 350
feet, and had found no bottom. From the usually deep trough-
shaped or gorge-like form of the bottom of the valleys, which is con-
structed solely of rock walls, it seems improbable that for a breadth
of 3,000 feet the mean depth should only be 350 feet; moreover, from
the manner in which ice moves, there must necessarily be a very thick
coherent mass beneath the crevassed part.

To render these magnitudes more intelligible by reference to more
familiar objects, imagine the valley of Heidelberg filled with ice up

a ,

4 A. P A^

Fig. 106

to the Molkencur, or higher, so that the whole town, with all its
steeples and the castle, is buried deeply beneath it; if, further, you
imagine this mass of ice, gradually extending in height, continued
from the mouth of the valley up to Neckargemiind, that would about
correspond to the lower united ice current of the Mer de Glace. Or,
instead of the Rhine and the Nahe at Bingen, suppose two ice
currents united which fill the Rhine valley to its upper border as
far as we can see from the river, and then the united currents stretch-
ing downwards to beyond Asmannshausen and Burg Rheinstein;
such a current would also about correspond to the size of the Mer
de Glace.

Fig. 107, which is a view of the magnificent Gorner Glacier seen
from below, also gives an idea of the size of the masses of ice of the
larger glaciers.

The surface of most glaciers is dirty, from the numerous pebbles
and sand which lie upon it, and which are heaped together the more
the ice under them and among them melts away. The ice of the

220

HELMHOLTZ

surface has been partially destroyed and rendered crumbly. In the
depths of the crevasses ice is seen of a purity and clearness with
which nothing that we are acquainted with on the plains can be
compared. From its purity it shows a splendid blue, like that of
the sky, only with a greenish hue. Crevasses in which pure ice is
visible in the interior occur of all sizes; in the beginning they form
slight cracks in which a knife can scarcely be inserted; becoming
gradually enlarged to chasms, hundreds or even thousands, of feet

Fig. 107

in length, and twenty, fifty, and as much as a hundred feet in breadth,
while some of them are immeasurably deep. Their vertical dark
blue walls of crystal ice, glistening with moisture from the trickling
water, form one of the most splendid spectacles which nature can
present to us; but, at the same time, a spectacle strongly impregnated
with the excitement of danger, and only enjoyable by the traveller
who feels perfectly free from the slightest tendency to giddiness. The
tourist must know how, with the aid of well-nailed shoes and a
pointed Alpenstock, to stand even on slippery ice, and at the edge
of a vertical precipice the foot of which is lost in the darkness of
night, and at an unknown depth. Such crevasses cannot always be
evaded in crossing the glacier; at the lower part of the Mer de Glace,
for instance, where it is usually crossed by travellers, we are com-
pelled to travel along some extent of precipitous banks of ice which

ICE AND GLACIERS 221

are occasionally only four to six feet in breadth, and on each side
of which is such a blue abyss. Many a traveller, who has crept along
the steep rocky slopes without fear, there feels his heart sink, and
cannot turn his eyes from the yawning chasm, for he must first
carefully select every step for his feet. And yet these blue chasms,
which lie open and exposed in the daylight, are by no means the
worst dangers of the glacier; though, indeed, we are so organised
that a danger which we perceive, and which therefore we can safely
avoid, frightens us far more than one which we know to exist, but
which is veiled from our eyes. So also it is with glacier chasms. In
the lower part of the glacier they yawn before us, threatening death
and destruction, and lead us, timidly collecting all our presence of
mind, to shrink from them; thus accidents seldom occur. On the
upper part of the glacier, on the contrary, the surface is covered with
snow; this, when it falls thickly, soon arches over the narrower crev-
asses of a breadth of from four to eight feet, and forms bridges which
quite conceal the crevasse, so that the traveller only sees a beautiful
plane snow surface before him. If the snow bridges are thick enough,
they will support a man; but they are not always so, and these are
the places where men, and even chamois, are so often lost. These
dangers may readily be guarded against if two or three men are
roped together at intervals of ten or twelve feet. If then one of them
falls into a crevasse, the two others can hold him, and draw him out
again.

In some places the crevasses may be entered, especially at the lower
end of a glacier. In the well-known glaciers of Grindelwald, Rosen-
laui, and other places, this is facilitated by cutting steps and arrang-
ing wooden planks. Then any one who does not fear the perpetually
trickling water may explore these crevasses, and admire the wonder-
fully transparent and pure crystal walls of these caverns. The
beautiful blue colour which they exhibit is the natural colour of
perfectly pure water; liquid water as well as ice is blue, though to an
extremely small extent, so that the colour is only visible in layers of
from ten to twelve feet in thickness. The water of the Lake of
Geneva and of the Lago di Garda exhibits the same splendid colour
as ice.

The glaciers are not everywhere crevassed; in places where the ice

222

HELMHOLTZ

meets with an obstacle, and in the middle of great glacier currents
the motion of which is uniform, the surface is perfectly coherent.

Fig. io8 represents one of the more level parts of the Mer de Glace
at the Montanvert, the little house of which is seen in the back-

FiG. io8

ground. The Gries Glacier, where it forms the height of the pass
from the Upper Rhone valley to the Tosa valley, may even be crossed
on horseback. We find the greatest disturbance of the surface of the
glacier in those places where it passes from a slightly inclined part
of its bed to one where the slope is steeper. The ice is there torn in
all directions into a quantity of detached blocks, which by melting

ICE AND GLACIERS 223

are usually changed into wonderfully shaped sharp ridges and pyra-
mids, and from time to time fall into the interjacent crevasses with a
loud rumbling noise. Seen from a distance such a place appears like
a wild frozen waterfall, and is therefore called a cascade; such a
cascade is seen in the Glacier du Talefre at I, another is seen in the

109

Glacier du Geant at g, Fig, iio, while a third forms the lower end
of the Mer de Glace. The latter, already mentioned as the Glacier
des Bois, which rises directly from the trough of the valley at
Chamouni to a height of 1,700 feet, the height of the Konigstuhl at
Heidelberg, affords at all times a chief object of admiration to the
Chamouni tourist. Fig 109 represents a view of its fantastically rent
blocks of ice.

224 HELMHOLTZ

We have hitherto compared the glacier with a current as regards
its outer form and appearance. This similarity, however, is not
merely an external one: the ice of the glacier does, indeed, move
forwards like the water of a stream, only more slowly. That this
must be the case follows from the considerations by which I have
endeavoured to explain the origin of a glacier. For as the ice is
being constantly diminished at the lower end by melting, it would
entirely disappear if fresh ice did not continually press forward from
above, which, again, is made up by the snowfalls on the mountain
tops.

But by careful ocular observation we may convince ourselves that
the glacier does actually move. For the inhabitants of the valleys,
who have the glaciers constantly before their eyes, often cross them,
and in so doing make use of the larger blocks of stone as sign posts —
detect this motion by the fact that their guide posts gradually descend
in the course of each year. And as the yearly displacement of the
lower half of the Mer de Glace at Chamouni amounts to no less than
from 400 to 600 feet, you can readily conceive that such displacements
must ultimately be observed, notwithstanding the slow rate at which
they take place, and in spite of the chaotic confusion of crevasses and
rocks which the glacier exhibits.

Besides rocks and stones, other objects which have accidentally
alighted upon the glacier are dragged along. In 1788 the celebrated
Genevese Saussure, together with his son and a company of guides
and porters, spent sixteen days on the Col du Geant. On descending
the rocks at the side of the cascade of the Glacier du Geant, they left
behind them a wooden ladder. This was at the foot of the Aiguille
Noire, where the fourth band of the Mer de Glace begins; this line
thus marks at the same time the direction in which ice travels from
this point. In the year 1832, that is, forty-four years after, fragments
of this ladder were found by Forbes and other travellers not far
below the junction of the three glaciers of the Mer de Glace, in the
same line (at s. Fig. no), from which it results that these parts of
the glacier must on the average have each year descended 375 feet.

In the year 1827 Hugi had built a hut on the central moraine of
the Unteraar Glacier for the purpose of making observations; the
exact position of this hut was determined by himself and afterwards

ICE AND GLACIERS 225

by Agassiz, and they found that each year it had moved downwards.
Fourteen years later, in the year 1841, it was 4,884 feet lower, so that
every year it had on the average moved through 349 feet. Agassiz
afterwards found that his own hut, which he had erected on the
same glacier, had moved to a somewhat smaller extent. For these
observations a long time was necessary. But if the motion of the
glacier be observed by means of accurate measuring instruments,

Fig. no

such as theodolites, it is not necessary to wait for years to observe
that ice moves — a single day is sufficient.

Such observations have in recent times been made by several
observers, especially by Forbes and by Tyndall. They show that in
summer the middle of the Mer de Glace moves through twenty
inches a day, while towards the lower terminal cascade the motion
amounts to as much as thirty-five inches in a day. In winter the
velocity is only about half as great. At the edges and in the lower
layers of the glacier, as in a flow of water, it is considerably smaller
than in the centre of the surface.

226 HELMHOLTZ

The upper sources of the Mer de Glace also have a slower motion,
the Glacier du Geant thirteen inches a day, and the Glacier du
Lechaud nine inches and a half. In different glaciers the velocity is
in general very various, according to the size, the inclination, the
amount of snowfall, and other circumstances.

Such an enormous mass of ice thus gradually and gently moves on,
imperceptibly to the casual observer, about an inch an hour — the ice
of the Col du Geant will take 120 years before it reaches the lower
end of the Mer de Glace — but it moves forward with uncontrollable
force, before which any obstacles that man could oppose to it yield
like straws, and the traces of which are distinctly seen even on the
granite walls of the valley. If, after a series of wet seasons, and an
abundant fall of snow on the heights, the base of a glacier advances,
not merely does it crush dwelling houses, and break the trunks of
powerful trees, but the glacier pushes before it the boulder walls
which form its terminal moraine without seeming to experience any
resistance. A truly magnificent spectacle is this motion, so gentle
and so continuous, and yet so powerful and so irresistible.

I will mention here that from the way in which the glacier moves
we can easily infer in what places and in what directions crevasses
will be formed. For as all layers of the glacier do not advance with
equal velocity, some points remain behind others; for instance, the
edges as compared with the middle. Thus if we observe the distance
from a given point at the edge to a given point of the middle, both
of which were originally in the same line, but the latter of which
afterwards descended more rapidly, we shall find that this distance
continually increases; and since the ice cannot expand to an extent
corresponding to the increasing distance, it breaks up and forms
crevasses, as seen along the edge of the glacier in Fig. hi, which
represents the Gorner Glacier at Zermatt. It would lead me too far
if I were here to attempt to give a detailed explanation of the forma-
tion of the more regular system of crevasses, as they occur in certain
parts of all glaciers; it may be sufficient to mention that the con-
clusions deducible from the considerations above stated are fully
borne out by observation.

I will only draw attention to one point — what extremely small
displacements are su£Scient to cause ice to form hundreds of crevasses.

ICE AND GLACIERS 227

The section of the Mer de Glace (Fig. 112, at g, c, h) shows places
where a scarcely perceptible change in the inclination of the surface
of the ice occurs of from two to four degrees. This is sufficient to
produce a system of cross crevasses on the surface. Tyndall more
especially has urged and confirmed by observation and measure-
ments, that the mass of ice of the glacier does not give way in the

Fig. Ill

smallest degree to extension, but when subjected to a pull is invari-
ably torn asunder.

The distribution of the boulders, too, on the surface of the glacier
is readily explained when we take their motion into account. These
boulders are fragments of the mountains between which the glacier
flows. Detached partly by the weathering of the stone, and partly by
the freezing of water in its crevices, they fall, and for the most part
on the edge of the mass of ice. There they either remain lying on
the surface, or if they have originally burrowed in the snow, they
ultimately reappear in consequence of the melting of the super-
ficial layers of ice and snow, and they accumulate especially at the
lower end of the glacier, where more of the ice between them has

228 HELMHOLTZ

been melted. The blocks which are gradually borne down to the
lower end of the glacier are sometimes quite colossal in size. Solid
rocky masses of this kind are met with in the lateral and terminal
moraines, which are as large as a two-storied house.

The masses of stone move in lines which are always nearly parallel
to each other and to the longitudinal direction of the glacier. Those,
therefore, that are already in the middle remain in the middle, and
those that lie on the edge remain at the edge. These latter are the
more numerous, for during the entire course of the glacier fresh
boulders are constantly falling on the edge, but cannot fall on the
middle. Thus are formed on the edge of the mass of ice the lateral
moraines, the boulders of which partly move along with the ice,
partly glide over its surface, and partly rest on the solid rocky base

oo««r

000'

Fig. 112

near the ice. But when two glacier streams unite, their coinciding
lateral moraines come to lie upon the centre of the united ice-stream,
and then move forward as central moraines parallel to each other
and to the banks of the stream, and they show, as far as the lower
end, the boundary line of the ice which originally belonged to one or
the other of the arms of the glacier. They are very remarkable as
displaying in what regular parallel bands the adjacent parts of the
ice-stream glide downwards. A glance at the map of the Mer de
Glace, and its four central moraines, exhibits this very distinctly.

On the Glacier du Geant and its continuation in the Mer de Glace,
the stones on the surface of the ice delineate, in alternately greyer
and whiter bands, a kind of yearly rings which were first observed
by Forbes. For since in the cascade at g, Fig. 112, more ice slides
down in summer than in winter, the surface of the ice below the
cascade forms a series of terraces as seen in the drawing, and as those
slopes of the terraces which have a northern aspect melt less than

ICE AND GLACIERS 229

their upper plane surfaces, the former exhibit purer ice than the lat-
ter. This, according to Tyndall, is the probable origin of these dirt
bands. At first they run pretty much across the glacier, but as after-
wards their centre moves somewhat more rapidly than the ends, they
acquire farther down a curved shape, as represented in the map,
Fig. 1 10. By their curvature they thus show to the observer with
what varying velocity ice advances in the different parts of its course.

A very peculiar part is played by certain stones which are imbedded
in the lower surface of the mass of ice, and which have partly fallen
there through crevasses, and may partly have been detached from
the bottom of the valley. For these stones are gradually pushed with
the ice along the base of the valley, and at the same time are pressed
against this base by the enormous weight of the superincumbent ice.
Both the stones imbedded in the ice as well as the rocky base are
equally hard, but by their friction against each other they are ground
to powder with a power compared to which any human exertion of
force is infinitely small. The product of this friction is an extremely
fine powder, which, swept away by water, appears lower down in
the glacier brook, imparting to it a whitish or yellowish muddy
appearance.

The rocks of the trough of the valley, on the contrary, on which
the glacier exerts year by year its grinding power, are polished as if
in an enormous polishing machine. They remain as rounded,
smoothly polished masses, in which are occasional scratches pro-
duced by individual harder stones. Thus we see them appear at the
edge of existing glaciers, when after a series of dry and hot seasons
the glaciers have somewhat receded. But we find such polished rocks
as remains of gigantic ancient glaciers to a far greater extent in the
lower parts of many Alpine valleys. In the valley of the Aar more
especially, as far down as Meyringen, the rock-walls polished to a
considerable height are very characteristic. There also we find the
celebrated polished plates, over which the way passes, and which are
so smooth that furrows have had to be hewn into them and rails
erected to enable men and animals to traverse them in safety.

The former enormous extent of glaciers is recognised by ancient
moraine-dykes and by transported blocks of stone, as well as by these
polished rocks. The blocks of stone which have been carried away

230 HELMHOLTZ

by the glacier are distinguished from those which water has rolled
down, by their enormous magnitude, by the perfect retention of all
their edges which are not at all rounded off, and finally by their
being deposited on the glacier in exactly the same order in which the
rocks of which they formed part stand in the mountain ridge; while
the stones which currents of water carry along are completely mixed
together.

From these indications, geologists have been able to prove that
the glaciers of Chamouni, of Monte Rosa, of the St. Gotthard, and
the Bernese Alps, formerly penetrated through the valley of the
Arve, the Rhone, the Aar, and the Rhine to the more level part of
Switzerland and the Jura, where they have deposited their boulders
at a height of more than a thousand feet above the present level of
the lake of Neufchatel. Similar traces of ancient glaciers are found
upon the mountains of the British Islands, and upon the Scandi-
navian Peninsula.

The drift ice too of the Arctic Sea is glacier ice; it is pushed down
into the sea by the glaciers of Greenland, becomes detached from the
rest of the glacier, and floats away. In Switzerland we find a similar
formation of drift ice, though on a far smaller scale, in the little
Marjelen See, into which part of the ice of the great Aletsch Glacier
pushes down. Blocks of stone which lie in drift ice may make long
voyages over the sea. The vast number of blocks of granite which
are scattered on the North German plains, and whose granite belongs
to the Scandinavian mountains, has been transported by drift ice at
the time when the European glaciers had such an enormous extent.

I must unfortunately content myself with these few references to
the ancient history of glaciers, and revert now to the processes at
present at work in them.

From the facts which I have brought before you it results that the
ice of a glacier flows slowly like the current of a very viscous sub-
stance, such for instance as honey, tar, or thick magma of clay. The
mass of ice does not merely flow along the ground like a solid which
glides over a precipice, but it bends and twists in itself; and although
even while doing this it moves along the base of the valley, yet the
parts which are in contact with the bottom and the sides of the valley
are perceptibly retarded by the powerful friction; the middle of the

ICE AND GLACIERS 23 1

surface of the glacier, which is most distant both from the bottom
and the sides, moving most rapidly. Rendu, a Savoyard priest, and
the celebrated natural philosopher Forbes, were the first to suggest
the similarity of a glacier with a current of a viscous substance.

Now you will perhaps inquire with astonishment how it is possible
that ice, which is the most brittle and fragile of substances, can flow
in the glacier like a viscous mass; and you may perhaps be disposed
to regard this as one of the wildest and most improbable statements
that have ever been made by philosophers. I will at once admit that
philosophers themselves were not a little perplexed by these results
of their investigations. But the facts were there, and could not be
got rid of. How this mode of motion originated was for a long time
quite enigmatical, the more so since the numerous crevasses in gla-
ciers were a sufficient indication of the well-known brittleness of
ice; and as Tyndall correctly remarked, this constituted an essential
difference between a stream of ice and the flow of lava, of tar, of
honey, or of a current of mud.

The solution of this strange problem was found, as is so often the
case in the natural sciences, in apparently recondite investigations
into the nature of heat, which form one of the most important con-
quests of modern physics, and which constitute what is known as
the mechanical theory of heat. Among a great number of deductions
as to the relations of the diverse natural forces to each other, the
principles of the mechanical theory of heat lead to certain con-
clusions as to the dependence of the freezing point of water on the
pressure to which ice and water are exposed.

Every one knows that we determine that one fixed point of our
thermometer scale which we call the freezing point or zero by plac-
ing the thermometer in a mixture of pure water and ice. Water, at
any rate when in contact with ice, cannot be cooled below zero with-
out itself being converted into ice; ice cannot be heated above
the freezing point without melting. Ice and water can exist in
each other's presence at only one temperature, the temperature of
zero.

Now, if we attempt to heat such a mixture by a flame beneath it,
the ice melts, but the temperature of the mixture is never raised above
that of 0° so long as some of the ice remains unmelted. The heat

232 HELMHOLTZ

imparted changes ice at zero into water at zero, but the thermom-
eter indicates no increase of temperature. Hence physicists say that
heat has become latent, and that water contains a certain quantity of
latent heat beyond that of ice at the same temperature.

On the other hand, when we withdraw more heat from the mix-
ture of ice and water, the water gradually freezes; but as long as
there is still liquid water, the temperature remains at zero. Water
at 0° has given up its latent heat, and has become changed into
ice at 0°.

Now a glacier is a mass of ice which is everywhere interpenetrated
by water, and its internal temperature is therefore everywhere that
of the freezing point. The deeper layers, even of the fields of neve,
appear on the heights which occur in our Alpine chain to have every-
where the same temperature. For, though the freshly fallen snow of
these heights is, for the most part, at a lower temperature than that
of 0°, the first hours of warm sunshine melt its surface and form
water, which trickles into the deeper colder layers, and there freezes,
until it has throughout been brought to the temperature of the
freezing point. This temperature then remains unchanged. For,
though by the sun's rays the surface of the ice may be melted, it
cannot be raised above zero, and the cold of winter penetrates as
little into the badly conducting masses of snow and ice as it does
into our cellars. Thus the interior of the masses of neve, as well as
of the glacier, remains permanently at the melting point.

But the temperature at which water freezes may be altered by
strong pressure. This was first deduced from the mechanical theory
of heat by James Thomson of Belfast, and almost simultaneously by
Clausius of Ziirich; and, indeed, the amount of this change may be
correctly predicted from the same reasoning. For each increase of a
pressure of one atmosphere the freezing point is lowered by the
i-ii5th part of a degree Centigrade. The brother of the former. Sir
W. Thomson, the celebrated Glasgow physicist, made an experi-
mental confirmation of this theoretical deduction by compressing in
a suitable vessel a mixture of ice and snow. This mixture became
colder and colder as the pressure was increased, and to the extent
required by the mechanical theory.

Now, if a mixture of ice and water becomes colder when it is

ICE AND GLACIERS 233

subjected to increased pressure without the withdrawal of heat, this
can only be effected by some free heat becoming latent; that is, some
ice in the mixture must melt and be converted into water. In this is
found the reason why mechanical pressure can influence the freezing
point. You know that ice occupies more space than the water from
which it is formed. When water freezes in closed vessels, it can
burst not only glass vessels, but even iron shells. Inasmuch, there-
fore, as in the compressed mixture of ice and water some of the ice
melts and is converted into water, the volume of the mass diminishes,
and the mass can yield more to the pressure upon it than it could
have done without such an alteration of the freezing point. Pressure
furthers in this case, as is usual in the interaction of various natural
forces, the occurrence of a change, that is fusion, which is favourable
to the development of its own activity.

In Sir W. Thomson's experiments water and ice were confined in
a closed vessel from which nothing could escape. The case is some-
what different when, as with glaciers, the water disseminated in the
compressed ice can escape through fissures. The ice is then com-
pressed, but not the water which escapes. The compressed ice
becomes colder in conformity with the lowering of its freezing point
by pressure; but the freezing point of water which is not compressed
is not lowered. Thus under these circumstances we have ice colder
than 0° in contact with water at 0°. The consequence is that around
the compressed ice water continually freezes and forms new ice,
while on the other hand part of the compressed ice melts.

This occurs, for instance, when only two pieces of ice are pressed
against each other. By the water which freezes at their surfaces of
contact they are firmly joined into one coherent piece of ice. With
powerful pressure, and the chilling therefore great, this is quickly
effected; but even with a feeble pressure it takes place, if sufficient
time be given. Faraday, who discovered this property, called it the
regelation of ice; the explanation of this phenomenon has been much
controverted; I have detailed to you that which I consider most
satisfactory.

This freezing together of two pieces of ice is very readily effected
by pieces of any shape, which must not, however, be at a lower
temperature than 0°, and the experiment succeeds best when the

234 HELMHOLTZ

pieces are already in the act of melting.' They need only be strongly
pressed together for a few minutes to make them adhere. The more
plane are the surfaces in contact, the more complete is their union.
But a very slight pressure is sufficient if the two pieces are left in
contact for some time.^

This property of melting ice is also utilised by boys in making
snowballs and snowmen. It is well known that this only succeeds
either when the snow is already melting, or at any rate is only so
much lower than o° that the warmth of the hand is sufficient to
raise it to this temperature. Very cold snow is a dry loose powder
which does not stick together.

The process which children carry out on a small scale in making
snowballs takes place in glaciers on the very largest scale. The
deeper layers of what was originally line loose neve are compressed
by the huge masses resting on them, often amounting to several
hundred feet, and under this pressure they cohere with an ever
firmer and closer structure. The freshly fallen snow originally con-
sisted of delicate microscopically fine ice spicules, united and forming
delicate six-rayed, feathery stars of extreme beauty. As often as the
upper layers of the snow fields are exposed to the sun's rays, some
of the snow melts; water permeates the mass, and on reaching the
lower layers of still colder snow, it again freezes; thus it is that the
firn first becomes granular and acquires the temperature of the
freezing point. But as the weight of the superincumbent masses of
snow continually increases by the firmer adherence of its individual
granules, it ultimately changes into a dense and perfectly hard mass.

This transformation of snow into ice may be artificially effected
by using a corresponding pressure.

We have here (Fig. 113) a cylindrical cast iron vessel, A A; the
base, B B, is held by three screws, and can be detached, so as to
remove the cylinder of ice which is formed. After the vessel has lain
for a while in ice water, so as to reduce it to the temperature of 0°,
it is packed full of snow, and then the cylindrical plug, C C, which
fits the inner aperture, but moves in it with gentle friction, is forced

' In the Lecture a series of small cylinders of ice, which had been prepared by a
method to be afterwards described, were pressed with their plane ends against each
other, and thus a cylindrical bar of ice produced.

^ Vide the additions at the end of this Lecture.

ICE AND GLACIERS 235

in with the aid of an hydraulic press. The press used was such that

the pressure to which the snow was exposed could be increased to

fifty atmospheres. Of course the looser snow contracts to a very

small volume under such a powerful pressure. The pressure is

removed, the cylindrical plug

taken out, the hollow again filled

up with snow, and the process re-

pea,ted until the entire form is

filled with the mass of ice, which

no longer gives way to pressure.

The compressed snow which I

now take out, you will see, has

been transformed into a hard,

angular, and translucent cylinder

of ice; and how hard it is appears

from the crash which ensues when

I throw it to the ground. Just as

the loose snow in the glaciers is

pressed together to solid ice, so

also in many places ready-formed

irregular pieces of ice are joined

and form clear and compact ice.

This is most remarkable at the

base of the glacier cascades. These

are glacier falls where the upper

part of the glacier ends at a steep rocky wall, and blocks of ice shoot

down as avalanches over the edge of this wall. The heap of shattered

blocks of ice which accumulate become joined at the foot of the

rock-wall to a compact, dense mass, which then continues its way

downwards as glacier. More frequent than such cascades, where

the glacier-stream is quite dissevered, are places where the base of

the valley has a steeper slope, as, for instance, the places in the Mer

de Glace (Fig. 105), at g, of the Cascade of the Glacier du Geant,

and at i and h of the great terminal cascade of the Glacier des Bois.

The ice splits there into thousands of banks and clifJs, which then

recombine towards the bottom of the steeper slope and form a

coherent mass.

Fig. 113

236 HELMHOLTZ

This also we may imitate in our ice mould. Instead of the snow
I take irregular pieces of ice, press them together; add new pieces of
ice, press them again, and so on, until the mould is full. When the
mass is taken out it forms a compact coherent cylinder of tolerably
clear ice, which has a perfectly sharp edge, and is an accurate copy
of the mould.

This experiment, which was first made by Tyndall, shows that a
block of ice may be pressed into any mould just like a piece of wax.
It might, perhaps, be thought that such a block had, by the pressure
in the interior, been first reduced to powder so fine that it readily
penetrated every crevice of the mould, and then that this powdered
ice, like snow, was again combined by freezing. This suggests itself
the more readily, since while the press is being worked a continual
creaking and cracking is heard in the interior of the mould. Yet the
mere aspect of the cylinders pressed from blocks of ice shows us that
it has not been formed in this manner; for they are generally clearer
than the ice which is produced from snow, and the individual larger
pieces of ice which have been used to produce them are recognised,
though they are somewhat changed and flattened. This is most
beautiful when clear pieces of ice are laid in the form and the rest
of the space stuffed full of snow. The cylinder is then seen to con-
sist of alternate layers of clear and opaque ice, the former arising
from the pieces of ice, and the latter from the snow; but here also
the pieces of ice seem pressed into flat discs.

These observations teach, then, that ice need not be completely
smashed to fit into the prescribed mould, but that it may give way
without losing its coherence. This can be still more completely
proved, and we can acquire a still better insight into the cause of the
pliability of ice, if we press the ice between two plane wooden boards,
instead of in the mould, into which we cannot see.

I place first an irregular cylindrical piece of natural ice, taken from
the frozen surface of the river, with its two plane terminal surfaces
between the plates of the press. If I begin to work, the block is
broken by pressure; every crack which forms extends through the
entire mass of the block; this splits into a heap of larger fragments,
which again crack and are broken the more the press is worked. If
the pressure is relaxed, all these fragments are, indeed, reunited by

ICE AND GLACIERS 237

freezing, but the aspect of the whole indicates that the shape of the
block has resulted less from pliability than from fracture, and that
the individual fragments have completely altered their mutual
positions.

The case is quite different when one of the cylinders which we
have formed from snow or ice is placed between the plates of the

Fig. 114 Fig. 115

press. As the press is worked the creaking and cracking is heard, but
it does not break; it gradually changes its shape, becomes lower and
at the same time thicker; and only when it has been changed into a
tolerably flat circular disc does it begin to give way at the edges and
form cracks, like crevasses on a small scale. Fig. 114 shows the height
and diameter of such a cylinder in its original condition; Fig. 115
represents its appearance after the action of the press.

A still stronger proof of the pliability of ice is afforded when one
of our cylinders is forced through a narrow aperture. With this
view I place a base on the previously described mould, which has a
conical perforation, the external aperture of which is only two thirds
the diameter of the cylindrical aperture of the form. Fig. 116 gives
a section of the whole. If now I insert into this one of the com-
pressed cylinders of ice, and force down the plug a, the ice is forced
through the narrow aperture in the base. It at first emerges as a
solid cylinder of the same diameter as the aperture; but as the ice
follows more rapidly in the centre than at the edges, the free terminal

238 HELMHOLTZ

surface of the cylinder becomes curved, the end thickens, so that it
could not be brought back through the aperture, and it ultimately

splits off. Fig. 117 exhibits a series
of shapes which have resulted in this
manner.'

Here also the cracks in the emerg-
ing cylinder of ice exhibit a surpris-
ing similarity with the longitudinal
rifts which divide a glacier current
where it presses through a narrow
rocky pass into a wider valley.

In the cases which we have de-
scribed we see the change in shape of
the ice taking place before our eyes,
whereby the block of ice retains its
coherence without breaking into individual pieces. The brittle mass
of ice seems rather to yield like a piece of wax.

A closer inspection of a clear cylinder of ice compressed from clear
pieces of ice, while the pressure is being applied, shows us what takes
place in the interior; for we then see an innumerable quantity of
extremely fine radiating cracks shoot through it like a turbid cloud,
which mostly disappear, though not completely, the moment the

Fig. 116

a

Fig. 117

pressure is suspended. Such a compressed block is distinctly more
opaque immediately after the experiment than it was before; and
the turbidity arises, as may easily be observed by means of a lens,
from a great number of whitish capillary lines crossing the interior

'In this experiment the lower temperature of the compressed ice sometimes
extended so far through the iron form, that the water in the slit between the base
plate and the cylinder froze and formed a thin sheet of ice, although the pieces of
ice as well as the iron mould had previously laid in ice water, and could not be
colder than 0°.

ICE AND GLACIERS 239

of the mass of what is otherwise clear. These Unes are the optical
expression of extremely fine cracks^ which interpenetrate the mass
of the ice. Hence we may conclude that the compressed block is
traversed by a great number of fine cracks and fissures which render
it pliable; that its particles become a little dispersed, and are there-
fore withdrawn from pressure, and that immediately afterwards the
greater part of the fissures disappear, owing to their sides freezing.
Only in those places in which the surfaces of the small displaced
particles do not accurately fit to each other some fissured spaces
remain open, and are discovered as white lines and surfaces by the
reflection of the light.

These cracks and lamina also become more perceptible when the
ice — which, as I before mentioned, is below zero immediately after
pressure has been applied — is again raised to this temperature and
begins to melt. The crevices then fill with water, and such ice then
consists of a quantity of minute granules from the size of a pin's
head to that of a pea, which are closely pushed into one another at
the edges and projections, and in part have coalesced, while the nar-
row fissures between them are full of water. A block of ice thus
formed of ice granules adheres firmly together; but if particles be
detached from its corners they are seen to consist of these angular
granules. Glacier ice, when it begins to melt, is seen to possess the
same structure, except that the pieces of which it consists are mostly
larger than in artificial ice, attaining the size of a pigeon's egg.

Glacier ice and compressed ice are thus seen to be substances of a
granular structure, in opposition to regularly crystallised ice, such as
is formed on the surface of still water. We here meet with the same
differences as between calcareous spar and marble, both of which
consist of carbonate of lime; but while the former is in large, regular
crystals, the latter is made up of irregularly agglomerated crystalline
grains. In calcareous spar, as well as in crystallised ice, the cracks

* These cracks are probably quite empty and free from air, for they are also formed
when perfectly clear and air-free pieces of ice are pressed in the form which has
been previously filled with water, and where, therefore, no air could gain access to
the pieces of ice. That such air-free crevices occur in glacier ice has been already
demonstrated by Tyndall. When the compressed ice afterwards melts, these crevices
fill up with water, no air being left. They are then, however, far less visible, and the
whole block is therefore clearer. And just for this reason they could not originally
have been filled with water.

240 HELMHOLTZ

produced by inserting the point o£ a knife extend through the mass,
while in granular ice a crack which arises in one of the bodies where
it must yield does not necessarily spread beyond the limits of the
granule.

Ice which has been compressed from snow, and has thus from the
outset consisted of innumerable very fine crystalline needles, is seen
to be particularly plastic. Yet in appearance it materially differs from
glacier ice, for it is very opaque, owing to the great quantity of air
which was originally inclosed in the flaky mass of snow, and which
remains there as extremely minute bubbles. It can be made clearer
by pressing a cylinder of such ice between wooden boards; the air
bubbles appear then on the top of the cylinder as a light foam. If
the discs are again broken, placed in the mould, and pressed into a
cylinder, the air may gradually be more and more eliminated, and
the ice be made clearer. No doubt in glaciers the originally whitish
mass of neve is thus gradually transformed into the clear, trans-
parent ice of the glacier.

Lastly, when streaked cylinders of ice formed from pieces of snow
and ice are pressed into discs, they become finely streaked, for both
their clear and their opaque layers are uniformly extended.

Ice thus striated occurs in numerous glaciers, and is no doubt
caused, as Tyndall maintains, by snow falling between the blocks
of ice; this mixture of snow and clear ice is again compressed in the
subsequent path of the glacier, and gradually stretched by the motion
of the mass: a process quite analogous to the artificial one which we
have demonstrated.

Thus to the eye of the natural philosopher the glacier, with its
wildly heaped ice blocks, its desolate, stony, and muddy surface, and
its threatening crevasses, has become a majestic stream whose peace-
ful and regular flow has no parallel; which according to fixed and
definite laws, narrows, expands, is heaped up, or, broken and shat-
tered, falls down precipitous heights. If we trace it beyond its
termination we see its waters, uniting to a copious brook, burst
through its icy gate and flow away. Such a brook, on emerging from
the glacier, seems dirty and turbid enough, for it carries away as
powder the stone which the glacier has ground. We are disenchanted
at seeing the wondrously beautiful and transparent ice converted

ICE AND GLACIERS 24 1

into such muddy water. But the water o£ the glacier streams is as
pure and beautiful as the ice, though its beauty is for the moment
concealed and invisible. We must search for these waters after they
have passed through a lake in which they have deposited this
powdered stone. The Lakes of Geneva, of Thun, of Lucerne, of
Constance, the Lago Maggiore, the Lake of Como, and the Lago
di Garda are chiefly fed with glacier waters; their clearness and their
wonderfully beautiful blue or blue-green colour are the delight of
all travellers.

Yet, leaving aside the beauty of these waters, and considering only
their utility, we shall have still more reason for admiration. The
unsightly mud which the glacier streams wash away forms a highly
fertile soil in the places where it is deposited; for its state of mechani-
cal division is extremely fine, and it is moreover an utterly unex-
hausted virgin soil, rich in the mineral food of plants. The fruitful
layers of fine loam which extend along the whole Rhine plain as far
as Belgium, and are known as Loess, are nothing more than the
dust of ancient glaciers.

Then, again, the irrigation of a district, which is effected by the
snow fields and glaciers of the mountains, is distinguished from that
of other places by its comparatively greater abundancy, for the moist
air which is driven over the cold mountain peaks deposits there most
of the water it contains in the form of snow. In the second place, the
snow melts most rapidly in summer, and thus the springs which flow
from the snow fields are most abundant in that season of the year
in which they are most needed.

Thus we ultimately get to know the wild, dead ice wastes from
another point of view. From them trickles in thousands of rills,
springs, and brooks the fructifying moisture which enables the
industrious dwellers of the Alps to procure succulent vegetation and
abundance of nourishment from the wild mountain slopes. On the
comparatively small surface of the Alpine chain they produce the
mighty streams the Rhine, the Rhone, the Po, the Adige, the Inn,
which for hundreds of miles form broad, rich river valleys, extending
through Europe to the German Ocean, the Mediterranean, the
Adriatic, and the Black Sea. Let us call to mind how magnificently
Goethe, in "Mahomet's Song," has depicted the course of the rocky

242 HELMHOLTZ

spring, from its origin beyond the clouds to its union with Father
Ocean. It would be presumptuous after him to give such a picture
in other than his own words: —

And along, in triumph rolling,
Names he gives to regions; cities
Grow amain beneath his feet.

On and ever on he rushes;
Spire and turret fiery crested
Marble palaces, the creatures
Of his wealth, he leaves behind.

Pine-buUt houses bears the Atlas
On his giant shoulders. O'er his
Head a thousand pennons rusde.
Floating far upon the breezes.
Tokens of his majesty.

And so beareth he his brothers,
And his treasures, and his children.
To their primal sire expectant,
All his bosom throbbing, heaving,
With a wild tumultuous joy.

Theodore Martin's Translation.

ADDITIONS

The theory of the regelation of ice has led to scientific discussions
between Faraday and Tyndall on the one hand, and James and Sir
W. Thomson on the other. In the text I have adopted the theory of
the latter, and must now accordingly defend it.

Faraday's experiments show that a very slight pressure, not more
than that produced by the capillarity of the layer of water between
two pieces of ice, is sufficient to freeze them together. James Thom-
son observed that in Faraday's experiments pressure which could
freeze them together was not utterly wanting. I have satisfied myself
by my own experiments that only very slight pressure is necessary. It
must, however, be remembered that the smaller the pressure the
longer will be the time required to freeze the two pieces, and that
then the junction will be very narrow and very fragile. Both these
points are readily explicable on Thomson's theory. For under a
feeble pressure the difference in temperature between ice and water
will be very small, and the latent heat will only be slowly abstracted
from the layers of water in contact with the pressed parts of the ice,
so that a long time is necessary before they freeze. We must further
take into account that we cannot in general consider that the two sur-
faces are quite in contact; under a feeble pressure which does not ap-
preciably alter their shape, they will only touch in what are practically
three points. A feeble total pressure on the pieces of ice concentrated
on such narrow surfaces will always produce a tolerably great local
pressure under the influence of which some ice will melt, and the
water thus formed will freeze. But the bridge which joins them will
never be otherwise than narrow.

Under stronger pressure, which may more completely alter the
shape of the pieces of ice, and fit them against each other, and which
will melt more of the surfaces that are first in contact, there will be
a greater difference between the temperature of the ice and water,
and the bridges will be more rapidly formed, and be of greater
extent.

243

^^^ t±t:,L,SS/Ltl{JL,ll,

In order to show the slow action of the small differences of tem-
perature which here come into play, I made the following experi-
ments.

A glass flask with a drawn-out neck was half filled with water,
which was boiled until all the air in the flask was driven out. The
neck of the flask was then hermetically sealed. When cooled, the
flask was void of air, and the water within it freed from the pressure
of the atmosphere. As the water thus prepared can be cooled con-
siderably below 0° C. before the first ice is formed, while when ice
is in the flask it freezes at o° C, the flask was in the first instance
placed in a freezing mixture until the water was changed into ice. It
was afterwards permitted to melt slowly in a place the temperature
of which was -\-2° C, until the half of it was liquefied.

The flask thus half filled with water, having a disc of ice swim-
ming upon it, was placed in a mixture of ice and water, being quite
surrounded by the mixture. After an hour, the disc within the flask
was frozen to the glass. By shaking the flask the disc was liberated,
but it froze again. This occurred as often as the shaking was
repeated.

The flask was permitted to remain for eight days in the mixture,
which was kept throughout at a temperature of o° C. During this
time a number of very regular and sharply defined ice crystals were
formed, and augmented very slowly in size. This is perhaps the
best method of obtaining beautifully formed crystals of ice.

While, therefore, the outer ice which had to support the pres-
sure of the atmosphere slowly melted, the water within the flask,
whose freezing point, on account of a defect of pressure, was 0.0075°
C. higher, deposited crystals of ice. The heat abstracted from the
water in this operation had, moreover, to pass through the glass of
the flask, which, together with the small difference of temperature,
explains the slowness of the freezing process.

Now as the pressure of one atmosphere on a square millimetre
amounts to about ten grammes, a piece of ice weighing ten grammes,
which lies upon another and touches it in three places, the total
surface of which is a square millimetre, will produce on these sur-
faces a pressure of an atmosphere. Ice will therefore be formed more
rapidly in the surrounding water than it was in the flask, where the

ICE AND GLACIERS 245

side of the glass was interposed between the ice and the water. Even
with a much smaller weight the same result will follow in the course
of an hour. The broader the bridges become, owing to the freshly
formed ice, the greater will be the surfaces over which the pressure
exerted by the upper piece of ice is distributed, and the feebler it
will become; so that with such feeble pressure the bridges can only
slowly increase, and therefore they will be readily broken when we
try to separate the pieces.

It cannot, moreover, be doubted that in Faraday's experiments, in
which two perforated discs of ice were placed in contact on a hori-
zontal glass rod, so that gravity exerted no pressure, capillary at-
traction is sufficient to produce a pressure of some grammes between
the plates, and the preceding discussions show that such a pressure,
if adequate time be given, can form bridges between the plates.

If, on the other hand, two of the above-described cylinders of ice
are powerfully pressed together by the hands, they adhere in a few
minutes so firmly that they can only be detached by the exertion of
a considerable force, for which indeed that of the hands is some-
times inadequate.

In my experiments I found that the force and rapidity with which
the pieces of ice united were so entirely proportional to the pressure
that I cannot but assign this as the actual and sufficient cause of
their union.

In Faraday's explanation, according to which regelation is due
to a contact action of ice and water, I find a theoretical difficulty. By
the water freezing, a considerable quantity of latent heat must be set
free, and it is not clear what becomes of this.

Finally, if ice in its change into water passes through an inter-
mediate viscous condition, a mixture of ice and water which was
kept for days at a temperature of 0° must ultimately assume this
condition in its entire mass, provided its temperature was uniform
throughout; this however is never the case.

As regards what is called the plasticity of ice, James Thomson has
given an explanation of it in which the formation of cracks in the
interior is not presupposed. No doubt when a mass of ice in dif-
ferent parts of the interior is exposed to different pressures, a por-
tion of the more powerfully compressed ice will melt; and the latent

246 HELMHOLTZ

heat necessary for this will be supplied by the ice which is less
strongly compressed, and by the water in contact with it. Thus ice
would melt at the compressed places, and water would freeze in
those which are not pressed: ice would thus be gradually transformed
and yield to pressure. It is also clear that, owing to the very small
conductivity for heat which ice possesses, a process of this kind must
be extremely slow, if the compressed and colder layers of ice, as in
glaciers, are at considerable distances from the less compressed ones,
and from the water which furnishes the heat for melting.

To test this hypothesis, I placed in a cylindrical vessel, between two
discs of ice of three inches in diameter, a smaller cylindrical piece of
an inch in diameter. On the uppermost disc I placed a wooden disc,
and this I loaded with a weight of twenty pounds. The section of
the narrow piece was thus exposed to a pressure of more than an
atmosphere. The whole vessel was packed between pieces of ice, and
left for five days in a room the temperature of which was a few
degrees above the freezing point. Under these circumstances the
ice in the vessel, which was exposed to the pressure of the weight,
should melt, and it might be expected that the narrow cylinder on
which the pressure was most powerful should have been most
melted. Some water was indeed formed in the vessel, but mostly at
the expense of the larger discs at the top and bottom, which being
nearest the outside mixture of ice and water, could acquire heat
through the sides of the vessel. A small welt, too, of ice, was formed
round the surface of contact of the narrower with the lower broad
piece, which showed that the water, which had been formed in con-
sequence of the pressure, had again frozen in places in which the
pressure ceased. Yet under these circumstances there was no appreci-
able alteration in the shape of the middle piece which was most
compressed.

This experiment shows that although changes in the shape of the
pieces of ice must take place in the course of time in accordance with
J. Thomson's explanation, by which the more strongly compressed
parts melt, and new ice is formed at the places which are freed from
pressure, these changes must be extremely slow when the thickness
of the pieces of ice through which the heat is conducted is at all con-
siderable. Any marked change in shape by melting in a medium the

ICE AND GLACIERS 247

temperature of which is everywhere 0°, could not occur without
access of external heat, or from the uncompressed ice and water; and
with the small differences in temperature which here come into play,
and from the badly conducting power of ice, it must be extremely
slow.

That on the other hand, especially in granular ice, the formation
of cracks, and the displacement of the surfaces of those cracks, ren-
der such a change of form possible, is shown by the above-described
experiments on pressure; and that in glacier ice changes of form thus
occur, follows from the banded structure, and the granular aggrega-
tion which is manifest on melting, and also from the manner in
which the layers change their position when moved, and so forth.
Hence, I doubt not that Tyndall has discovered the essential and
principal cause of the motion of glaciers, in referring it to the forma-
tion of cracks and to regelation.

I would at the same time observe that a quantity of heat, which
is far from inconsiderable, must be produced by friction in the larger
glaciers. It may be easily shown by calculation that when a mass of
firn moves from the Col du Geant to the source of the Arveyron, the
heat due to the mechanical work would be sufficient to melt a four-
teenth part of the mass. And as the friction must be greatest in those
places that are most compressed, it will at any rate be sufficient to
remove just those parts of the ice which offer most resistance to
motion.

I will add, in conclusion, that the above-described granular struc-
ture of ice is beautifully shown in polarised light. If a small clear
piece is pressed in the iron mould, so as to form a disc of about five
inches in thickness, this is sufficiently transparent for investigation.
Viewed in the polarising apparatus, a great number of variously
coloured small bands and rings are seen in the interior; and by the
arrangement of their colours it is easy to recognise the limits of the
ice granules, which, heaped on one another in irregular order of
their optical axes, constitute the plate. The appearance is essentially
the same when the plate has just been taken out of the press, and
the cracks appear in it as whitish lines, as afterwards when these
crevices have been filled up in consequence of the ice beginning to
melt.

248 HELMHOLTZ

In order to explain the continued coherence of the piece of ice
during its change of form, it is to be observed that in general the
cracks in the granular ice are only superficial, and do not extend
throughout its entire mass. This is directly seen during the pressing
of the ice. The crevices form and extend in different directions, like
cracks produced by a heated wire in a glass tube. Ice possesses a
certain degree of elasticity, as may be seen in a thin flexible plate. A
fissured block of ice of this kind will be able to undergo a displace-
ment at the two sides which form the crack, even when these con-
tinue to adhere in the unfissured part of the block. If then part of
the fissure at first formed is closed by regelation, the fissure can
extend in the opposite direction without the continuity of the block
being at any time disturbed. It seems to me doubtful, too, whether
in compressed ice and in glacier ice, which apparently consists of
interlaced polyhedral granules, these granules, before any attempt is
made to separate them, are completely detached from each other, and
are not rather connected by ice bridges which readily give way; and
whether these latter do not produce the comparatively firm coherence
of the apparent heap of granules.

The properties of ice here described are interesting from a physical
point of view, for they enable us to follow so closely the transition
from a crystalline body to a granular one; and they give the causes
of the alteration of its properties better than in any other well-known
example. Most natural substances show no regular crystalline struc-
ture; our theoretical ideas refer almost exclusively to crystallised and
perfectly elastic bodies. It is precisely in this relationship that the
transition from fragile and elastic crystalline ice into plastic granular
ice is so very instructive.

THE WAVE THEORY OF LIGHT
THE TIDES

BY

SIR WILLIAM THOMSON
(LORD KELVIN)

INTRODUCTORY NOTE

William Thomson, Baron Kelvin of Largs, was born in Belfast, Ire-
land, June 24, 1824. He was the son of the professor of mathematics at
Glasgow University, and himself entered that institution at the age of
eleven. By the time he was twenty-one he graduated from Cambridge
as Second Wrangler, and, after studying in Paris, he returned to Scotland
to become, as professor of natural philosophy, the colleague of his father
and elder brother. The story of his life thenceforth is the record of
amazingly brilliant and fruitful scientific work, recognized by the award
of almost all the honors appropriate to such service, from learned societies,
universities, and governments at home and abroad. His part in laying the
Adantic Cable was the occasion of his receiving knighthood, and in 1892
he was raised to the peerage. He held his professorship at Glasgow for
fifty-three years, and later was chosen its Chancellor. He died on Decem-
ber 17, 1907, and was buried in Westminster Abbey.

Lord Kelvin's activities were remarkable for both profundity and
range. A large number of his results are to be appreciated only by the
highly skilled mathematician and physicist; but his speculations on the
ultimate constitution of matter; his statement of the principle of the dissi-
pation of energy, with its bearing upon the age of life on the earth; his
calculations as to the age of the earth itself, and much more, are of great
general interest. His fertility in practical invention was no less notable.
He contrived a large number of instruments; his services to navigation
and ocean telegraphy being especially valuable. Long before his death
he was recognized as the most distinguished man of science of his time
and country, and he was also the most loved.

The lectures which follow are favorable examples of his power of
exposition in subjects in which he had no superior.

THE WAVE THEORY OF
LIGHT

[A LECTURE DELIVERED AT THE ACADEMY OF MUSIC,

PHILADELPHIA, UNDER THE AUSPICES OF

THE FRANKLIN INSTITUTE,

SEPTEMBER 29th, 1884.]

THE subject upon which I am to speak to you this evening
is happily for me not new in Philadelphia. The beautiful
lectures on light which were given several years ago by Presi-
dent Morton, of the Stevens' Institute, and the succession of lectures
on the same subject so admirably illustrated by Professor Tyndall,
which many now present have heard, have fully prepared you for
anything I can tell you this evening in respect to the wave theory of
light.

It is indeed my humble part to bring before you only some mathe-
matical and dynamical details of this great theory. I cannot have
the pleasure of illustrating them to you by anything comparable
with the splendid and instructive experiments which many of you
have already seen. It is satisfactory to me to know that so many
of you, now present, are so thoroughly prepared to understand any-
thing I can say, that those who have seen the experiments will not
feel their absence at this time. At the same time I wish to make
them intelligible to those who have not had the advantages to be
gained by a systematic course of lectures. -I must say, in the first
place, without further preface, as time is short and the subject is
long, simply that sound and light are both due to vibrations propa-
gated in the manner of waves; and I shall endeavour in the first
place to define the manner of propagation and the mode of motion
that constitute those two subjects of our senses, the sense of sound
and the sense of light.

251

252 KELVIN

Each is due to vibrations, but the vibrations of Ught differ widely
from the vibrations of sound. Something that I can tell you more
easily than anything in the way of dynamics or mathematics re-
specting the two classes of vibrations is, that there is a great difference
in the frequency of the vibrations of light when compared with the
frequency of the vibrations of sound. The term "frequency" applied
to vibrations is a convenient term, appUed by Lord Rayleigh in his
book on sound to a definite number of full vibrations of a vibrating
body per unit of time. Consider, then, in respect to sound, the fre-
quency of the vibrations of notes, which you all know in music
represented by letters, and by the syllables for singing, the do, re, mi,
&c. The notes of the modern scale correspond to different frequencies
of vibrations. A certain note and the octave above it, correspond to
a certain number of vibrations per second, and double that number.

I may conveniently explain in the first place the note called
'C; I mean the middle 'C; I believe it is the C of the tenor voice,
that most nearly approaches the tones used in speaking. That note
corresponds to two hundred and fifty-six full vibrations per sec-
ond — two hundred and fifty-six times to and fro per second
of time.

Think of one vibration per second of time. The seconds pendulum
of the clock performs one vibration in two seconds, or a half vibration
in one direction per second. Take a ten-inch pendulum of a drawing-
room clock, which vibrates twice as fast as the pendulum of an
ordinary eight-day clock, and it gives a vibration of one per second,
a full period of one per second to and fro. Now think of three vibra-
tions per second. I can move my hand three times per second easily,
and by a violent effort I can move it to and fro five times per second.
With four times as great force, if I could apply it, I could move it
twice five times per second.

Let us think, then, of an exceedingly muscular arm that would
cause it to vibrate ten times per second, that is, ten times to the
left and ten times to the right. Think of twice ten times, that is,
twenty times per second, which would require four times as much
force; three times ten, or thirty times a second, would require nine
times as much force. If a person were nine times as strong as the
most muscular arm can be, he could vibrate his hand to and fro thirty

THE WAVE THEORY OF LIGHT 253

times per second, and without any other musical instrument could
make a musical note by the movement of his hand which would
correspond to one of the pedal notes of an organ.

If you want to know the length of a pedal pipe, you can calculate
it in this way. There are some numbers you must remember, and
one of them is this. You, in this country, are subjected to the British
insularity in weights and measures; you use the foot and inch and
yard. I am obliged to use that system, but I apologise to you for
doing so, because it is so inconvenient, and I hope all Americans
will do everything in their power to introduce the French metrical
system. I hope the evil action performed by an English minister
whose name I need not mention, because I do not wish to throw
obloquy on any one, may be remedied. He abrogated a useful rule,
which for a short time was followed, and which I hope will soon be
again enjoined, that the French metrical system be taught in all
our national schools. I do not know how it is in America. The school
system seems to be very admirable, and I hope the teaching of the
metrical system will not be let slip in the American schools any
more than the use of the globes. I say this seriously: I do not think
any one knows how seriously I speak of it. I look upon our English
system as a wickedly brain-destroying piece of bondage under which
we suffer. The reason why we continue to use it is the imaginary
difficulty of making a change, and nothing else; but I do not think
in America that any such difficulty should stand in the way of
adopting so splendidly useful a reform.

I know the velocity of sound in feet per second. If I remember
rightly, it is 1,089 ^^^t per second in dry air at the freezing tempera-
ture, and 1,115 fss' psr second in air of what we would call moderate
temperature, 59 or 60 degrees — (I do not know whether that tem-
perature is ever attained in Philadelphia or not; I have had no ex-
perience of it, but people tell me it is sometimes 59 or 60 degrees in
Philadelphia, and I believe them) — in round numbers let us call
the speed 1,000 feet per second. Sometimes we call it a thousand
musical feet per second, it saves trouble in calculating the length of
organ pipes; the time of vibration in an organ pipe is the time it
takes a vibration to run from one end to the other and back. In an
organ pipe 500 feet long the period would be one per second; in an

254 KELVIN

organ pipe ten feet long the period would be 50 per second; in
an organ pipe twenty feet long the period would be 25 per second at
the same rate. Thus 25 per second, and 50 per second of frequencies
correspond to the periods of organ pipes of 20 feet and 10 feet.

The period of vibration of an organ pipe, open at both ends, is
approximately the time it takes sound to travel from one end to the
other and back. You remember that the velocity in dry air in a pipe
10 feet long is a little more than 50 periods per second; going up to
256 f)eriods per second, the vibrations correspond to those of a pipe
two feet long. Let us take 512 periods per second; that corresponds
to a pipe about a foot long. In a flute, open at both ends, the holes
are so arranged that the length of the sound-wave is about one foot,
for one of the chief "open notes." Higher musical notes correspond
to greater and greater frequency of vibration, viz., 1,000, 2,000, 4,000,
vibrations per second; 4,000 vibrations per second correspond to a
piccolo flute of exceedingly small length; it would be but one and
a half inches long. Think of a note from a little dog-call, or other
whistle, one and a half inches long, open at both ends, or from a little
key having a tube three quarters of an inch long, closed at one end;
you will then have 4,000 vibrations per second.

A wave-length of sound is the distance traversed in the period
of vibration. I will illustrate what the vibrations of sound are by this
condensation travelling along our picture on the screen. Alternate
condensations and rarefactions of the air are made continuously by
a sounding body. When I pass my hand vigorously in one direction,
the air before it becomes dense, and the air on the other side becomes
rarefied. When I move it in the other direction these things become
reversed; there is a spreading out of condensation from the place
where my hand moves in one direction and then in the reverse. Each
condensation is succeeded by a rarefaction. Rarefaction succeeds
condensation at an interval of one-half what we call "wave-lengths."
Condensation succeeds condensation at the full interval of a wave-
length.

We have here these luminous particles on this scale,' representing
portions of air close together, more dense; a little higher up, por-

' Alluding to a moving diagram of wave motion of sound produced by a working
slide for lantern projection.

THE WAVE THEORY OF LIGHT 255

tions of air less dense. I now slowly turn the handle of the apparatus
in the lantern, and you see the luminous sectors showing condensa-
tion travelling slowly upwards on the screen; now you have another
condensation making one wave-length.

This picture or chart represents a wave-length of four feet. It
represents a wave of sound four feet long. The fourth part of a
thousand is 250. What we see now of the scale represents the lower
note C of the tenor voice. The air from the mouth of a singer is
alternately condensed and rarefied just as you see here. But that
process shoots forward at the rate of about one thousand feet per
second; the exact period of the motion being 256 vibrations per
second for the actual case before you.

Follow one particle of the air forming part of a sound wave, as
represented by these moving spots of light on the screen; now it goes
down, then another portion goes down rapidly; now it stops going
down; now it begins to go up; now it goes down and up again. As
the maximum of condensation is approached it is going up with
diminishing maximum velocity. The maximum of rarefaction has
now reached it, and the particle stops going up and begins to move
down. When it is of mean density the particles are moving with
maximum velocity, one way or the other. You can easily follow these
motions, and you will see that each particle moves to and fro and
the thing that we call condensation travels along.

I shall show the distinction between these vibrations and the vi-
brations of light. Here is the fixed appearance of the particles when
displaced but not in motion. You can imagine particles of some-
thing, the thing whose motion constitutes light. This thing we call
the luminiferous ether. That is the only substance we are confident
of in dynamics. One thing we are sure of, and that is the reality
and substantiality of the luminiferous ether. This instrument is
merely a method of giving motion to a diagram designed for the
purpose of illustrating wave motion of light. I will show you the
same thing in a fixed diagram, but this arrangement shows the
mode of motion.

Now follow the motion of each particle. This represents a particle
of the luminiferous ether, moving at the greatest speed when it is
at the middle position.

256 KELVIN

You see the two modes of vibration,^ sound and light now moving
together; the travelUng of the wave of condensation and rarefaction,
and the travelling of the wave of transverse displacement. Note
the direction of propagation. Here it is from your left to your
right, as you look at it. Look at the motion when made faster. We
have now the direction reversed. The propagation of the wave is
from right to left, again the propagation of the wave is from left
to right; each particle moves perpendicularly to the line of propa-
gation.

I have given you an illustration of the vibration of sound waves,
but I must tell you that the movement illustrating the condensation
and rarefaction represented in that moving diagram are necessarily
very much exaggerated, to let the motion be perceptible, whereas
the greatest condensation in actual sound motion is not more than
one or two per cent, or a small fraction of a per cent. Except that
the amount of condensation was exaggerated in the diagram for
sound, you have in the chart a correct representation of what actually
takes place in sounding the low note C.

On the other hand, in the moving diagram representing light
waves what had we? We had a great exaggeration of the inclina-
tion of the line of particles. You must first imagine a line of particles
in a straight line, and then you must imagine them disturbed into
a wave-curve, the shape of the curve corresponding to the disturb-
ance. Having seen what the propagation of the wave is, look at this
diagram and then look at that one. This, in light, corresponds to the
different sounds I spoke of at first. The wave-length of light is the
distance from crest to crest of the wave, or from hollow to hollow.
I speak of crests and hollows, because we have a diagram of ups and
downs as the diagram is placed.

Here, then, you have a wave-length.' In this lower diagram (Fig.
119) you have a wave-length of violet light. It is but one-half the
length of the upper wave of red light; the period of vibration is but
half as long. Now there, on an enormous scale, exaggerated not only
as to slope, but immensely magnified as to wave-length, we have an

^ showing two moving diagrams, simultaneously, on the screen, one depicting a
wave motion of light, the other a sound vibration.

' Exhibiting a large drawing, or chart, representing a red and a violet wave of light
(reproduced in Figs. 118 and 119).

THE WAVE THEORY OF LIGHT 257

illustration of the waves of violet light. The drawing marked "red"
(Fig. 118) corresponds to red light, and this lower diagram corre-
sponds to violet light. The upper curve really corresponds to some-
thing a Uttle below the red ray of light in the spectrum, and the
lower curve to something beyond the violet light. The variation in
wave-length between the most extreme rays is in the proportion of
four and a half of red to eight of the violet, instead of four and
eight; the red waves are nearly as one to two of the violet.

To make a comparison between the number of vibrations for each
wave of sound and the number of vibrations constituting light waves,
I may say that 30 vibrations per second is about the smallest number
which will produce a musical sound; 50 per second gives one of the

Fig. 118 — Waves of Red Light

Fig. 119 — ^Waves of Violet Light

grave pedal notes of an organ, 100 or 200 per second give the low
notes of the bass voice, higher notes with 250 per second, 300 per
second, 1,000, 4,000 up to 8,000 per second give about the shrillest
notes audible to the human ear.

Instead of the numbers, which we have, say in the most com-
monly used part of the musical scale, /. e., from 200 or 300 to 600 or
700 per second, we have millions of millions of vibrations per second
in Ught waves: that is to say, 400 per second, instead of 400 million
million per second, which is the number of vibrations performed
when we have red light produced.

An exhibition of red light travelling through space from the
remotest star is due to propagation by waves or vibrations, in which
(each individual particle of the transmitting medium vibrates to and
'fro 400 million million times in a second.

258 KELVIN

Some people say they cannot understand a million million. Those
people cannot understand that twice two makes four. That is the
way I put it to people who talk to me about the incomprehensibility
of such large numbers. I say finitude is incomprehensible, the infi-
nite in the universe is comprehensible. Now apply a little logic to
this. Is the negation of infinitude incomprehensible? What would
you think of a universe in which you could travel one, ten, or a thou-
sand miles, or even to California, and then find it come to an end?
Can you suppose an end of matter or an end of space ? The idea is
incomprehensible. Even if you were to go millions and millions of
miles the idea of coming to an end is incomprehensible. You can
understand one thousand per second as easily as you can understand
one per second. You can go from one to ten, and ten times ten and
then to a thousand without taxing your understanding and then you
can go on to a thousand million and a million million. You can all
understand it.

Now 400 million million vibrations per second is the kind of
thing that exists as a factor in the illumination by red light. Violet
light, after what we have seen and have had illustrated by that curve
(Fig. 119), I need not tell you corresponds to vibrations of about
800 million million per second. There are recognisable qualities of
light caused by vibrations of much greater frequency and much less
frequency than this. You may imagine vibrations having about twice
the frequency of violet light, and others having about one-fifteenth
the frequency of red light, and still you do not pass the limit of the
range of continuous phenomena, only a part of which constitutes
visible light.

When we go below visible red light what have we? We have
something we do not see with the eye, something that the ordinary
photographer does not bring out on his photographically sensitive
plates. It is light, but we do not see it. It is something so closely
continuous with visible light, that we may define it by the name of
invisible light. It is commonly called radiant heat; invisible radiant
heat. Perhaps, in this thorny path of logic, with hard words flying in
our faces, the least troublesome way of speaking of it is to call it
radiant heat. The heat effect you experience when you go near a
bright hot coal fire, or a hot steam boiler; or when you go near, but

THE WAVE THEORY OF LIGHT 259

not over, a set of hot water pipes used for heating a house; the thing
we perceive in our faces and hands when we go near a boiling pot
and hold the hand on a level with it, is radiant heat; the heat of the
hands and face caused by a hot fire, or by a hot ketde when held
under the kettle, is also radiant heat.

You might readily make the experiment with an earthen teapot;
it radiates heat better than polished silver. Hold your hands below
the teapot and you perceive a sense of heat; above it you get more
heat; either way you perceive heat. If held over the teapot you
readily understand that there is a little current of hot air rising;
if you put your hand under the teapot you find cold air rising, and
the upper side of your hand is heated by radiation while the lower
side is fanned and is actually cooled by virtue of the heated kettle
above it.

That perception by the sense of heat, is the perception of some-
thing actually continuous with light. We have knowledge of rays
of radiant heat perceptible down to (in round numbers) about four
times the wave-length, or one-fourth the period, of visible or red
light. Let us take red light at 400 million million vibrations per sec-
ond, then the lowest radiant heat, as yet investigated, is about 100
million million per second of frequency of vibration.

I had hoped to be able to give you a lower figure. Professor
Langley has made splendid experiments on the top of Mount Whit-
ney, at the height of 15,000 feet above the sea-level, with his "Bolom-
eter," and has made actual measurements of the wave-length of
radiant heat down to exceedingly low figures. I will read you one of
the figures; I have not got it by heart yet, because I am expecting
more from him.* I learned a year and a half ago that the lowest
radiant heat observed by the diffraction method of Professor Langley
corresponds to 28 one hundred thousandths of a centimetre of wave-
length, 28 as compared with red light, which is 7.3; or nearly four-
fold. Thus wave-lengths of four times the amplitude, or one-fourth

* Since my lecture I have heard from Professor Langley that he has measured the
refrangibility by a rock salt prism, and inferred the wave-length of heat rays from a
"Leslie cube" (a metal vessel filled with hot water and radiating heat from a blackened
side). The greatest wave-length he has thus found is one-thousandth of a centimetre,
which is seventeen times that of sodium light — the corresponding period being about
thirty million million per second.

November, 1884.— W. T.

26o KELVIN

the frequency per second of red light, have been experimented on
by Professor Langley and recognised as radiant heat.

Everybody knows the "photographer's hght," and has heard of
invisible Hght producing visible effects upon the chemically pre-
pared plate in the camera. Speaking in round numbers, I may say
that, in going up to about twice the frequency I have mentioned for
violet light, you have gone to the extreme end of the range of known
light of the highest rates of vibration; I mean to say that you have
reached the greatest frequency that has yet been observed. Photo-
graphic, or actinic light, as far as our knowledge extends at present,
takes us to a little less than one-half the wave-length of violet light.

You will thus see that while our acquaintance with wave motion
below the red extends down to one quarter of the slowest rate which
affects the eye, our knowledge of vibrations at the other end of the
scale only comprehends those having twice the frequency of violet
light. In round numbers we have 4 octaves of light, corresponding
to 4 octaves of sound in music. In music the octave has a range to a
note of double frequency. In light we have one octave of visible
light, one octave above the visible range and two octaves below the
visible range. We have 100 per second, 200 per second, 400 per
second (million million understood) for invisible radiant heat; 800
per second for visible light, and 1,600 per second for invisible or
actinic light.

One thing common to the whole is the heat effect. It is extremely
small in moonlight, so small that until recently nobody knew there
was any heat in the moon's rays. Herschel thought it was perceptible
in our atmosphere by noticing that it dissolved away very light
clouds, an effect which seemed to show in full moonlight more than
when we have less than full moon. Herschel, however, pointed this
out as doubtful; but now, instead of its being a doubtful question,
we have Professor Langley giving as a fact that the light from the
moon drives the indicator of his sensitive instrument clear across the
scale, showing a comparatively prodigious heating effect!

I must tell you that if any of you want to experiment with the
heat of the moonlight, you must measure the heat by means of
apparatus which comes within the influence of the moon's rays only.
This is a very necessary precaution; if, for instance, you should take

THE WAVE THEORY OF LIGHT 261

your Bolometer or other heat detector from a comparatively warm
room into the night air, you would obtain an indication of a fall in
temperature owing to this change. You must be sure that your
apparatus is in thermal equilibrium with the surrounding air, then
take your burning-glass, and first point it to the moon and then to
space in the sky beside the moon; you thus get a differential measure-
ment in which you compare the radiation of the moon with the
radiation of the sky. You will then see that the moon has a distinctly
heating effect.

To continue our study of visible light, that is undulations extend-
ing from red to violet in the spectrum (which I am going to show

iiiiiiniiiiii|iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii|iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiigiiiii
J I i I

AaBCD Eb ¥ G H

Fio. 120 — ^The Solar Spectrum

you presently), I would first point out on this chart (Fig. 120) that
in the section from letter A to letter D we have visual effect and
heating effect only; but no ordinary chemical or photographic effect.
Photographers can leave their usual sensitive chemically prepared
plates exposed to yellow light and red light without experiencing any
sensible effect; but when you get toward the blue end of the spectrum
the photographic effect begins to tell, and more and more strongly
as you get towards the violet end. When you get beyond the violet
there is the invisible light known chiefly by its chemical action. From
yellow to violet we have visual effect, heating effect, and chemical
effect, all three; above the violet only chemical and heating effects,
and so little of the heating effect that it is scarcely perceptible.

The prismatic spectrum is Newton's discovery of the composition
of white light. White light consists of every variety of colour from
red to violet. Here, now, we have Newton's prismatic spectrum, pro-
duced by a prism. I will illustrate a little in regard to the nature of
colour by putting something before the light which is like coloured
glass; it is coloured gelatin. I will put in a plate of red gelatin which
is carefully prepared of chemical materials and see what that will do.
Of all the light passing to it from violet to red it only lets through
the red and orange, giving a mixed reddish colour. Here is a plate

262 KELVIN

of green gelatin: the green absorbs all the red, giving only green.
Here is a plate absorbing something from each portion of the spec-
trum, taking away a great deal of the violet and giving a yellow or
orange appearance to the light. Here is another absorbing the green
and all the violet, leaving red, orange, and a very little faint green.

When the spectrum is very carefully produced, far more carefully
than Newton knew how to show it, we have a homogeneous spec-
trum. It must be noticed that Nevrton did not understand what we
call a homogeneous spectrum; he did not produce it, and does not
point out in his writings the conditions for producing it. With an
exceedingly fine line of light we can bring it out as in sunlight, like
this upper picture — red, orange, yellow, green, blue, indigo, and
violet, according to Newton's nomenclature. Newton never used a
narrow beam of light, and so could not have had a homogeneous
spectrum.

This is a diagram painted on glass and showing the colours as we
know them. It would take two or three hours if I were to explain
the subject of spectrum analysis to-night. We must tear ourselves
away from it. I will just read out to you the wave-lengths corre-
sponding to the different positions of the sun's spectrum of certain
dark lines commonly called "Fraunhofer's lines." I will take as a
unit the one hundred thousandth of a centimetre. A centimetre is .4
of an inch; it is a rather small half an inch. I take the thousandth
of a centimetre and the hundredth of that as a unit. At the red end
of the spectrum the light in the neighbourhood of that black line A
(Fig. 120) has for its wave-length 7.6; B has 6.87; D has 5.89; the
"frequency" for A is 3.9 times 100 million million, the freauency of
D light is 5.1 times 100 million million per second.

Now what force is concerned in those vibrations as compared vwth
sound at the rate of 400 vibrations per second? Suppose for a
moment the same matter was to move to and fro through the same
range but 400 million million times per second. The force required
is as the square of the number expressing the frequency. Double
frequency would require quadruple force for the vibration of the
same body. Suppose I vibrate my hand again, as I did before. If I
move it once per second a moderate force is required; for it to
vibrate ten times per second 100 times as much force is required; for

THE WAVE THEORY OF LIGHT 263

400 vibrations per second 160,000 times as much force. If I move
my hand once per second through a space of a quarter of an inch a
very small force is required; it would require very considerable force
to move it ten times a second, even through so small a range; but
think of the force required to move a tuning fork 400 times a second,
and compare that with the force required for a motion of 400 million
million times a second. If the mass moved is the same, and the range
of motion is the same, then the force would be one million million
miUion million times as great as the force required to move the
prongs of the tuning fork — it is as easy to understand that number
as any number like 2, 3, or 4. Consider now what that number
means and what we are to infer from it. What force is there in the
space between my eye and that light? What forces are there in the
space between our eyes and the sun, and our eyes and the remotest
visible star? There is matter and there is motion, but what magni-
tude of force may there be ?

I move through this "luminiferous ether" as if it were nothing.
But were there vibrations with such frequency in a medium of steel
or brass, they would be measured by millions and millions and
millions of tons' action on a square inch of matter. There are no
such forces in our air. Comets make a disturbance in the air, and
perhaps the luminiferous ether is split up by the motion of a comet
through it. So when we explain the nature of electricity, we explain
it by a motion of the luminiferous ether. We cannot say that it is
electricity. What can this luminiferous ether be? It is something
that the planets move through with the greatest ease. It permeates
our air; it is nearly in the same condition, so far as our means of
judging are concerned, in our air and in the inter-planetary space.
The air disturbs it but little; you may reduce air by air-pumps to the
hundred thousandth of its density, and you make little effect in the
transmission of light through it. The luminiferous ether is an elastic
solid, for which the nearest analogy I can give you is this jelly which
you see,' and the nearest analogy to the waves of light is the motion,
which you can imagine, of this elastic jelly, with a ball of wood
floating in the middle of it. Look there, when with my hand I

' Exhibiting a large bowl of clear jelly with a srr\all red wooden ball embedded
in the surface near the centre.

264 KELVIN

vibrate the little red ball up and down, or when I turn it quickly
round the vertical diameter, alternately in opposite directions; — that
is the nearest representation I can give you of the vibrations of lumi-
niferous ether.

Another illustration is Scottish shoemakers' wax or Burgundy
pitch, but I know Scottish shoemakers' wax better. It is heavier than
water, and absolutely answers my purpose. I take a large slab of the
wax, place it in a glass jar filled with water, place a number of corks
on the lower side and bullets on the upper side. It is brittle like the
Trinidad pitch or Burgundy pitch which I have in my hand — you
can see how hard it is — but when left to itself it flows like a fluid.
The shoemakers' wax breaks with a brittle fracture, but it is viscous
and gradually yields.

What we know of the luminiferous ether is that it has the rigidity
of a solid and gradually yields. Whether or not it is brittle and cracks
we cannot yet tell, but I believe the discoveries in electricity and the
motions of comets and the marvellous spurts of light from them, tend
to show cracks in the luminiferous ether — show a correspondence
between the electric flash and the aurora borealis and cracks in the
luminiferous ether. Do not take this as an assertion, it is hardly more
than a vague scientific dream: but you may regard the existence of
the luminiferous ether as a reality of science; that is, we have an all-
pervading medium, an elastic solid, with a great degree of rigidity —
a rigidity so prodigious in proportion to its density that the vibra-
tions of light in it have the frequencies I have mentioned, with the
wave-lengths I have mentioned. The fundamental question as to
whether or not luminiferous ether has gravity has not been answered.
We have no knowledge that the luminiferous ether is attracted by
gravity; it is sometimes called imponderable because some people
vainly imagine that it has no weight: I call it matter with the same
kind of rigidity that this elastic jelly has.

Here are two tourmalines; if you look through them toward the
light you see the white light all round, i. e. they are transparent. If
I turn round one of these tourmalines the light is extinguished, it is
absolutely black, as though the tourmalines were opaque. This is an
illustration of what is called polarisation of light. I cannot speak
to you about qualities of light without speaking of the polarisation of

THE WAVE THEORY OF LIGHT 265

light. I want to show you a most beautiful eflect of polarising light,
before illustrating a little further by means of this large mechanical
illustration which you have in the bowl of jelly. What you saw first
were two plates of the crystal tourmaline (which came from Brazil,
I believe) having the property of letting light pass when both plates
are placed in one particular direction as regards their axes of crystal-
lisation, and extinguishing it when it passes through them with one
of the plates held in another direction. Now I put in the lantern
an instrument called a "Nicol prism," which also gives rays of
polarised light. A Nicol prism is a piece of Iceland spar, cut in two
and turned one part relatively to the other in a very ingenious way,
and put together again and cemented into one by Canada balsam.
The Nicol prism takes advantage of the property which the spar has
of double refraction, and produces the phenomenon which I now
show you. I turn one prism round in a certain direction and you
get light — a maximum of light. I turn it through a right angle
and you get blackness. I turn it one quarter round again, and get
maximum light; one quarter more, maximum blackness; one quar-
ter more, and bright light. We rarely have a grand specimen of a
Nicol prism as this.

There is another way of producing polarised light. I stand before
that light and look at its reflection in a plate of glass on the table
through one of the Nicol prisms, which I turn round, so. Now if I
incline that plate of glass at a particular angle — rather more than
fifty-five degrees — I find a particular position in which, if I look at it
and then turn the prism round in the hand, the effect is absolutely
to extinguish the light in one position of the prism and to give it
maximum brightness in another position. I use the term "absolute"
somewhat rashly. It is only a reduction to a very small quantity of
light, not an absolute annulment as we have in the case of the two
Nicol prisms used conjointly. As to the mechanics of the thing, those
of you who have never heard of this before would not know what I
was talking about; it could only be explained to you by a course of
lectures in physical optics. The thing is this, vibrations of light must
be in a definite direction relatively to the line in which the light
travels.

Look at this diagram, the light goes from left to right; we have

266 KELVIN

vibrations perpendicular to the line of transmission. There is a line
up and down which is the line of vibration. Imagine here a source
of light, violet light, and here in front of it is the line of propagation.
Sound-vibrations are to and fro in, this is transverse to, the line of
propagation. Here is another, perpendicular to the diagram, still
following the law of transverse vibration; here is another, circular
vibration. Imagine a long rope, you whirl one end of it and you see
a screw-like motion running along, and you can get this circular
motion in one direction or in the opposite.

Plane-polarised light is light with the vibrations all in a single
plane, perpendicular to the plane through the ray which is technically
called the "plane of polarisation." Circularly polarised light consists
of undulations of luminiferous ether having a circular motion. Ellip-
tically polarised light is something between the two, not in a straight
line, and not in a circular line; the course of vibration is an ellipse.
Polarised light is light that performs its motions continually in one
mode or direction. If in a straight line it is plane-polarised; if in a
circular direction it is circularly polarised light; when elliptical it is
elliptically polarised light.

With Iceland spar, one unpolarised ray of light divides on entering
it into two rays of polarised light, by reason of its power of double
refraction, and the vibrations are perpendicular to one another in the
two emerging rays. Light is always polarised when it is reflected
from a plate of unsilvered glass, or from water, at a certain definite
angle of fifty-six degrees for glass, fifty-two degrees for water, the
angle being reckoned in each case from a perpendicular to the sur-
face. The angle for water is the angle whose tangent is 1.4. I wish
you to look at the polarisation with your own eyes. Light from glass
at fifty-six degrees and from water at fifty-two degrees goes away
vibrating perpendicularly to the plane of incidence and plane of re-
flection.

We can distinguish it without the aid of an instrument. There
is a phenomenon well known in physical optics as "Haidinger's
Brushes." The discoverer is well known in Philadelphia as a miner-
alogist, and the phenomenon I speak of goes by his name. Look at
the sky in a direction of ninety degrees from the sun, and you will
see a yellow and blue cross, with the yellow toward the sun, and

THE WAVE THEORY OF LIGHT 267

from the sun, spreading out like two foxes' tails with blue between,
and then two red brushes in the space at right angles to the blue. If
you do not see it, it is because your eyes are not sensitive enough,
but a little training will give them the needed sensitiveness. If you
cannot see it in this way try another method. Look into a pail of
water with a black bottom; or take a clear glass dish of water, rest it
on a black cloth, and look down at the surface of the water on a day
with a white cloudy sky (if there is such a thing ever to be seen in
Philadelphia). You will see the white sky reflected in the basin of
water at an angle of about fifty degrees. Look at it with the head
tipped on one side and then again with the head tipped to the
other side, keeping your eyes on the water, and you will see Haiding-
er's brushes. Do not do it fast or you will make yourself giddy. The
explanation of this is the refreshing of the sensibility of the retina.
The Haidinger's brush is always there, but you do not see it because
your eye is not sensitive enough. After once seeing it you always see
it; it does not thrust itself inconveniently before you when you do
not want to see it. You can also readily see it in a piece of glass with
a dark cloth below it, or in a basin of water.

I am going to conclude by telling you how we know the wave-
lengths of light, and how we know the frequency of the vibrations,
and we shall actually make a measurement of the wave-length of
yellow light. I am now going to show you the diffraction spectrum.

You see on the screen,^ on each side of a central white bar of light,
a set of bars of light, of variegated colours, the first one on each side
showing blue or indigo colour, about four inches from the central
white bar, and red about four inches farther, with vivid green be-
tween the blue and the red. That effect is produced by a grating with
400 lines to the centimetre, engraved on glass, which I now hold in
my hand. The next grating that we shall try has 3,000 lines on a
Paris inch. You see the central space and on each side a large
number of spectrums, blue at one end and red at the other. The
fact that, in the first spectrum, red is about twice as far from the
centre as the blue, proves that a wave-length of red light is double
that of blue light.

I will now show you the operation of measuring the length of a

' Showing the chromatic bands thrown upon the screen from a diffraction grating.

268 KELVIN

wave of sodium light, that is a light like that marked D on the
spectrum (Fig. 120), a light produced by a spirit-lamp with salt in it.
The sodium vapour is heated up to several thousand degrees, when
it becomes self-luminous and gives such a light as we get by throwing
salt upon a spirit-lamp in the game of snap-dragon.

I hold in my hand a beautiful grating of glass silvered by Liebig's
process with metallic silver, a grating with 6,480 lines to the inch,
belonging to my friend Professor Barker, which he has kindly
brought here for us this evening. You will see the brilliancy of colour
as I turn the light reflected from the grating toward you and pass
the beam round the room. You have now seen directly with your
own eyes these brilliant colours reflected from the grating, and you
have also seen them thrown upon the screen from a grating placed
in the lantern. Now with a grating of 17,000 lines per inch — a much
greater number than the other — you will see how much further from
the central bright space the first spectrum is; how much more this
grating changes the direction, or diffraction, of the beam of light.
Here is the centre of the grating, and there is the first spectrum. You
v\dll note that the violet light is least diffracted and the red light is
most diffracted. This diffraction of light first proved to us definitely
the reality of the undulatory theory of light.

You ask why does not light go round a corner as sound does.
Light does go round a corner in these diffraction spectrums; and it
is shown going round a corner, since it passes through these bars
and is turned round an angle of thirty degrees. The phenomena of
light going round a corner seen by means of instruments adapted
to show the result and to measure the angles through which it is
turned, is called the diffraction of light.

I can show you an instrument which will measure the wave-lengths
of light. Without proving the formula, let me tell it to you. A spirit-
lamp with salt sprinkled on the wick gives very nearly homogeneous
light, that is to say, light of one wave-length, or all of the same period.
I have here a little grating which I take in my hand. I look through
this grating and see that candle before me. Close behind it you see
a blackened slip of wood with two white marks on it ten inches
asunder. The line on which they are marked is placed perpendicular
to the line at which I shall go from it. When I look at this salted

THE WAVE THEORY OF LIGHT 269

spirit-lamp I see a series o£ spectrums of yellow light. As I am some-
what short-sighted I am making my eye see with this eye-glass and
the natural lenses of the eye what a long-sighted person would make
out without an eye-glass. On that screen you saw a succession of
spectrums. I now look direct at the candle and what do I see? I see
a succession of five or six brilliantly coloured spectrums on each side
of the candle. But when I look at the salted spirit-lamp, now I see
ten spectrums on one side and ten on the other, each of which is a
monochromatic band of light.

I will measure the wave-length of the light thus. I walk away to a
considerable distance and look at the spirit-lamp and marks. I see a
set of spectrums. The first white line is exactly behind the flame. I
want the first spectrum to the right of that white line to fall exactly
on the other white line, which is ten inches from the first. As I walk
away from it I see it is now very near it; it is now on it. Now the
distance from my eye is to be measured, and the problem is again to
reduce feet to inches. The distance from the spectrum of the flame
to my eye is thirty-four feet nine inches. Mr. President, how many
inches is that? 417 inches, in round numbers 420 inches. Then we
have the proportion, as 420 is to 10 so is the length from bar to bar
of the grating to the wave-length of sodium light. That is to say as
forty-two is to one. The distance from bar to bar is the four hun-
dredth of a centimetre: therefore the forty-second part of the four
hundredth of a centimetre is the wave-length according to our
simple, and easy and hasty experiment. The true wave-length of
sodium light, according to the most accurate measurement, is about
a 17,000th of a centimetre, which differs by scarcely more than one
per cent, from our result!

The only apparatus you see is this little grating — a piece of glass
having a space four-tenths of an inch wide ruled with 400 fine lines.
Any of you who will take the trouble to buy one may measure the
wave-length of a candle flame himself. I hope some of you will be
induced to make the experiment for yourselves.

If I put salt on the flame of a spirit-lamp, what do I see through
this grating? I see merely a sharply defined yellow light, constituting
the spectrum of vaporised sodium, while from the candle flame I see
an exquisitely coloured spectrum, far more beautiful than that I

270 KELVIN

showed you on the screen. I see in fact a series of spectrums on the
two sides with the blue toward the candle flame and the red further
out. I cannot get one definite thing to measure from in the spec-
trum from the candle flame, as I can with the flame of a spirit-lamp
with the salt thrown on it, which gives as I have said a simple yellow
light. The highest blue light I see in the candle flame is now exactly
on the line. Now measure to my eye, it is forty-four feet four inches,
or 532 inches. The length of this wave then is the 532d part of the
four hundredth of a centimetre which would be the 21,280th of a
centimetre, say the 21,000th of a centimetre. Then measure for the
red and you will find something like the ii,oooth for the lowest of
the red light.
Lastly, how do we know the frequency of vibration.'
Why, by the velocity of light. How do we know that ? We know
it in a number of different ways, which I cannot explain now because
time forbids, and I can now only tell you shortly that the frequency of
vibration for any particular ray is equal to the velocity of light
divided by the wave-length for that ray. The velocity of light is
about 187,000 British statute miles per second, but it is much better
to take the kilometre — which is about six-tenths of a mile— for the
unit, when we find the velocity is very accurately 300,000 kilometres,
or 30,000,000,000 centimetres, per second. Take now the wave-length
of sodium light, as we have j ust measured it by means of the salted
spirit-lamp, to be one 17,000th of a centimetre, and we find the fre-
quency of vibration of the sodium light to be 510 million million per
second. There, then, you have a calculation of the frequency from a
simple observation which you all can make for yourselves.

Lastly, I must tell you about the colour of the blue sky which is
illustrated by this spherule imbedded in an elastic solid (Fig. 121).
I want to explain to you in two minutes the mode of vibration. Take
the simplest plane-polarised light. Here is a spherule which is pro-
ducing it in an elastic solid. Imagine the solid to extend miles
horizontally and miles up and down, and imagine this spherule to
vibrate up and down. It is quite clear that it will make transverse
vibrations similarly in all horizontal directions. The plane of polari-
sation is defined as a plane perpendicular to the line of vibration.
Thus, light produced by a molecule vibrating up and down, as this

THE WAVE THEORY OF LIGHT 27I

red globe in the jelly before you, is polarised in a horizontal plane
because the vibrations are vertical.

Here is another mode of vibration. Let me twist this spherule in
the jelly as I am now doing, and that will produce vibrations, also
spreading out equally in all horizontal directions. When I twist this
globe round it draws the jelly round with it; twist it rapidly back
and the jelly flies back. By the inertia of the jelly the vibrations
spread in all directions and the lines of vibration are horizontal all
through the jelly. Everywhere, miles away that solid is placed in

// f \ \

\

Fio. 121 — Vibrating Spherule Imbedded in an Elastic Solid

vibration. You do not see the vibrations, but you must understand
that they are there. If it flies back it makes vibration, and we have
waves of horizontal vibrations travelling out in all directions from
the exciting molecule.

I am now causing the red globe to vibrate to and fro horizontally.
That will cause vibrations to be produced which will be parallel to
the line of motion at all places of the plane perpendicular to the
range of the exciting molecule. What makes the blue sky? These
are exactly the motions that make the blue light of the sky, which is
due to spherules in the luminiferous ether, but little modified by the
air. Think of the sun near the horizon, think of the light of the sun
streaming through and giving you the azure blue and violet over-
head. Think first of any one particle and think of it moving in such
a way as to give horizontal and vertical vibrations and circular and
elliptic vibrations.

You see the blue sky in high pressure steam blown into the air;
you see it in the experiment of Tyndall's blue sky in which a delicate
condensation of vapour gives rise to exactly the azure blue of the sky.

Now the motion of the luminiferous ether relatively to the spherule
gives rise to the same effect as would an opposite motion impressed
upon the spherule quite independently by an independent force. So

272 KELVIN

you may think of the blue colour coming from the sky as being
produced by to and fro vibrations of matter in the air, which vibrates
much as this little globe vibrates imbedded in the jelly.

The result in a general way is this: The light coming from the
blue sky is polarised in a plane through the sun, but the blue light
of the sky is complicated by a great number of circumstances and
one of them is this, that the air is illuminated not only by the sun but
by the earth. If we could get the earth covered by a black cloth
then we could study the polarised light of the sky with a simplicity
which we cannot do now. There are, in nature, reflections from the
seas and rocks and hills and waters in an infinitely complicated man-
ner.

Let observers observe the blue sky not only in winter when the
earth is covered with snow, but in summer when it is covered with
dark green foliage. This will help to unravel the complicated phe-
nomena in question. But the azure blue of the sky is light produced
by the reaction on the vibrating ether of little spherules of water, of
perhaps a fifty thousandth or a hundred thousandth of a centimetre
diameter, or perhaps litde motes, or lumps, or crystals of common
salt, or particles of dust, or germs of vegetable or animal species
wafted about in the air. Now what is the luminiferous ether? It is
matter prodigiously less dense than air — millions and millions and
millions of times less dense than air. We can form some sort of idea
of its limitations. We believe it is a real thing, with great rigidity in
comparison with its density: it may be made to vibrate 400 million
miUion times per second; and yet be of such density as not to produce
the slightest resistance to any body going through it.

Going back to the illustration of the shoemakers' wax; if a cork
will, in the course of a year, push its way up through a plate of
that wax when placed under water, and if a lead bullet will penetrate
downwards to the bottom, what is the law of the resistance? It
clearly depends on time. The cork slowly in the course of a year
works its way up through two inches of that substance; give it one or
two thousand years to do it and the resistance will be enormously
less; thus the motion of a cork or bullet, at the rate of one inch in
2,000 years, may be compared with that of the earth, moving at the
rate of six times ninety-three million miles a year, or nineteen miles

THE WAVE THEORY OF LIGHT 273

per second, through the luminiferous ether; but when we can have
actually before us a thing elastic like jelly and yielding like pitch,
surely we have a large and solid ground for our faith in the specu-
lative hypothesis of an elastic luminiferous ether, which constitutes
the wave theory of light.

THE TIDES

[EVENING LECTURE TO THE BRITISH ASSOCIATION

AT THE SOUTHAMPTON MEETING, FRIDAY,

AUGUST 25, 1882]

THE subject on which I have to speak this evening is the tides,
and at the outset I feel in a curiously difficult position. If
I were asked to tell what I mean by the Tides I should feel
it exceedingly difficult to answer the question. The tides have some-
thing to do with motion of the sea. Rise and fall of the sea is some-
times called a tide; but I see, in the Admiralty Chart of the Firth
of Clyde, the whole space between Ailsa Craig and the Ayrshire
coast marked "very little tide here." Now, we find there a good ten
feet rise and fall, and yet we are authoritatively told there is very little
tide. The truth is, the word "tide" as used by sailors at sea means
horizontal motion of the water; but when used by landsmen or sail-
ors in port, it means vertical motion of the water. I hope my friend
Sir Frederick Evans will allow me to say that we must take the
designation in the chart, to which I have referred, as limited to the
instruction of sailors navigating that part of the sea, and to say that
there is a very considerable landsman's tide there — a rise and fall
of the surface of the water relatively to the land — though there is
exceedingly little current.

One of the most interesting points of tidal theory is the determina-
tion of the currents by which the rise and fall is produced, and so far
the sailor's idea of what is most noteworthy as to tidal motion is
correct: because before there can be a rise and fall of the water
anywhere it must come from some other place, and the water cannot
pass from place to place without moving horizontally, or nearly
horizontally, through a great distance. Thus the primary phenom-
enon of the tides is after all the tidal current; and it is the tidal
currents that are referred to on charts where we have arrow-heads

274

THE TIDES 275

marked with the statement that we have "very httle tide here," or
that we have "strong tides" there.

One instance of great interest is near Portland. We hear of the
"race of Portland" which is produced by an exceedingly strong tidal
current; but in Portland harbour there is exceedingly little rise and
fall, and that little is much confused, as if the water did not know
which way it was going to move. Sometimes the water rises, sinks,
seems to think a litde while about it, and then rises again. The rise
of the tide at Portland is interesting to the inhabitants of Southamp-
ton in this, that whereas here, at Southampton, there is a double high
water, there, at Portland, there is a double low water. The double
high water seems to extend across the Channel. At Havre, and on
the bar off the entrance to Havre, there is a double high water very
useful to navigation; but Southampton I believe is pre-eminent
above all the ports in the British Islands with respect to this con-
venience. There is here (at Southampton) a good three hours of
high water; — a little dip after the first high water, and then the water
rises again a very little more for an hour and a half or two hours,
before it begins to fall to low water.

I shall endeavour to refer to this subject again. It is not merely the
Isle of Wight that gives rise to the phenomenon. The influence ex-
tends to the east as far as Christchurch, and is reversed at Portland,
and we have the double or the prolonged high water also over at
Havre; therefore, it is clearly not, as it has been supposed to be, due
to the Isle of Wight.

But now I must come back to the question, What are the "Tides" ?
Is a "tidal wave" a tide? What is called in the newspapers a "tidal
wave" rises sometimes in a few minutes, does great destruction, and
goes down again, somewhat less rapidly. There are frequent in-
stances in all parts of the world of the occurrence of that phenom-
enon. Such motions of the water, however, are not tides; they are
usually caused by earthquakes. But we are apt to call any not very
short-time rise and fall of the water a tide, as when standing on the
coast of a slandng shore where there are long ocean waves, we see the
gradual sinkings and risings produced by them, and say that it is a
wave we see, not a tide, till one comes which is exceptionally slow,
and then we say "that is liker a tide than a wave." The fact is,

276 KELVIN

there is something perfectly continuous in the species of motion
called wave, from the smallest ripple in a musical glass, whose
period may be a thousandth of a second, to a "lop of water" in the
Solent, whose period is one or two seconds, and thence on to the great
ocean wave with a period of from fifteen to twenty seconds, where
end the phenomenon which we commonly call waves (Fig. 122), and

Fig. 122 — Wave forms

not tides. But any rise and fall which is manifestly of longer period,
or slower in its rise from lowest to highest, than a wind wave, we are
apt to call a tide; and some of the phenomena that are analysed for,
and worked out in this very tidal analysis that I am going to explain,
are in point of fact more properly wind waves than true tides.

Leaving these complicated questions, however, I will make a short
cut, and assuming the cause without proving it, define the thing by
the cause. I shall therefore define tides thus: Tides are motions of
water on the earth, due to the attractions of the sun and of the moon.
I cannot say tides are motions due to the actions of the sun and of the
moon; for so I would include, under the designation of tide, every
ripple that stirs a puddle or a millpond, and waves in the Solent or
in the English Channel, and the long Atlantic wind waves, and the
great swell of the ocean from one hemisphere to the other and back
again (under the name which I find in the harmonic reduction of
tidal observations), proved to take place once a year, and which I can
only explain as the result of the sun's heat.

But while the action of the sun's heat by means of the wind pro-
duces ripples and waves of every size, it also produces a heaping-up
of the water as illustrated by this diagram (Fig. 123). Suppose we
have wind blowing across one side of a sheet of water, the wind
ruffles the surface, the waves break if the wind is strong, and the

THE TIDES 277

result is a strong tangential force exerted by the wind on the surface
water. If a ship is sailing over the water there is strong tangential
force; thus the water is found going fast to leeward for a long dis-
tance astern of a great ship sailing with a side wind: and, just as
the sails of a ship standing high above the sea give a large area for
the wind to act upon, every wave standing up gives a surface, and
we have horizontal tangential force over the whole surface of a
troubled sea. The result is that water is dragged along the surface
from one side of the ocean to the other — from one side of the Atlantic
to the other — and is heaped up on the side towards which the wind
is blowing. To understand the dynamics of this phenomenon, think
of a long straight canal with the wind blowing lengthwise along it.
In virtue of the tangential force exerted on the surface of the water
by the wind, and which increases with the speed of the wind, the
water will become heaped up at one end of the canal, as shown in
the diagram (Fig. 123), while the surface water throughout the

Fig. 123 — Showing the heaping-up o£ water produced by wind

whole length will be observed moving in the direction of the wind —
say in the direction of the two arrows near to the surface of the water
above and below it. But to re-establish the disturbed hydrostatic
equilibrium, the water so heaped up will tend to flow back to the
end from which it has been displaced, and as the wind prevents this
taking place by a surface current, there will be set up a return cur-
rent along the bottom of the canal, in a direction opposite to that of
the wind, as indicated by the lowermost arrow in the diagram (Fig.
123). The return current in the ocean, however, is not always an
under current, such as I have indicated in the diagram, but may
sometimes be a lateral current. Thus a gale of wind blowing over
ten degrees of latitude will cause a drag of water at the surface,
but the return current may be not an under current but a current on

278 KELVIN

one side or the other of the area affected by the wind. Suppose, for
instance, in the Mediterranean there is a strong east wind blowing
along the African coast, the result will be a current from east to west
along that coast, and return current along the northern coasts of
the Mediterranean.

The rise and fall of the water due to these motions are almost in-
extricably mixed up with the true tidal rise and fall.

There is another rise and fall, also connected with the heating
effect of the sun, that I do not call a true tide, and that is a rise and
fall due to change of atmospheric pressure. When the barometer is
high over a large area of ocean, then, there and in neighboring places,
the tendency to hydrostatic equilibrium causes the surface of the
water to be lower, where it is pushed down by the greater weight of
air, and to be higher where there is less weight over it. It does not
follow that in every case it is lower, because there may not be time
to produce the effect, but there is this tendency. It is very well known
that two or three days of low barometer make higher tides on our
coast. In Scotland and England and Ireland, two or three days of
low barometer generally produce all round the shore higher water
than when the barometer is high; and this effect is chiefly noticed
at the time of tidal high water, because people take less notice of
low water — as at Portland where they think nothing of the double
low water. Hence we hear continually of very high tides — very
high water noticed at the time of high tides — when the barometer is
low. We have not always, however, in this effect of barometric
pressure really great tidal rise and fall. On the contrary we have the
curious phenomenon that sometimes when the barometer is very
low, and there are gales in the neighbourhood, there is very little
rise and jail, as the water is \ept heaped up and does not sink by
anything like its usual amount from the extra high level that it has
at high water. But I fear I have got into questions which are leading
me away from my subject, and as I cannot get through them I must
just turn back.

Now think of the definition which I gave of the "tides," and
think of the sun alone. The action of the sun cannot be defined as
the cause of the solar tides. Solar tides are due to action of the sun,
but all risings and fallings of the water due to the action of the sun

THE TIDES 279

are not tides. We want the quantification of the predicate here
very badly. We have a true tide depending on the sun, the mean solar
diurnal tide, having for its period twenty-four solar hours, which is
inextricably mixed up with those meteorological tides that I have
just been speaking of — tides depending on the sun's heat, and on the
variation of the direction of the wind, and on the variation of baro-
metric pressure according to the time of day. The consequence is
that in tidal analysis, when we come to the solar tides, we cannot
know how much of the analysed result is due to attraction, and how
much to heating effect directly or indirectly, whether on water, or
on air, or on water as affected by air. As to the lunar tides we are
quite sure of them; — they are gravitational, and nothing but gravita-
tional; but I hope to speak later of the supposed relation of the moon
to the weather, and the relation that has to the tides.

I have defined the tides as motions of water on the earth due to
the attractions of the sun and of the moon. How are we to find out
whether an observed motion of the water is a tide or is not a tide as
thus defined? Only by the combination of theory and observation: if
these afford sufficient reason for believing that the motion is due to
attraction of the sun or of the moon, or of both, then we must call
it a tide.

It is curious to look back on the knowledge of the tides possessed
in ancient times, and to find as early as two hundred years before
the Christian era a very clear account given of the tides at Cadiz.
But the Romans generally, knowing only the Mediterranean, had
not much clear knowledge of the tides. At a much later time than
that, we hear from the ancient Greek writers and explorers — Posi-
donius, Strabo, and others — that in certain remote parts of the
world, in Thule, in Britain, in Gaul, and on the distant coasts of
Spain, there were motions of the sea — a rising and falling of the
water — which depended in some way on the moon. Julius Caesar
came to know something about it; but it is certain the Roman
Admiralty did not supply Julius Csesar's captains with tide tables
when he sailed from the Mediterranean with his expeditionary
force, destined to put down anarchy in Britain. He says, referring
to the fourth day after his first landing in Britain — "That night it
happened to be full moon, which time is accustomed to give the

28o KELVIN

greatest risings o£ water in the ocean, though our people did not
know it." It has been supposed however that some of his people did
know it — some of his quartermasters had been in England before
and did know — ^but that the discipline in the Roman navy was so
good that they had no right to obtrude their knowledge; and so,
although a storm was raging at the time, he was not told that the
water would rise in the night higher than usual, and nothing was
done to make his transports secure higher up on the shore while he
was fighting the Britons. After the accident Caesar was no doubt
told — "Oh, we knew that before, but it might have been ill taken
if we had said so."

Strabo says — "Soon after moonrise the sea begins to swell up and
flow over the earth till the moon reaches mid heaven. As she de-
scends thence the sea recedes till about moonset, when the water is
lowest. It then rises again as the moon, below the horizon, sinks
to mid heaven under the earth." It is interesting here to find the
tides described simply with reference to the moon. But there is some-
thing more in this ancient account of Strabo; he says, quoting Posi-
donius — "This is the daily circuit of the sea. Moreover, there is a
regular monthly course, according to which the greatest rise and
fall takes place about new moon, then diminishing rise and fall till
half moon, and again increasing till full moon." And lastly he refers
to a hearsay report of the Gaditani (Cadizians) regarding an annual
period in the amount of the daily rise and fall of the sea, which
seems to be not altogether right, and is confessedly in part conjectural.
He gave no theory, of course, and he avoided the complication of
referring to the sun. But the mere mention of an annual period is
interesting in the history of tidal theory, as suggesting that the
rises and falls are due not to the moon alone but to the sun also.
The account given by Posidonius is fairly descriptive of what occurs
at the present day at Cadiz. Exactly the opposite would be true at
many places; but at Cadiz the time of high water at new and full
moon is nearly twelve o'clock. Still, I say we have only definition
to keep us clear of ambiguities and errors; and yet, to say that those
motions of the sea which we call tides depend on the moon, was con-
sidered, even by Galileo, to be a lamentable piece of mysticism which
he read with regret in the writings of so renowned an author as
Kepler.

THE TIDES 281

It is indeed impossible to avoid theorising. The first who gave a
theory was Newton; and I shall now attempt to speak of it suf-
ficiently to allow us to have it as a foundation for estimating the
forces with which we are concerned, in dealing with some of the
very perplexing questions which tidal phenomena present.

We are to imagine the moon as attracting the earth, subject to the
forces that the different bodies exert upon each other. We are not
to take Hegel's theory — that the Earth and the Planets do not move
like stones, but move along like blessed gods, each an independent
being. If Hegel had any grain of philosophy in his ideas of the solar
system, Newton is all wrong in his theory of the tides. Newton con-
sidered the attraction of the sun upon the earth and the moon, of
the earth upon the moon, and the mutual attractions of different parts
of the earth; and left it for Cavendish to complete the discovery of
gravitation, by exhibiting the mutual attraction of two pieces of
lead in his balance. Tidal theory is one strong link in the grand
philosophic chain of the Newtonian theory of gravitation. In ex-
plaining the tide-generating force we are brought face to face with
some of the subtleties, and with some of the mere elements, of
physical astronomy. I will not enter into details, as it would be
useless for those who already understand the tidal theory, and un-
intelligible to those who do not.

I may just say that the moon attracts a piece of matter, for example
a pound-weight, here on the earth, with a force which we compare
with the earth's attraction thus. Her mass is ^ of the earth's, and
she is sixty times as far away from the earth's centre as we are
here. Newton's theory of gravitation shows, that when you get
outside the mass of the earth the resultant attraction of the earth on
the pound weight, is the same as if the whole mass of the earth were
collected at the centre, and that it varies inversely as the square of
the distance from the centre. The same law is inferred regarding
the moon's attraction from the general theory. The moon's attraction

on this pound weight is therefore _I5^ , or ags^.ooo of the attraction

'^ ° 60X60

of the earth on the same mass. But that is not the tide-generating
force. The moon attracts any mass at the nearest parts of the earth's
surface with greater force than an equal mass near the centre; and
attracts a mass belonging to the remoter parts with less force.

282 KELVIN

Imagine a point where the moon is overhead, and imagine another
point on the surface of the earth at the other end of a diameter
passing through the first point and the centre of the earth (illustrated
by B and A of Fig. 124, p. 284). The moon attracts the nearest
point (B) with a force which is greater than that with which it
attracts the farther point (A) in the ratio of the square of 59 to
the square of 61. Hence the moon's attraction on equal masses at
the nearest and farthest points differs by one fifteenth part of her
attraction on an equal mass at the earth's centre, or about a 4,320,-
oooth, or, roughly, a four-millionth, of the earth's attraction on an
equal mass at its surface. Consequently the water tends to protrude
towards the moon and from the moon. If the moon and earth were
held together by a rigid bar the water would be drawn to the side
nearest to the moon— drawn to a prodigious height of several hun-
dred feet. But the earth and moon are not so connected. We may
imagine the earth as falling towards the moon, and the moon as fall-
ing towards the earth, but never coming nearer; the bodies, in reality,
revolving round their common centre of gravity. A point nearest to
the moon is as it were dragged away from the earth, and thus the
result is that apparent gravity differs by about one four-millionth
at the points nearest to and farthest from the moon. At the inter-
mediate points of the circle C, D (Fig. 124, p. 284), there is a some-
what complicated action according to which gravitation is increased
by about one 17-millionth, and its direction altered by about one 17-
millionth, so that a pendulum 17,000 feet long, a plummet rather
longer than from the top of Mont Blanc to sea level, would, if show-
ing truly the lunar disturbing force, be deflected through a space of
one thousandth of a foot. It seems quite hopeless by a plummet to
exhibit the lunar disturbance of gravity. A spring balance to show
the alteration of magnitude, and a plummet to show the change of
direction are conceivable; but we can scarcely believe that either
can ever be produced, with sufficient delicacy and consistency and
accuracy to indicate these results.

A most earnest and persevering effort has been made by Mr.
George Darwin and Mr. Horace Darwin to detect variations in grav-
ity due to lunar disturbance, and they have made apparatus which,
notwithstanding the prodigious smallness of the effect to be observed,

THE TIDES 283

is in point of delicacy and consistency capable of showing it; but
when they had got their delicate pendulum— their delicate plummet
about the length of an ordinary seconds' pendulum — and their deli-
cate multiplying gear to multiply the motion of its lower end by
about a million times, and to show the result on a scale by the reflec-
tion of a ray of light, they found the little image incessantly moving
backward and forward on the scale with no consistency or regu-
larity; and they have come to the conclusion that there are continual
local variations of apparent gravity taking place for which we know
no rule, and which are considerably greater than the lunar disturb-
ance for which they were seeking. That which they found — continual
motions of the surface of the earth, and which was not the primary
object of their investigation — is in some respects more interesting than
what they sought and did not find. The delicate investigation thus
opened up promises a rich harvest of knowledge. These disturbances
are connected with earthquakes such as have been observed in a
very scientific and accurate manner by Milne, Thomas Gray, and
Ewing in Japan, and in Italy by many accurate observers. All such
observations agree in showing continual tremor and palpitation of
the earth in every part.

One other phenomenon that I may just refer to now as coming
out from tide-gauge observations, is a phenomenon called seiches by
Forel, and described by him as having been observed in the lakes
of Geneva and Constance. He attributes them to differences of
barometric pressure at the ends of the lake, and it is probable that
part of the phenomenon is due to such differences. I think it is
certain, however, that the whole is not due to such differences. The
Portland tide curve and those of many other places, notably the tide
curve for Malta, taken about ten years ago by Sir Cooper Key, and
observations on the Atlantic coasts and in many other parts of the
world, show something of these phenomena; a ripple or roughness
on the curve traced by the tide gauge, which, when carefully looked
to, indicates a variation not regular but in some such period as
twenty or twenty-five minutes. It has been suggested that they are
caused by electric action! Whenever the cause of a thing is not
known it is immediately put down as electrical!

I would like to explain to you the equilibrium theory, and the

284 KELVIN

kinetic theory, of the tides, but I am afraid I must merely say there
are such things; and that Laplace in his great work, his Mecanique
Celeste, first showed that the equilibrium theory was utterly insuffi-
cient to account for the phenomena, and gave the true principles of
the dynamic action on which they depend. The resultant effect of
the tide-generating force is to cause the water to tend to become
protuberant towards the moon and the sun and from them, when
they are in the same straight line, and to take a regular spheroidal

Fig. 124 — Spring Tides

Fig. 125 — Spring Tides

form, in which the difference between the greatest and the least
semi-diameter is about 2 feet for lunar action alone, and i foot for
the action of the sun alone — that is a tide which amounts to 3 feet
when the sun and moon act together (Figs. 124 and 125), and to i
foot only when they act at cross purposes (Figs. 126 and 127), so
as to produce opposite effects. These diagrams. Figs. 124 to 127, illus-
trate spring and neap tides: the dark shading around the globe, E,
representing a water envelope surrounding the earth. There has
been much discussion on the origin of the word neap. It seems to be
an Anglo-Saxon word meaning scanty. Spring seems to be the same
as when we speak of plants springing up. I well remember at the
meeting of the British Association at Edinburgh a French mem-
ber who, meaning spring tides, spoke of the grandes marees du prin-
temps. Now you laugh at this; and yet, though he did not mean it,
he was quite right, for the spring tides in the spring time are greater

THE TIDES

285

Fig. 126 — ^Neap Tides

Fig. 127 — ^Neap Tides

286 KELVIN

on the whole than those at other times, and we have the greatest
spring tides in the spring of the year. But there the analogy ceases,
for we have also very high spring tides in autumn. Still the meaning
of the two words is the same etymologically. Neap tides are scanty
tides, and spring tides are tides which spring up to remarkably
great heights.

The equilibrium theory of the tides is a way of putting tidal
phenomena. We say the tides would be so and so if the water took
the figure of equilibrium. Now the water does not cover the whole
earth, as we have assumed in the diagrams (Figs. 124-127), but
the surface of the water may be imagined as taking the same figure,
so far as there is water, that it would take if there were water over
the whole surface of the earth. But here a difficult question comes
in — namely, the attraction of the water for parts of itself. If we
consider the water flowing over the whole earth this attraction must
be taken into account. If we imagine the water of exceedingly small
density so that its attraction on itself is insensible compared with
that of the earth, we have thus to think of the equilibrium theory.
But, on the other hand, if the water had the same density as the
earth, the result would be that the solid nucleus would be almost
ready to float; and now imagine that the water is denser than the
earth, and we put the tides out of consideration altogether. Think
of the earth covered over with mercury instead of water — a layer of
mercury a foot deep. The solid earth would tend to float, and would
float, and the result would be that the denser liquid would run to,
and cover one side up to a certain depth, and the earth would be
as it were floating out of the sea. That explains one curious result
that Laplace seems to have been much struck with: the stability
of the ocean requires that the density of the water should be less
than that of the solid earth. But take the sea as having the specific
gravity of water, the mean density of the earth is only 5.6 times that
of water, and this is not enough to prevent the attraction of water
for water from being sensible. Owing therefore to the attraction of
the water for parts of itself the tidal phenomena are somewhat
larger than they would be without it, but neglecting this, and
neglecting the deformation of the solid earth, we have the ordinary
equilibrium theory.

THE TIDES 287

Why does the water not follow the equilibrium theory? Why
have we tides of 20 feet or 30 feet or 40 feet in some places, and
only of 2 or 3 feet in others ? Because the water has not time in the
course of 12 hours to take the equilibrium figure, and because after
tending towards it, the water runs beyond it.

I ask you to think of the oscillations of water in a trough. Look at
this diagram (Fig. 128), which will help you to understand how the

Fig. 128 — Oscillations of Water in a Trough

tidal efJect is prodigiously magnified by a dynamical action due to the
inertia of the water. The tendency of water in motion to keep its
motion prevents it from taking the figure of equilibrium. [A chart
showing the tides of the English Channel was exhibited, from which
it was seen that while at Dover there were tides of 21 feet, there was
at Portland very little rise and fall.] Imagine a canal instead of the
English Channel, a canal stopped at the Straits of Dover and at the
opposite end at Land's End, and imagine somehow a disturbing force
causing the water to be heaped up at one end. There would be a
swing of water from one end to the other, and if the period of the
disturbing force approximately agreed with the period of free oscil-
lation, the effect would be that the rise and fall would go vastly
above and below the range due to equilibrium action. Hence it is
we have the 21 feet rise and fall at Dover. The very little rise and fall
at Portland is also illustrated in the uppermost figure of this diagram
(Fig. 128). Thus high water at Dover is low water at Land's End,
and the water seesaws as it were about a line going across from Port-
land to Havre (represented by N in the figure) ; not a line going

288 KELVIN

directly across, however, for on the other side of the Channel there
is a curious complication.

At the time of high water at Dover there is hardly any current
in the Channel. As soon as the water begins to fall at Dover the
current begins to flow west through the whole of the Channel. When
it is mid-tide at Dover the tide is flowing fastest in the Channel.
This was first brought to light by Admiral Beechey.

I wish I had time to show the similar theory as to the tides in
the Irish Channel. The water runs up the English Channel to
Dover, and up the Irish Channel to fill up the basin round the Isle
of Man. Take the northern mouth of the Irish Channel between the
Mull of Cantire and the northeast coast of Ireland. The water
rushes in through the straits between Cantire and Rathlin Island,
to fill up the Bay of Liverpool and the great area of water round
the Isle of Man. This tidal wave entering from the north, running
southward through the Channel, meets in the Liverpool basin with
the tidal stream coming from the south entrance, and causes the
time of high water at Liverpool to be within half-an-hour of the time
of no currents in the northern and southern parts of the Channel.

I would like to read you the late Astronomer-Royal's appreciation
of Laplace's splendid work on the tides.

Airy says of Laplace: "If now, putting from our thoughts the
details of the investigation, we consider its general plan and objects,
we must allow it to be one of the most splendid works of the greatest
mathematician of the past age. To appreciate this, the reader must
consider, first, the boldness of the writer, who, having a clear under-
standing of the gross imperfections in the methods of his predeces-
sors, had also the courage deliberately to take up the problem on
grounds fundamentally correct (however it might be limited by sup-
positions afterwards introduced) ; secondly, the general difficulty of
treating the motion of fluids; thirdly, the pecuHar difficulty of treat-
ing the motions when the fluids cover an area which is not plane
but convex; and fourthly, the sagacity of perceiving that it was
necessary to consider the earth as a revolving body, and the skill of
correctly introducing this consideration. This last point alone, in
our opinion, gives a greater claim for reputation than the boasted
explanation of the long inequaUty of Jupiter and Saturn."

THE TIDES 289

Tidal theory must be carried on along with tidal observations. In-
struments for measuring and recording the height of the water at any

Fig. 129 — ^Tide Gauge

time give us results of observations.' Here is such an instrument — a
tide gauge (Fig. 129) . The floater is made of thin sheet copper, and

'The various instruments and tide-curves referred to in this lecture are fully
described and illustrated in a paper on "The Tide Gauge, Tidal Harmonic Analyser,
and Tide Predicter," read before the Institution of Civil Engineers, on ist March,
1881, and published in their Proceedings for that date.

290 KELVIN

is suspended by a fine platinum wire. The vertical motion of the
floater, as the water rises and falls, is transmitted, in a reduced pro-
portion by a single pinion and wheel, to this frame or marker, which
carries a small marking pencil. The paper on which the pencil
marks the recording curve, is stretched on this cylinder, which, by
means of the clockwork, is caused to make one revolution every
twenty-four hours. The leaning-tower-of-Pisa arrangement of the
paper-cylinder, and the extreme simplicity of the connection between

Fig. 130 — Facsimile of weekly sheet of curves traced by Tide Gauge

marker and floater, constitute the chief novelty. This tide gauge is
similar to one now in actual use, recording the rise and fall of the
water in the River Clyde, at the entrance to the Queen's Dock,
Glasgow. A sheet bearing the curves (Fig. 130) traced by that ma-
chine during a week is exhibited.

After the observations have been taken, the next thing is to make
use of them. Hitherto this has been done by laborious arithmetical
calculation. I hold in my hand the Reports of the late Tidal Com-
mittee of the British Association with the results of the harmonic
analysis — about eight years' work carried on with great labour, and
by aid of successive grants from the British Association. The Indian
Government has continued the harmonic analysis for the seaports
of India. The Tide Tables for Indian Ports for the Year 1882, issued
under the authority of the Indian Government, show this analysis
as in progress for the following ports, viz.: Aden, Kurrachee, Okha
Point and Beyt Harbour at the entrance to the Gulf of Cutch, Bom-
bay, Karwar, Beypore, Paumben Pass, Madras, Vizagapatam, Dia-

THE TIDES 291

mond Harbour, Fort Gloster and Kidderpore on the River Hooghly,
Rangoon, Moulmein, and Port Blair. Mr. Roberts, who was first
employed as calculator by the Committee of the British Association,
has been asked to carry on the work for the Indian Government,
and latterly, in India, native calculators under Major Baird, have
worked by the methods and forms by which Mr. Roberts had worked
in England for the British Association.^ The object is to find the
values of the different tidal constituents. We want to separate out
from the whole rise and fall of the ocean the part due to the sun,

Fig. 131 — Declinational Tide

the part due to the moon, the part due to one portion of the moon's
effect, and the part due to another. There are complications depend-
ing on the moon's position — declinational tides according as the
moon is or is not in the plane of the earth's equator — and also on
that of the sun. Thus we have the diurnal declinational tides. When
the moon is in the north declination (Fig. 131) we have (in the
equilibrium theory) higher water at lunar noon than at lunar mid-
night. That difference in the height of high water, and the cor-
responding solar noon tides and solar midnight tides, due to the sun
not being in the earth's equator, constitute the lunar and solar diur-
nal declinational tides. In summer the noon high water might be

^ Note of September 17, 1887. On the subject of Tidal Harmonic Analysis see
"Manual of Instructions for Tidal Observation," by Major Baird, published by Messrs.
Taylor and Francis, London, 1886; also the Reports of the British Association Com-
mittee "On Harmonic Analysis of Tidal Observations." — W. T.

292 KELVIN

expected to be higher than the mid-
night high water, because the sun is
nearer overhead to us than to our Ai>
tipodes.

By kind permission of Sir Fred-
erick Evans, I am able to place before
you these diagrams of curves drawn
by Captain Harris, R. N., of the
Hydrographic Department of the Ad-
miralty, exhibiting the rise and fall of
tides in Princess Royal Harbour, King
George Sound, Western Australia, from
January ist to December 31st, 1877, and
in Broad Sound, Queensland, Australia,
from July 15th, 1877, to July 23rd, 1878.
Look at this one of these diagrams, a
diagram of the tides at the northeast
corner of Australia. For several days
high water always at noon. When the
tides are noticeable at all we have high
water at noon, and when the tides are
not at noon they are so small that they
are not taken notice of at all. It thus
appears as if the tides were irrespective
of the moon, but they are not really so.
When we look more closely, it is a full
moon if we have a great tide at noon; or
else it is new moon. It is at half moon
that we have the small tides, and when
they are smallest we have high water at
six. There is also a great difference be-
tween day and night high water; the
difference between them is called the
diurnal tide. A similar phenomenon
is shown on a smaller scale in this
curve, drawn by the first tide-predict-

r

THE TIDES 293

ing machine. At a certain time the two high waters become equal-
ised, and the two low waters very unequal (see p. 290 for real
examples) .

The object of the harmonic analysis is to analyse out from the com-
plicated curve traced by the tide gauge the simplest harmonic ele-
ments. A simple harmonic motion may be imagined as that of a
body which moves simply up and down in a straight line, keep-
ing level with the end of a clock hand, moving uniformly round.
The exceedingly complicated motion that we have in the tides
is analysed into a series of simple harmonic motions in different
periods and with different amplitudes or ranges; and these sim-
ple harmonic constituents added together give the complicated
tides.

All the work hitherto done has been accomplished by sheer cal-
culation; but calculation of so methodical a kind that a machine
ought to be found to do it. The Tidal Harmonic Analyser consists
of an application of Professor James Thomson's disk-globe-and-
cylinder integrator to the evaluation of the integrals required for
the harmonic analysis. The principle of the machine and the essen-
tial details are fully described and explained in papers communicated
by Professor James Thomson and the author to the Royal Society, in
1876 and 1878, and published in the Proceedings for those years;'
also reprinted, with a postscript dated April, 1879, in Thomson and
Tait's Natural Philosophy, Second Edition, Appendix B. It remains
now to describe and explain the actual machine referred to in the
last of these communications, which is the only tidal harmonic an-
alyser hitherto made. It may be mentioned, however, in passing, that
a similar instrument, with the simpler construction wanted for the
simpler harmonic analysis of ordinary meteorological phenomena,
has been constructed for the Meteorological Committee, and is now
regularly at work at their office, harmonically analysing the results
of meteorological observations, under the superintendence of Mr.
R. H. Scott.

Fig. 132 represents the tidal harmonic analyser, constructed under
the author's direction, with the assistance of a grant from the Gov-

' Vide vol. xxiv. p. 262, and vol. xxvii. p. 371.

294 KELVIN

ernment Grant Fund of the Royal Society. The eleven cranks of

this instrument are allotted as follows: —

Dis-

tm-

Cranks.

Object.

guish-

ing
Letter.

Speed.

I and 2

To find the mean lunar semi-diurnal tide . . .

M

2 {y—a)

3 .. 4

„ mean solar „ „ . . .

S

2 (7—,)

5 ,. 6

„ luni-solar declinational diurnal tide

K,

y

7 ., 8

„ slower lunar „ „

(7—2.7)

9 .. lo

„ slower solar „ „

P

(7—27;)

II

„ mean water level

A,

The general arrangement of the several parts may be seen from
Fig. 132. The large circle at the back, near the center, is merely a
counter to count the days, months, and years for four years, being the
leap year period. It is driven by a worm carried on an intermediate
shaft, with a toothed wheel geared on another on the solar shaft.
In front of the centre is the paper drum, which is on the solar shaft,
and goes round in the period corresponding to twelve mean solar
hours. On the extreme left, the first pair of disks, with globes and
cylinders, and crank shafts with cranks at right angles between them,
driving their two cross-heads, corresponds to the Ki, or luni-solar
diurnal tide. The next pair of disk-globe-and-cylinders corresponds
to M, or the mean lunar semi-diurnal tide, the chief of all the tides.
The next pair lie on the two sides of the main shaft carrying the
paper drum, and correspond to S, the mean solar semi-diurnal tide.
The first pair on the right correspond to O, or the lunar diurnal tide.
The second pair on the right correspond to P, the solar diurnal tide.
The last disk on the extreme right is simply Professor James Thom-
son's disk-globe-and-cylinder integrator, applied to measure the area
of the curve as it passes through the machine.

The idle shafts for the M and the O tides are seen in front
respectively on the left and right of the centre. The two other longer
idle shafts for the K and the P tides are behind, and therefore not
seen. That for the P tide serves also for the simple integrator on the
extreme right.

The large hollow square brass bar, stretching from end to end

THE TIDES 295

along the top of the instrument, and carrying the eleven forks rigidly
attached to it, projecting downwards, is moved to and fro through
the requisite range by a rack and pinion, worked by a handle and
crank in front above the paper cylinder, a little to the right of its
centre. Each of these eleven forks moves one of the eleven globes
of the eleven disk-globe-and-cylinder integrators of which the ma-
chine is composed. The other handle and crank in front, lower
down and a little to the left of the centre, drives by a worm, at a con-
veniently slow speed, the solar shaft, and through it, and the four
idle shafts, the four other tidal shafts.

To work the machine the operator turns with his left hand the
driving crank, and with his right hand the tracing crank, by which
the fork-bar is moved. His left hand he turns always in one direc-
tion, and at as nearly constant a speed as is convenient to allow his
right hand, alternately in contrary directions, to trace exactly with
the steel pointer the tidal curve on the paper, which is carried across
the line of to-and-fro motion of the pointer by the revolution of the
paper drum, of which the speed is in simple proportion to the speed
of the operator's left hand.

The eleven little counters of the cylinders in front of the disks
are to be set each at zero at the commencement of an operation, and
to be read off from time to time during the operation, so as to give
the value of the eleven integrals for as many particular values of the
time as it is desired to have them.

A first working model harmonic analyser, which served for
model and for the meteorological analyser, now at work in the
Meteorological Office, is here before you. It has five disk-globe-and-
cylinders, and shafting geared for the ratio i : 2. Thus it serves to
determine, from the deviation curve, the celebrated "A B C D E" of
the Admiralty Compass Manual, this is to say, the coefficients in the
harmonic expression

A + B sin + C cosfl + D sin 20 + E cos 2B.

for the deviation of the compass in an iron ship.

The first instrument which I designed and constructed for use as
a Tide Predictor was described in the Catalogue of the Loan Col-
lection of Scientific Apparatus at South Kensington in 1876; and the

296 KELVIN

instrument itself was presented by the British Association to the
South Kensington Museum, where it now is. The second instrument
constructed on the same principle is in London, and is being worked
under the direction of Mr. Roberts, analysing the tides for the Indian
ports. The result of this work is these books {Tide Tables for In-

FiG. 133 — ^Tide Predictor

dian Ports) in which we have, for the first time, tables of the times
and heights of high water and low water for fourteen of the Indian
ports.

To predict the tides for the India and China Seas and Australia
we have a much more difficult thing to do than for the British
ports. The Admiralty Tide Tables give all that is necessary for the
British ports, practically speaking; but for other parts of the world
generally the diurnal tide comes so much into play that we have
exceedingly complicated action. The most complete thing would be
a table showing the height of the water every hour of the twenty-
four. No one has yet ventured to do that generally for all parts of

THE TIDES 297

the world; but for the comparatively complicated tides of the India
Seas, the curves traced by the Tide Predictor from which is ob-
tained the information given in these Indian tide tables, do actually
tell the height of the water for every instant of the twenty-four
hours.

The mechanical method which I have utilised in this machine is
primarily due to the Rev. F. Bashforth who, in 1845, when he was a
Bachelor of Arts and Fellow of St. John's College, Cambridge, de-
scribed it to Section A of the 1845 (Cambridge) meeting of the
British Association in a communication entitled "A Description of a
Machine for finding the Numerical Roots of Equations and tracing
a Variety of Useful Curves," of which a short notice appears in
the British Association Report for that year. The same subject was
taken up by Mr. Russell in a communication to the Royal Society in
1869, "On the Mechanical Description of Curves,"^ which contains
a drawing showing mechanism substantially the same as that of
the Tide Predictor. Here is the principle as embodied in No. 3 Tide
Predictor (represented in Fig. 133, p. 296), now actually before
you: —

A long cord of which one end is held fixed passes over one pulley,
under another, and so on. These eleven pulleys are all moved up and
down by cranks, and each pulley takes in or lets out cord according
to the direction in which it moves. These cranks are all moved by
trains of wheels gearing into the eleven wheels fixed on this driving
shaft. The greatest number of teeth on any wheel is 802 engaging
with another of 423. All the other wheels have comparatively small
numbers of teeth. The machine is finished now, except a cast-iron
sole and cast-iron back. A flywheel of great inertia enables me to
turn the machine fast, without jerking the pulleys, and so to run off a
year's curve in about twenty-five minutes. This machine is arranged
for fifteen constituents in all and besides that there is an arrange-
ment for analysing out the long period tides.

The following table shows how close an approximation to astro-
nomical accuracy is given by the numbers chosen for the teeth of the
several wheels. These numbers I have found by the ordinary arith-
metical progress of converging fractions.

'^Proc. Royal Society, June 17, 1869; (vol. xviii. p. 72).

298

KELVIN

Tidal

Speed in Degrees per Mean Solar Hour.

Losses of Angle in
Machine.

Con-
stitu-
ents.

Accurate.

As given by Machine.

Per Mean
Solar Hour.

Per
Half
Year.

M,

28°'984i042

15X^^5 -

2SI

28°"984o630

-|- 0°'00004I2

o°-i8o

K,

iS°'04io686

xsxf =
36s

i5°-O4i09S9

— o°"oooo267

o°-ii7

i3''"94303S6

--^ =

i3°-943o894

— o°-oooo538

o°-237

P

I4°-95893I4

'< -

i4°-958904o

-|- 0°'0000242

o°ii9

N

28°-4397296

,,802
iSX — =

423

28°-4397i63

-1- o°-ooooi33

o°-os9

L

29°-S284788

159

29°-S283oi8

-|- o°'oooi77

o''■^?,

V

28°-si2s830

i5X^ =
121

28°-si23966

+ o''"oooi864

o°-82

M S

58°'984I042

-x^xgr-

= S8''-98366oo

-|- o°'ooo444

i°-9S

li

27°-9682o84

«xf =

27°-968i27S

-f o°"oooo8o9

o°-36

X

29°-4Ss6254

i^X*^ -

'^^248 ~

29°-4S5645i

— o°'ooooig7

o°-o87

Q

i3°"39866o9

iSX^ =

459

i3°-3986928

— o°'oooo3i8

o°-i4

To-day (Aug. 25, 1882), a committee, consisting of only two
members, Mr. George Darwin and Professor Adams of Cambridge,
have been appointed, and one of their chief objects will be to exam-
ine the long period tides.

There is one very interesting point I said I would endeavour to
speak of if I had time; I have not time, but still I must speak of it —
the influence of the moon on the weather. "We almost laugh when
we hear of the influence of the moon on the weather," Sir F. Evans
said to me, "but there is an influence." Gales of wind are remarkably

THE TIDES 299

prevalent in Torres Straits and the neighbourhood about the time
of new and full moon. This was noticed by Dr. Rattray, a surgeon
in the navy, in connection with observations made by the surveying
ship. Fly, during the three years 1841-44. Dr. Rattray noticed that
at those times there was a large area of coral reef uncovered at the
very low water of the spring tides, extending out some sixty or
seventy miles from land. This large area becomes highly heated,
and the great heating of that large portion of land gives rise to a
tendency to gales at the full and change, that is at the new and full
moon. This indirect effect of the moon upon the weather through
the tides is exceedingly interesting; but it does not at all invalidate
the scientific conclusion that there is no direct influence, and the
general effect of the moon on the weather — the changes in the moon
and the changes in the weather, and their supposed connection —
remains a mere chimaera.

The subject of elastic tides in which the yielding of the solid earth
is taken into account is to be one of the primary objects of Mr. G.
Darwin's committee. The tide-generating force which tends to pull

Fig. 134 — Elastic Tides

the water to and from the moon, tends to pull the earth also.
Imagine the earth made of india-rubber and pulled out to and from
the moon. It will be made prolate (Fig. 134) . If the earth were of
india-rubber the tides would be nothing, the rise and fall of the
water relatively to the solid would be practically nil. If the earth
(as has long been a favourite hypothesis of geologists) had a thin
shell 20 or 30 miles thick with liquid inside, there would be no such
thing as tides of water rising and falling relatively to land, or sea-
bottom. The earth's crust would yield to and from the moon, and the
water would not move at all relatively to the crust. If the earth were
even as rigid as glass all through, calculation shows that the solid
would yield so much that the tides could only be about one third of
what they would be if the earth were perfectly rigid. Again, if the

300 KELVIN

earth were two or three times as rigid as glass, about as rigid as a
sohd globe of steel, it would still, considering its great dimensions,
yield two or three feet to that great force, which elastic yielding
would be enough to make the tides only two thirds of what they
would be if the earth were perfectly rigid. Mr. G. Darwin has made
the investigation by means of the lunar fortnightly tides, and the
general conclusion, subject to verification, is that the earth does
seem to yield somewhat, and may have something like the rigidity
of a solid globe of steel.

APPENDIX A

[Extracts from a Lecture on "The Tides," given to the Glasgow
Science Lectures Association, not hitherto published, and now in-
cluded as explaining in greater detail certain paragraphs of the
preceding Lecture. \

(i) Gravitation. — The great theory of gravitation put before us
by Newton asserts that every portion of matter in the universe at-
tracts every other portion; and that the force depends on the masses
of the two portions considered, and on the distance between them.
Now, the first great point of Newton's theory is, that bodies which
have equal masses are equally attracted by any other body, a body
of double mass experiencing double force. This may seem only what
is to be expected. It would take more time than we have to spare
were I to point out all that is included in this statement; but let me
first explain to you how the motions of different kinds of matter
depend on a property called inertia. I might show you a mass of iron
as here. Consider that if I apply force to it, it gets into a state of
motion; greater force applied to it, during the same time, gives it
increased velocity, and so on. Now, instead of a mass of iron, I
might hang up a mass of lead, or a mass of wood, to test the equality
of the mass by the equality of the motion which is produced in the
same time by the action of the same force, or in equal times by the
action of equal forces. Thus, quite irrespectively of the kind of
matter concerned, we have a test of the quantity of matter. You
might weigh a pound of tea against a pound of brass without ever
putting them into the balance at all. You might hang up one body by
a proper suspension, and you might, by a spring, measure the force
applied, first to the one body, and then to the other. If the one body
is found to acquire equal velocity under the influence of equal force
for equal times as compared with the other body, then the mass of
the one is said to be equal to the mass of the other.

301

302 KELVIN

I have spoken of mutual forces between any two masses. Let us
consider the weight or heaviness of a body on the earth's surface.
Newton explained that the attraction of the whole earth upon a body
— ^for example, this 56 pounds mass of iron — causes its heaviness or
weight. Well, now, take 56 pounds of iron here, and take a mass of
lead, which, when put in the balance, is found to be of equal weight.
You see we have quite a hew idea here. You weigh this mass of iron
against a mass of lead, or to weigh out a commodity for sale; as, for
instance, to weigh out pounds of tea, to weigh them with brass
weights is to compare their gravitations towards the earth — to com-
pare the heavinesses of the different bodies. But the first subject that
I asked you to think of had nothing to do with heaviness. The first
subject was the mass of the different bodies as tested by their resist-
ance to force tending to set them in motion. I may just say that
the property of resistance against being set into motion, and again
against resistance to being stopped when in motion, is the property
of matter called inertia.

The first great point in Newton's discovery shows, then, that if
the property of inertia is possessed to an equal degree by two dif-
ferent substances, they have equal heaviness. One of his proofs was
founded on the celebrated guinea and feather experiment, showing
that the guinea and feather fall at the same rate when the resistance
of the air is removed. Another was founded upon making pendu-
lums of different substances — lead, iron, and wood — to vibrate, and
observing their times of vibration. Newton thus discovered that
bodies which have equal heaviness have equal inertia.

The other point of the law of gravitation is, that the force between
any two bodies diminishes as the distance increases, according to the
law of the inverse square of the distance. That law expresses that,
with double distance, the force is reduced one quarter, at treble
distance the force is reduced to one-ninth part. Suppose we compare
forces at the distance of one million miles, then again at the distance
of two and a half million miles, we have to square the one number,
then square the other, and find the proportion of the square of the one
number to the square of the other. The forces are inversely as the
squares of the distance, that is the most commonly quoted part of
the law of gravitation; but the law is incomplete without the first

THE TIDES 303

part, which establishes the relation between two apparently different
properties of matter. Newton founded this law upon a great variety
of different natural phenomena. The motion of the planets round
the sun, and the moon round the earth, proved that for each planet
the force varies inversely as the square of its distance from the sun;
and that from planet to planet the forces on equal portions of their
masses are inversely as the squares of their distances. The last
link in the great chain of this theory is the tides.

(2) Tide-Generating Force. — And now we are nearly ready to
complete the theory of tide-generating force. The first rough view of
the case, which is not always incorrect, is that the moon attracts the
waters of the earth towards herself and heaps them up, therefore, on
one side of the earth. It is not so. It would be so if the earth
and moon were at rest and prevented from falling together by a rigid
bar or column. If the earth and moon were stuck on the two ends of
a strong bar, and put at rest in space, then the attraction of the moon
would draw the waters of the earth to the side of the earth next to
the moon. But in reality things are very different from that supposi-
tion. There is no rigid bar connecting the moon and the earth. Why
then does not the moon fall towards the earth ? According to New-
ton's theory, the moon is always falling towards the earth. Newton
compared the fall of the moon, in his celebrated statement, with the
fall of a stone at the earth's surface, as he recounted, after the fall of
an apple from the tree, which he perceived when sitting in his gar-
den musing on his great theory. The moon is falling towards the
earth, and falls in an hour as far as a stone falls in a second. It
chances that the number 60 is nearly enough, as I have said before,
a numerical expression for the distance of the moon from the earth
in terms of the earth's radius. It is only by that chance that the com-
parison between the second and hour can be here introduced. Since
there are 60 times 60 seconds in an hour, and about 60 radii of the
earth in the distance from the moon, we are led to the comparison
now indicated, but I am inverting the direction of Newton's com-
parison. He found by observation that the moon falls as far in
an hour as a stone falls in a second, and hence inferred that the force
on the moon is a 60th of the 60th of the force per equal mass on the
earth's surface. Then he learned from accurate observations, and

304 KELVIN

from the earth's dimensions, what I have mentioned as the moon's
distance, and perceived the law of variation between the weight of a
body at the earth's surface and the force that keeps the moon in her
orbit. The moon in Newton's theory was always falling towards the
earth. Why does it not come down? Can it be always falling and
never come down ? That seems impossible. It is always falling, but
it has also a motion perpendicular to the direction in which it is
falling, and the result of that continual falling is simply a change
of direction of this motion.

It would occupy too much of our time to go into this theory. It
is simply the dynamical theory of centrifugal force. There is a con-
tinual falling away from the line of motion, as illustrated in a stone
thrown from the hand describing an ordinary curve. You know that
if a stone is thrown horizontally it describes a parabola — the stone
falling away from the line in which it was thrown. The moon is
continually falling away from the line in which it moves at any
instant, falling away towards the point of the earth's centre, and
falling away towards that point in the varying direction from itself.
You can see it may be always falling, now from the present direc-
tion, now from the altered direction, now from the farther altered
direction in a further altered line; and so it may be always falling
and never coming down. The parts of the moon nearest to the
earth tend to fall most rapidly, the parts furthest from the earth,
least rapidly; in its own circle, each is falling away and the result
is as if we had the moon falling directly.

But while the moon is always falling towards the earth, the earth
is always falling towards the moon; and each preserves a constant
distance, or very nearly a constant distance from the common centre
of gravity of the two. The parts of the earth nearest to the moon are
drawn towards the moon with more force than an equal mass at
the average distance; the most distant parts are drawn towards the
moon with less force than corresponds to the average distance. The
solid mass of the earth, as a whole, experiences, according to its mass,
a force depending on the average distance; while each portion of
the water on the surface of the earth experiences an attractive force
due to its own distance from the moon. The result clearly is, then, a
tendency to protuberance towards the moon and from the moon;

THE TIDES 305

and thus, in a necessarily most imperfect manner, I have explained
to you how it is that the waters are not heaped up on the side next
the moon, but are drawn up towards the moon and left away from
the moon so as to tend to form an oval figure. The diagram (Fig,
125, p. 284) shows the protuberance of water towards and from the
moon. It shows also the sun on the far side, I need scarcely say,
with an enormous distortion of proportions, because without that it
would be impossible in a diagram to show the three bodies. This
illustrates the tendency of the tide-generating forces.

(3) Elastic Tides. — But another question arises. This great force
of gravity operating in different directions, pulling at one place,
pressing in at another, will it not squeeze the earth out of shape?
I perceive signs of incredulity; you think it impossible it can pro-
duce any sensible effect. Well, I will just tell you that instead of
being impossible, instead of it not producing any such effect, we
have to suppose the earth to be of exceedingly rigid material, in order
that the effect of these distorting influences on it may not mask the
phenomenon of the tides altogether.

There is a very favourite geological hypothesis which I have no
doubt many here present have heard, which perhaps till this moment
many here present have believed, but which I hope no one will go
out of this room believing, and that is that the earth is a mere crust,
a solid shell thirty, or forty, or fifty miles thick at the most, and that
it is filled with molten liquid lava. This is not a supposition to be
dismissed as absurd, as ludicrous, as absolutely unfounded and un-
reasonable. It is a theory based on hypothesis which requires most
careful weighing. But it has been carefully weighed and found
wanting in conformity to the truth. On a great many different es-
sential points it has been found at variance with the truth. One of
these points is, that unless the material of this supposed shell were
preternaturally rigid, were scores of times more rigid than steel, the
shell would yield so freely to the tide-generating forces that it would
take the figure of equilibrium, and there would be no rise and fall
of the water, relatively to the solid land, left to show us the phe-
nomena of the tides.

Imagine that this (Fig. 134, p. 299) represents a solid shell with
water outside, you can understand if the solid shell yields with suf-

306 KELVIN

ficiently great freedom, there will be exceedingly little tidal yielding
left for the water to show. It may seem strange when I say that
hard steel would yield so freely. But consider the great hardness of
steel and the smaller hardness of india-rubber. Consider the great-
ness of the earth, and think of a little hollow india-rubber ball, how
freely it yields to the pressure of the hand, or even to its own weight
when laid on a table. Now, take a great body like the earth: the
greater the mass the more it is disposed to yield to the attraction of
distorting forces when these forces increase with the whole mass. I
cannot just now fully demonstrate to you this conclusion; but I say
that a careful calculation of the forces shows that in virtue of the
greatness of the mass it would require an enormously increased
rigidity in order to keep in shape. So that if we take the actual di-
mensions of the earth at forty-two million feet diameter, and the
crust at fifty miles thick, or two hundred and fifty thousand feet, and
with these proportions make the calculation, we find that something
scores of times more rigid than steel would be required to keep the
shape so well as to leave any appreciable degree of difference from
the shape of hydrostatic equilibrium, and allow the water to indicate,
by relative displacement, its tendency to take the figure of equilib-
rium; that is to say, to give us the phenomena of tides. The geologi-
cal inference from this conclusion is, that not only must we deny the
fluidity of the earth and the assertion that it is encased by a thin
shell, but we must say that the earth has, on the whole, a rigidity
greater than that of a solid globe of glass of the same dimensions;
and perhaps greater than that of a globe of steel of the same dimen-
sions. But that it cannot be less rigid than a globe of glass, we are
assured. It is not to be denied that there may be a very large space
occupied by liquid. We know there are large spaces occupied by
lava; but we do not know how large they may be, although we can
certainly say that there are no such spaces, as can in volume be
compared with the supposed hollow shell, occupied by liquid con-
stituting the interior of the earth. The earth as a whole must be
rigid, and perhaps exceedingly rigid, probably rendered more rigid
than it is at the surface strata by the greater pressure in the greater
depths.

THE TIDES 307

The phenomena of underground temperature, which led geologists
to that supposition, are explained otherwise than by their assumption
of a thin shell full of liquid; and further, every view we can take of
underground temperature, in the past history of the earth, confirms
the statement that we have no right to assume interior fluidity.

THE EXTENT OF THE UNIVERSE

BY
SIMON NEWCOMB

INTRODUCTORY NOTE

In spite of the fertility of America in mechanical invention and applied
science, there are few branches of pure science in which she can be
regarded as among the leading nations. Her nearest approach to pre-
eminence has probably been in astronomy; and in this field Simon New-
comb was, at his death, the most distinguished figure.

Newcomb was born in the village of Wallace, Nova Scotia, March 12,
1835. His father, who was a teacher, gave him his elementary education;
and at the age of eighteen we find him teaching a country school in
Maryland. Two years later, a position as computer on the "Nautical
Almanac" brought him to Cambridge, Mass., where he studied in Har-
vard University till 1861, when he was appointed professor of mathe-
matics in the United States Navy. He remained in the government
service till he was retired as a rear admiral in 1897, having served besides
as professor of mathematics and astronomy in Johns Hopkins University,
Baltimore, from 1884.

Newcomb's chief labors were in the department of mathematical
astronomy, and were directed toward the explanation of the observed
movements of the heavenly bodies. The difficulty and complexity of the
calculations involved are beyond the conception of the layman; and the
achievements which brought Newcomb honors from the learned of
almost all civilized countries have to be taken on trust by the general.
He had, nevertheless, an admirable power of clear exposition of those
parts of his subject which were capable of popularization; and the accom-
panying paper is a good example of the simple treatment of a large
subject.

Newcomb's interests extended beyond his special field, and he wrote
with vigor and originality on finance and economics, and played a lead-
ing part in the general intellectual life of his time. When he died in the
midst of his labors on July 11, 1909, he left a place at the head of
American science that will not easily be filled.

THE EXTENT OF THE
UNIVERSE

WE cannot expect that the wisest men o£ our remotest pos-
terity, who can base their conclusions upon thousands of
years of accurate observation, will reach a decision on this
subject without some measure of reserve. Such being the case, it
might appear the dictate of wisdom to leave its consideration to some
future age, when it may be taken up with better means of informa-
tion than we now possess. But the question is one which will refuse
to be postponed so long as the propensity to think of the possibilities
of creation is characteristic of our race. The issue is not whether we
shall ignore the question altogether, like Eve in the presence of
Raphael; but whether in studying it we shall confine our specula-
tions within the limits set by sound scientific reasoning. Essaying
to do this, I invite the reader's attention to what science may suggest,
admitting in advance that the sphere of exact knowledge is small
compared with the possibilities of creation, and that outside this
sphere we can state only more or less probable conclusions.

The reader who desires to approach this subject in the most recep-
tive spirit should begin his study by betaking himself on a clear,
moonless evening, when he has no earthly concern to disturb the
serenity of his thoughts, to some point where he can lie on his back
on bench or roof, and scan the whole vault of heaven at one view.
He can do this with the greatest pleasure and profit in late summer
or autumn — winter would do equally well were it possible for the
mind to rise so far above bodily conditions that the question of
temperature should not enter. The thinking man who does this
under circumstances most favorable for calm thought will form a new
conception of the wonder of the universe. If summer or autumn be
Copyright, 1906, by Harper & Brothers.
3"

312 NEWCOMB

chosen, the stupendous arch of the Milky Way will pass near the
zenith, and the constellation Lyra, led by its beautiful blue Vega of
the first magnitude, may be not very far from that point. South of it
will be seen the constellation Aquila, marked by the bright Altair,
between two smaller but conspicuous stars. The bright Arcturus will
be somewhere in the west, and, if the observation is not made too
early in the season, Aldebaran will be seen somewhere in the east.
When attention is concentrated on the scene the thousands of stars on
each side of the Milky Way will fill the mind with the consciousness
of a stupendous and all-embracing frame, beside which all human
affairs sink into insignificance. A new idea will be formed of such
a well-known fact of astronomy as the motion of the solar system in
space, by reflecting that, during all human history, the sun, carrying
the earth with it, has been flying towards a region in or just south
of the constellation Lyra, with a speed beyond all that art can pro-
duce on earth, without producing any change apparent to ordinary
vision in the aspect of the constellation. Not only Lyra and Aquila,
but every one of the thousand stars which form the framework of the
sky, were seen by our earliest ancestors just as we see them now.
Bodily rest may be obtained at any time by ceasing from our labors,
and weary systems may find nerve rest at any summer resort; but I
know of no way in which complete rest can be obtained for the
weary soul — in which the mind can be so entirely relieved of the
burden of all human anxiety — as by the contemplation of the spec-
tacle presented by the starry heavens under the conditions just
described. As we make a feeble attempt to learn what science can
tell us about the structure of this starry frame, I hope the reader will
allow me to at least fancy him contemplating it in this way.

The first question which may suggest itself to the inquiring reader
is: How is it possible by any methods of observation yet known to
the astronomer to learn anything about the universe as a whole?
We may commence by answering this question in a somewhat com-
prehensive way. It is possible only because the universe, vast though
it is, shows certain characteristics of a unified and bounded whole.
It is not a chaos, it is not even a collection of things, each of which
came into existence in its own separate way. If it were, there would
be nothing in common between two widely separate regions of the

THE EXTENT OF THE UNIVERSE 313

universe. But, as a matter of fact, science shows unity in the whole
structure, and diversity only in details. The Milky Way itself will
be seen by the most ordinary observer to form a single structure.
This structure is, in some sort, the foundation on which the universe
is built. It is a girdle which seems to span the whole of creation, so
far as our telescopes have yet enabled us to determine what creation
is; and yet it has elements of similarity in all its parts. What has
yet more significance, it is in some respects unlike those parts of the
universe which lie without it, and even unlike those which lie in that
central region within it where our system is now situated. The
minute stars, individually far beyond the limits of visibility to the
naked eye, which form its cloudlike agglomerations, are found to be
mostly bluer in color, from one extreme to the other, than the gen-
eral average of the stars which make up the rest of the universe.

In the preceding essay on the structure of the universe, we have
pointed out several features of the universe showing the unity of the
whole. We shall now bring together these and other features with
a view of showing their relation to the question of the extent of the
universe.

The Milky Way being in a certain sense the foundation on which
the whole system is constructed, we have first to notice the symmetry
of the whole. This is seen in the fact that a certain resemblance is
found in any two opposite regions of the sky, no matter where we
choose them. If we take them in the Milky Way, the stars are more
numerous than elsewhere; if we take opposite regions in or near the
Milky Way, we shall find more stars in both of them than elsewhere;
if we take them in the region anywhere around the poles of the
Milky Way, we shall find fewer stars, but they will be equally nu-
merous in each of the two regions. We infer from this that whatever
cause determined the number of the stars in space was of the same
nature in every two antipodal regions of the heavens.

Another unity marked with yet more precision is seen in the
chemical elements of which stars are composed. We know that the
sun is composed of the same elements which we find on the earth
and into which we resolve compounds in our laboratories. These
same elements are found in the most distant stars. It is true that
some of these bodies seem to contain elements which we do not find

314 NEWCOMB

on earth. But as these unknown elements are scattered from one
extreme of the universe to the other, they only serve still further to
enforce the unity which runs through the whole. The nebulae are
composed, in part at least, of forms of matter dissimilar to any with
which we are acquainted. But, different though they may be, they
are alike in their general character throughout the whole field we
are considering. Even in such a feature as the proper motions of the
stars, the same unity is seen. The reader doubtless knows that each
of these objects is flying through space on its own course with a speed
comparable with that of the earth around the sun. These speeds
range from the smallest limit up to more than one hundred miles
a second. Such diversity might seem to detract from the unity of
the whole; but when we seek to learn something definite by taking
their average, we find this average to be, so far as can yet be deter-
mined, much the same in opposite regions of the universe. Quite
recently it has become probable that a certain class of very bright
stars known as Orion stars — because there are many of them in the
most brilliant of our constellations — which are scattered along the
whole course of the Milky Way, have one and all, in the general
average, slower motions than other stars. Here again we have a
definable characteristic extending through the universe. In drawing
attention to these points of similarity throughout the whole universe,
it must not be supposed that we base our conclusions directly upon
them. The point they bring out is that the universe is in the nature
of an organized system; and it is upon the fact of its being such a
system that we are able, by other facts, to reach conclusions as to its
structure, extent, and other characteristics.

One of the great problems connected with the universe is that of
its possible extent. How far away are the stars? One of the unities
which we have described leads at once to the conclusion that the
stars must be at very different distances from us; probably the more
distant ones are a thousand times as far as the nearest; possibly even
farther than this. This conclusion may, in the first place, be based
on the fact that the stars seem to be scattered equally throughout
those regions of the universe which are not connected with the
Milky Way. To illustrate the principle, suppose a farmer to sow a
wheat field of entirely unknown extent with ten bushels of wheat.

THE EXTENT OF THE UNIVERSE 315

We visit the field and wish to have some idea of its acreage. We
may do this if we know how many grains of wheat there are in the
ten bushels. Then we examine a space two or three feet square in
any part of the field and count the number of grains in that space.
If the wheat is equally scattered over the whole field, we find its
extent by the simple rule that the size of the field bears the same
proportion to the size of the space in which the count was made
that the whole number of grains in the ten bushels sown bears to
the number of grains counted. If we find ten grains in a square
foot, we know that the number of square feet in the whole field is
one-tenth that of the number of grains sown. So it is with the uni-
verse of stars. If the latter are sown equally through space, the
extent of the space occupied must be proportional to the number of
stars which it contains.

But this consideration does not tell us anything about the actual
distance of the stars or how thickly they may be scattered. To do
this we must be able to determine the distance of a certain number of
stars, just as we suppose the farmer to count the grains in a certain
small extent of his wheat field. There is only one way in which we
can make a definite measure of the distance of any one star. As the
earth swings through its vast annual circuit round the sun, the direc-
tion of the stars must appear to be a little different when seen from
one extremity of the circuit than when seen from the other. This
difference is called the parallax of the stars; and the problem of
measuring it is one of the most delicate and difficult in the whole
field of practical astronomy.

The nineteenth century was well on its way before the instru-
ments of the astronomer were brought to such perfection as to admit
of the measurement. From the time of Copernicus to that of Bessel
many attempts had been made to measure the parallax of the stars,
and more than once had some eager astronomer thought himself
successful. But subsequent investigation always showed that he
had been mistaken, and that what he thought was the effect of
parallax was due to some other cause, perhaps the imperfections of
his instrument, perhaps the effect of heat and cold upon it or upon
the atmosphere through which he was obliged to observe the star,
or upon the going of his clock. Thus things went on until 1837,

3l6 NEWCOMB

when Bessel announced that measures with a heliometer — the most
refined instrument that has ever been used in measurement — showed
that a certain star in the constellation Cygnus had a parallax of one-
third of a second. It may be interesting to give an idea of this
quantity. Suppose one's self in a house on top of a mountain looking
out of a window one foot square, at a house on another mountain
one hundred miles away. One is allowed to look at that distant house
through one edge of the pane of glass and then through the opposite
edge; and he has to determine the change in the direction of the dis-
tant house produced by this change of one foot in his own position.
From this he is to estimate how far off the other mountain is. To do
this, one would have to measure just about the amount of parallax
that Bessel found in his star. And yet this star is among the few
nearest to our system. The nearest star of all, Alpha Centauri, visible
only in latitudes south of our middle ones, is perhaps half as far as
Bessel's star, while Sirius and one or two others are nearly at the
same distance. About loo stars, all told, have had their parallax
measured with a greater or less degree of probability. The work is
going on from year to year, each successive astronomer who takes
it up being able, as a general rule, to avail himself of better instru-
ments or to use a better inethod. But, after all, the distances of even
some of the loo stars carefully measured must still remain quite
doubtful.

Let us now return to the idea of dividing the space in which the
universe is situated into concentric spheres drawn at various dis-
tances around our system as a centre. Here we shall take as our
standard a distance 400,000 times that of the sun from the earth.
Regarding this as a unit, we imagine ourselves to measure out in any
direction a distance twice as great as this — then another equal dis-
tance, making one three times as great, and so indefinitely. We then
have successive spheres of which we take the nearer one as the unit.
The total space filled by the second sphere will be 8 times the unit;
that of the third space 27 times, and so on, as the cube of each
distance. Since each sphere includes all those within it, the volume
of space between each two spheres will be proportional to the differ-
ence of these numbers — that is, to i, 7, 19, etc. Comparing these
volumes with the number of stars probably within them, the general

THE EXTENT OF THE UNIVERSE $IJ

result up to the present time is that the number of stars in any o£
these spheres will be about equal to the units o£ volume which they
comprise, when we take for this unit the smallest and innermost of
the spheres, having a radius 400,000 times the sun's distance. We are
thus enabled to form some general idea of how thickly the stars are
sown through space. We cannot claim any numerical exactness for
this idea, but in the absence of better methods it does afford us some
basis for reasoning.

Now we can carry on our computation as we supposed the farmer
to measure the extent of his wheat field. Let us suppose that there
are 125,000,000 stars in the heavens. This is an exceedingly rough
estimate, but let us make the supposition for the time being. Accept-
ing the view that they are nearly equally scattered throughout space,
it will follow that they must be contained within a volume equal
to 125,000,000 times the sphere we have taken as our unit. We find
the distance of the surface of this sphere by extracting the cube root
of this number, which gives us 500. We may, therefore, say, as the
result of a very rough estimate, that the number of stars we have
supposed would be contained within a distance found by multiply-
ing 400,000 times the distance of the sun by 500; that is, that they are
contained within a region whose boundary is 200,000,000 times the
distance of the sun. This is a distance through which light would
travel in about 3,300 years.

It is not impossible that the number of stars is much greater than
that we have supposed. Let us grant that there are eight times as
many, or 1,000,000,000. Then we should have to extend the boundary
of our universe twice as far, carrying it to a distance which light
would require 6,600 years to travel.

There is another method of estimating the thickness with which
stars are sown through space, and hence the extent of the universe,
the result of which will be of interest. It is based on the proper
motion of the stars. One of the greatest triumphs of astronomy of
our time has been the measurement of the actual speed at which
many of the stars are moving to or from us in space. These measures
are made with the spectroscope. Unfortunately, they can be best
made only on the brighter stars — ^becoming very difficult in the case
of stars not plainly visible to the naked eye. Still the motions of

3l8 NEWCOMB

several hundreds have been measured and the number is constantly
increasing.

A general result of all these measures and of other estimates may
be summed up by saying that there is a certain average speed with
which the individual stars move in space; and that this average is
about twenty miles per second. We are also able to form an estimate
as to what proportion of the stars move with each rate of speed from
the lowest up to a limit which is probably as high as 150 miles per
second. Knowing these proportions we have, by observation of the
proper motions of the stars, another method of estimating how
thickly they are scattered in space; in other words, what is the volume
of space which, on the average, contains a single star. This method
gives a thickness of the stars greater by about twenty-five per cent,
than that derived from the measures of parallax. That is to say, a
sphere like the second we have proposed, having a radius 800,000
times the distance of the sun, and therefore a diameter 1,600,000
times this distance, would, judging by the proper motions, have ten
or twelve stars contained within it, while the measures of parallax
only show eight stars within the sphere of this diameter having the
sun as its centre. The probabilities are in favor of the result giving
the greater thickness of the stars. But, after all, the discrepancy does
not change the general conclusion as to the limits of the visible
universe. If we cannot estimate its extent with the same certainty
that we can determine the size of the earth, we can still form a
general idea of it.

The estimates we have made are based on the supposition that the
stars are equally scattered in space. We have good reason to believe
that this is true of all the stars except those of the Milky Way. But,
after all, the latter probably includes half the whole number of
stars visible with a telescope, and the question may arise whether our
results are seriously wrong from this cause. This question can best
be solved by yet another method of estimating the average distance
of certain classes of stars.

The parallaxes of which we have heretofore spoken consist in the
change in the direction of a star produced by the swing of the earth
from one side of its orbit to the other. But we have already remarked
that our solar system, with the earth as one of its bodies, has been

THE EXTENT OF THE UNIVERSE 319

journeying straightforward through space during all historic times.
It follows, therefore, that we are continually changing the position
from which we view the stars, and that, if the latter were at rest,
we could, by measuring the apparent speed with which they are
moving in the opposite direction from that of the earth, determine
their distance. But since every star has its own motion, it is impos-
sible, in any one case, to determine how much of the apparent motion
is due to the star itself, and how much to the motion of the solar
system through space. Yet, by taking general averages among groups
of stars, most of which are probably near each other, it is possible
to estimate the average distance by this method. When an attempt
is made to apply it, so as to obtain a definite result, the astronomer
finds that the data now available for the purpose are very deficient.
The proper motion of a star can be determined only by comparing
its observed position in the heavens at two widely separate epochs.
Observations of sufficient precision for this purpose were commenced
about 1750 at the Greenwich Observatory, by Bradley, then Astron-
omer Royal of England. But out of 3,000 stars which he determined,
only a few are available for the purpose.- Even since his time, the
determinations made by each generation of astronomers have not
been sufficiently complete and systematic to furnish the material for
anything like a precise determination of the proper motions of stars.
To determine a single position of any one star involves a good deal
of computation, and if we reflect that, in order to attack the problem
in question in a satisfactory way, we should have observations of
1,000,000 of these bodies made at intervals of at least a considerable
fraction of a century, we see what an enormous task the astronomers
dealing with this problem have before them, and how imperfect must
be any determination of the distance of the stars based on our motion
through space. So far as an estimate can be made, it seems to agree
fairly well with the results obtained by the other methods. Speaking
roughly, we have reason, from the data so far available, to believe
that the stars of the Milky Way are situated at a distance between
100,000,000 and 200,000,000 times the distance of the sun. At dis-
tances less than this it seems likely that the stars are distributed
through space with some approach to uniformity. We may state as
a general conclusion, indicated by several methods of making the

320 NEWCOMB

estimate, that nearly all the stars which we can see with our tele-
scopes are contained within a sphere not likely to be much more than
200,000,000 times the distance of the sun.

The inquiring reader may here ask another question. Granting
that all the stars we can see are contained within this limit, may
there not be any number of stars outside the limit which are invisible
only because they are too far away to be seen ?

This question may be answered quite definitely if we grant that
light from the most distant stars meets with no obstruction in reach-
ing us. The most conclusive answer is afforded by the measure of
starlight. If the stars extended out indefinitely, then the number of
those of each order of magnitude would be nearly four times that
of the magnitude next brighter. For example, we should have nearly
four times as many stars of the sixth magnitude as of the fifth; nearly
four times as many of the seventh as of the sixth, and so on indefi-
nitely. Now, it is actually found that while this ratio of increase is
true for the brighter stars, it is not so for the fainter ones, and that
the increase in the number of the latter rapidly falls off when we
make counts of the fainter telescopic stars. In fact, it has long been
known that, were the universe infinite in extent, and the stars equally
scattered through all space, the whole heavens would blaze with
the light of countless millions of distant stars separately invisible
even with the telescope.

The only way in which this conclusion can be invalidated is by
the possibility that the light of the stars is in some way extinguished
or obstructed in its passage through space. A theory to this effect
was propounded by Struve nearly a century ago, but it has since
been found that the facts as he set them forth do not justify the con-
clusion, which was, in fact, rather hypothetical. The theories of
modern science converge towards the view that, in the pure ether of
space, no single ray of light can ever be lost, no matter how far it
may travel. But there is another possible cause for the extinction
of light. During the last few years discoveries of dark and there-
fore invisible stars have been made by means of the spectroscope
with a success which would have been quite incredible a very few
years ago, and which, even to-day, must excite wonder and admira-
tion. The general conclusion is that, besides the shining stars which

THE EXTENT OF THE UNIVERSE 321

exist in space, there may be any number of dark ones, forever invis-
ible in our telescopes. May it not be that these bodies are so numer-
ous as to cut off the light which we would otherwise receive from
the more distant bodies of the universe ? It is, of course, impossible
to answer this question in a positive way, but the probable conclusion
is a negative one. We may say with certainty that dark stars are not
so numerous as to cut off any important part of the light from the
stars of the Milky Way, because, if they did, the latter would not be
so clearly seen as it is. Since we have reason to believe that the
Milky Way comprises the more distant stars of our system, we may
feel fairly confident that not much light can be cut off by dark
bodies from the most distant region to which our telescopes can
penetrate. Up to this distance we see the stars just as they are. Even
within the limit of the universe as we understand it, it is Ukely that
more than one-half the stars which actually exist are too faint to be
seen by human vision, even when armed with the most powerful
telescopes. But their invisibility is due only to their distance and the
faintness of their intrinsic light, and not to any obstructing agency.
The possibility of dark stars, therefore, does not invalidate the
general conclusions at which our survey of the subject points. The
universe, so far as we can see it, is a bounded whole. It is sur-
rounded by an immense girdle of stars, which, to our vision, appears
as the Milky Way. While we cannot set exact limits to its distance,
we may yet confidently say that it is bounded. It has uniformities
running through its vast extent. Could we fly out to distances equal
to that of the Milky Way, we should find comparatively few stars
beyond the limits of that girdle. It is true that we cannot set any
definite limit and say that beyond this nothing exists. What we can
say is that the region containing the visible stars has some approxi-
mation to a boundary. We may fairly anticipate that each successive
generation of astronomers, through coming centuries, will obtain a
little more light on the subject — will be enabled to make more defi-
nite the boundaries of our system of stars, and to draw more and
more probable conclusions as to the existence or non-existence of any
object outside of it. The wise investigator of to-day will leave to
them the task of putting the problem into a more positive shape.

GEOGRAPHICAL EVOLUTION

BY

SIR ARCHIBALD GEIKIE

INTRODUCTORY NOTE

Sir Archibald Geikie was born in Edinburgh, Scotland, in 1835, and
educated at the university in that city. He became a member of the
Geological Survey of Scotland, of which, in 1867, he was made director;
and from 1871 to 1882 was Murchison professor of geology and min-
eralogy in his own university. At the latter date he resigned to take up
the general directorship of the Geological Survey of the United Kingdom,
which he held till 1901. He has been an extremely productive investi-
gator, and his achievements have brought him many distinctions from
learned societies at home and abroad, including the honor of knighthood,
the presidency of the British Association, and the secretaryship of the
Royal Society.

Among his more important writings may be mentioned "The Scenery
of Scotland, Viewed in Connection With its Physical Geography" (1865),
"Text-book of Geology" (1882), "Ancient Volcanoes of Britain" (1897),
"Types of Scenery and Their Influence on Literature" (1898), and
"Landscape in History" (1905).

The following paper on "Geographical Evolution," published among
his "Geological Sketches at Home and Abroad" (1882), might be entitled
with less ambiguity "Geological Evolution," since it is in this sense
rather than in the broader modern signification that the word "geographi-
cal" is employed throughout the essay.

The interest in literature indicated by the names of several of Geikie's
books has not been without influence on his writings, for there are few
scientific authors to-day who can command so fascinating a style. His
power of exposition is shown by the skill with which, in the present
paper, he maps out in comparatively few pages, the views of his school
as to the geological history of the earth.

GEOGRAPHICAL EVOLUTION^

IN the quaint preface to his Navigations and Voyages of the
English Nation, Hakluyt calls geography and chronology "the
sunne and moone, the right eye and the left of all history."
The position thus claimed for geography three hundred years ago by
the great English chronicler was not accorded by his successors, and
has hardly been admitted even now. The functions of the geog-
rapher and the traveller, popularly assumed to be identical, have been
supposed to consist in descriptions of foreign countries, their climate,
productions, and inhabitants, bristling on the one hand with dry
statistics, and relieved on the other by as copious an introduction as
may be of stirring adventure and personal anecdote. There has
indeed been much to justify this popular assumption. It was not
until the key-note of its future progress vvas struck by Karl Ritter,
within the present century, that geography advanced beyond the
domain of travellers' tales and desultory observation into that o£
orderly, methodical, scientific progress. This branch of inquiry,
however, is now no longer the pursuit of mere numerical statistics,
nor the chronicle of marvellous and often questionable adventures
by flood and fell. It seeks to present a luminous picture of the
earth's surface, its various forms of configuration, its continents,
islands, and oceans, its mountains, valleys, and plains, its rivers and
lakes, its climates, plants, and animals. It thus endeavours to pro-
duce a picture which shall not be one of mere topographical detail.
It ever looks for a connection between scattered facts, tries to ascer-
tain the relations which subsist between the different parts of the
globe, their reactions on each other and the function of each in the
general economy of the whole. Modern geography studies the dis-
tribution of vegetable and animal life over the earth's surface, with
the action and reaction between it and the surrounding inorganic

' A Lecture delivered at the Evening Meeting of the Royal Geographical Society,
24th March, 1879.

325

326 GEIKIE

world. It traces how man, alike unconsciously and knowingly, has
changed the face of nature, and how, on the other hand, the con-
ditions of his geographical environment have moulded his own
progress.

With these broad aims geography comes frankly for assistance to
many different branches of science. It does not, however, claim in
any measure to occupy their domain. It brings to the consideration
of their problems a central human interest in which these sciences
are sometimes apt to be deficient; for it demands first of all to know
how the problems to be solved bear upon the position and history
of man and of this marvellously-ordered world wherein he finds
himself undisputed lord. Geography freely borrows from meteor-
ology, physics, chemistry, geology, zoology, and botany; but the debt
is not all on one side. Save for the impetus derived from geograph-
ical research, many of these sciences would not be in their present
advanced condition. They gain in vast augmentation of facts, and
may cheerfully lend their aid in correlating these for geographical
requirements.

In no respect does modern geography stand out more prominently
than in the increased precision and fulness of its work. It has fitted
out exploratory expeditions, and in so doing has been careful to see
them provided with the instruments and apparatus necessary to
enable them to contribute accurate and definite results. It has guided
and fostered research, and has been eager to show a generous appre-
ciation of the labours of those by whom our knowledge of the earth
has been extended. Human courage and endurance are not less
enthusiastically applauded than they once were; but they must be
united to no common powers of observation before they will now
raise a traveller to the highest rank. When we read a volume of
recent travel, while warmly appreciating the spirit of adventure,
fertility of resource, presence of mind, and other moral qualities of
its author, we instinctively ask ourselves, as we close its pages, what
is the sum of its additions to our knowledge of the earth ? From the
geographical point of view — and it is to this point alone that these
remarks apply — we must rank an explorer according to his success
in widening our knowledge and enlarging our views regarding the
aspects of nature.

GEOGRAPHICAL EVOLUTION 327

The demands of modern geography are thus becoming every year
more exacting. It requires more training in its explorers abroad,
more knowledge on the part of its readers at home. The days are
drawing to a close when one can gain undying geographical renown
by struggling against man and beast, fever and hunger and drought,
across some savage and previously unknown region, even though
little can be shown as the outcome of the journey. All honour to the
pioneers by whom this first exploratory work has been so nobly
done! They will be succeeded by a race that will find its laurels
more difficult to win — a race from which more will be expected, and
which will need to make up in the variety, amount, and value of its
detail, what it lacks in the freshness of first glimpses into new lands.

With no other science has geography become more intimately
connected than with geology, and the connection is assuredly des-
tined to become yet deeper and closer. These two branches of
human knowledge are, to use Hakluyt's phrase, "the sunne and
moone, the right eye and the left," of all fruitful inquiry into the
character and history of the earth's surface. As it is impossible to
understand the genius and temperament of a people, its laws and
institutions, its manners and customs, its buildings and its industries,
unless we trace back the history of that people, and mark the rise
and effect of each varied influence by which its progress has been
moulded in past generations; so it is clear that our knowledge of
the aspect of a continent, its mountains and valleys, rivers and
plains, and all its surface-features, cannot be other than singularly
feeble and imperfect, unless we realise what has been the origin of
these features. The land has had a history, not less than the human
races that inhabit it.

One can hardly consider attentively the future progress of geog-
raphy without being convinced that in the wide development yet in
store for this branch of human inquiry, one of its main lines of
advance must be in the direction of what may be termed geographi-
cal evolution. The geographer will no longer be content to take
continents and islands, mountain chains and river valleys, table-lands
and plains, as initial or aboriginal outlines of the earth's surface. He
will insist on knowing what the geologist can tell him regarding the
growth of these outlines. He will try to trace out the gradual evolu-

328 GEIKIE

tion of a continent, and may even construct maps to show its succes-
sive stages of development. At the same time, he will seek for infor-
mation regarding the history of the plants and animals of the region,
and may find much to reward his inquiry as to the early migrations
of the fauna and flora, including those even of man himself. Thus
his pictures of the living world of to-day, as they become more
detailed and accurate, will include more and more distinctly a back-
ground of bygone geographical conditions, out of which, by con-
tinuous sequence, the present conditions will be shown to have arisen.

I propose this evening to sketch in mere outline the aspects of one
side of this evolutional geography. I wish to examine, in the first
place, the evidence whereby we establish the fundamental fact that
the present surface of any country or continent is not that which it
has always borne, and the data by which we may trace backward the
origin of the land; and, in the second place, to consider, by way of
illustration, some of the more salient features in the gradual growth
of the framework of Europe.

The first of these two divisions of the subject deals with general
principles, and may be conveniently grouped into two parts: 1st, The
Materials of the Land. 2d, The Building of the Land.

L General Principles of Continental Evolution,
i. — The Materials of the Land.

Without attempting to enter into detailed treatment of this branch
of the subject, we may, for the immediate purpose in view, content
ourselves with the broad, useful classification of the materials of the
land into two great series — Fragmental and Crystalline.

§ I. Fragmental. — A very cursory examination of rocks in almost
any part of the world suffices to show that by far the larger portion
of them consists of compacted fragmentary materials. Shales, sand-
stones, and conglomerates, in infinite variety of texture and colour,
are piled above each other to form the foundation of plains and the
structure of mountains. Each of these rocks is composed of distinct
particles, worn by air, rain, frost, springs, rivers, glaciers, or the sea,
from previously existing rocks. They are thus derivative formations,
and their source, as well as their mode of origin, can be determined.

GEOGRAPHICAL EVOLUTION 329

Their component grains are for the most part rounded, and bear
evidence of having been rolled about in water. Thus we easily and
rapidly reach a first and fundamental conclusion — that the substance
of the main part of the solid land has been originally laid down and
assorted under water.

The mere extent of the area covered by these water-formed rocks
would of itself suggest that they must have been deposited in the
sea. We cannot imagine rivers or lakes of magnitude sufficient to
have spread over the sites of the present continents. The waters of
the ocean, however, may easily be conceived to have rolled at differ-
ent times over all that is now dry land. The fragmental rocks con-
tain, indeed, within themselves proof that they were mainly of
marine, and not of lacustrine or fluviatile origin. They have pre-
served in abundance the remains of foraminifera, corals, crinoids,
molluscs, annelides, crustaceans, fishes, and other organisms of
undoubtedly marine habitat, which must have lived and died in the
places where their traces remain still visible.

But not only do these organisms occur scattered through sedi-
mentary rocks; they actually themselves form thick masses of min-
eral matter. The Carboniferous or Mountain Limestone of Central
England and Ireland, for example, reaches a thickness of from 2,000
to 3,000 feet, and covers thousands of square miles of surface. Yet it
is almost entirely composed of congregated stems and joints and
plates of crinoids, with foraminifera, corals, bryozoans, brachiopods,
lamellibranchs, gasteropods, fish-teeth, and other unequivocally
marine organisms. It must have been for ages the bottom of a clear
sea, over which generation after generation lived and died, until
their accumulated remains had gathered into a deep and compact
sheet of rock. From the internal evidence of the stratified formations
we thus confidently announce a second conclusion — that a great
portion of the solid land consists of materials which have been laid
down on the floor of the sea.

From these familiar and obvious deductions we may proceed fur-
ther to inquire under what conditions these marine formations,
spreading so widely over the land, were formed. According to a
popular belief, shared in perhaps by not a few geologists, land and
sea have been continually changing places. It is supposed that while,

330 GEIKIE

on the one hand, there is no part of a continent over which sea
waves may not have rolled, so, on the other hand, there is no lonely
abyss of the ocean where a wide continent may not have bloomed.
That this notion rests upon a mistaken interpretation of the facts
may be shown from an examination — (i) of the rocks of the land,
and (2) of the bottom of the present ocean.

(i) Among the thickest masses of sedimentary rock — those of the
ancient palaeozoic systems — no features recur more continually than
alterations of different sediments, and surfaces of rock covered with
well-preserved ripple-marks, trails and burrows of annelides, poly-
gonal and irregular desiccation marks, like the cracks at the bottom
of a sun-dried muddy pool. These phenomena unequivocally point
to shallow and even littoral waters. They occur from bottom to top
of deposits which reach a thickness of several thousand feet. They
can be interpreted only in one way, viz., that their deposition began
in shallow water; that during their formation the area of deposit
gradually subsided for thousands of feet; yet that the rate of accumu-
lation of sediment kept pace on the whole with this depression; and
hence, that the original shallow water character of the deposits
remained, even after the original sea bottom had been buried under
a vast mass of sedimentary matter. Now, if this explanation be true,
even for the enormously thick and comparatively uniform systems
of older geological periods, the relatively thin and much more varied
stratified groups of later date can offer no difficulty. In short, the
more attentively the stratified rocks of the crust of the earth are
studied, the more striking becomes the absence of any deposits among
them which can legitimately be considered those of a deep sea. They
have all been deposited in comparatively shallow water.

The same conclusion may be arrived at from a consideration of
the circumstances under which the deposition must have taken place.
It is evident that the sedimentary rocks of all ages have been derived
from degradation of land. The gravel, sand, and mud, of which they
consist, existed previously as part of mountains, hills, or plains.
These materials carried down to the sea would arrange themselves
there as they do still, the coarser portions nearest the shore, the finer
silt and mud farthest from it. From the earliest geological times the
great area of deposit has been, as it still is, the marginal belt of sea

GEOGRAPHICAL EVOLUTION 33 1

floor skirting the land. It is there that nature has always strewn "the
dust of continents to be." The decay of old rocks has been unceas-
ingly in progress on the land, and the building up of new rocks has
been unintermittently going on underneath the adjoining sea. The
two phenomena are the complementary sides of one process, which
belongs to the terrestrial and shallow oceanic parts of the earth's sur-
face and not to the wide and deep ocean basins.

(2) Recent explorations of the bottom of the deep sea all over the
world have brought additional light to this question. No part of the
results obtained by the Challenger Expedition has a profounder
interest for geologists and geographers than the proof which they
furnish that the floor of the ocean basins has no real analogy among
the sedimentary formations that form most of the framework of the
land. We now know by actual dredging and inspection that the
ordinary sediment washed off the land sinks to the sea bottom before
it reaches the deeper abysses, and that, as a rule, only the finer
particles are carried more than a few score of miles from the shore.
Instead of such sandy and pebbly material as we find so largely
among the sedimentary rocks of the land, wide tracts of the sea
bottom at great depths are covered with various kinds of organic
ooze, composed sometimes of minute calcareous foraminifera, some-
times of siliceous radiolaria or diatoms. Over other areas vast sheets
of clay extend, derived apparently from the decomposition of vol-
canic detritus, of which large quantities are floated away from vol-
canic islands, and much of which may be produced by submarine
volcanoes. On the tracts farthest removed from any land the sedi-
ment seems to settle scarcely so rapidly as the dust that gathers over
the floor of a deserted hall. Mr. Murray of the Challenger staff, has
described how from these remote depths large numbers of shark's
teeth and ear-bones of whales were dredged up. We cannot suppose
the number of sharks and whales to be much greater in these regions
than in others where their relics were found much less plentifully.
The explanation of the abundance of their remains was supplied by
their varied condition of decay and preservation. Some were com-
paratively fresh, others had greatly decayed, and were incrusted with
or even completely buried in a deposit of earthy manganese. Yet the
same cast of the dredge brought up these different stages of decay

332 GEIKIE

from the same surface of the sea floor. While generation after gener-
ation of sea creatures drops its bones to the bottom, now here, now
there, so exceedingly feeble is the rate of deposit of sediment that they
lie uncovered, mayhap for centuries, so that the remains which sink
to-day may lie side by side with the mouldered and incrusted bones
that found their way to the bottom hundreds of years ago.

Another striking indication of the very slow rate at which sedi-
mentation takes place in these abysses has also been brought to notice
by Mr. Murray. In the clay from the bottom he found numerous
minute spherical granules of native iron, which, as he suggests, are
almost certainly of meteoric origin — fragments of those falling stars
which, coming to us from planetary space, burst into fragments when
they rush into the denser layers of our atmosphere. In tracts where
the growth of silt upon the sea floor is excessively tardy, the fine
particles, scattered by the dissipation of these meteorites, may remain
in appreciable quantity. In this case, again, it is not needful to sup-
pose that meteorites have disappeared over these ocean depths more
numerously than over other parts of the earth's surface. The iron
granules have no doubt been as plentifully showered down else-
where, though they cannot be so readily detected in accumulating
sediment. I know no recent observation in physical geography more
calculated to impress deeply the imagination than the testimony of
this presumably meteoric iron from the most distant abysses of the
ocean. To be told that mud gathers on the floor of these abysses at
an extremely slow rate conveys but a vague notion of the tardiness
of the process. But to learn that it gathers so slowly, that the very
star-dust which falls from outer space forms an appreciable part
of it, brings home to us, as hardly anything else could do, the idea
of undisturbed and excessively slow accumulation.

From all this evidence we may legitimately conclude that the
present land of the globe, though formed in great measure of marine
formations, has never lain under the deep sea; but that its site must
always have been near land. Even its thick marine limestones are
the deposits of comparatively shallow water. Whether or not any
trace of aboriginal land may now be discoverable, the characters of
the most unequivocally marine formations bear emphatic testimony
to this proximity of a terrestrial surface. The present continental

GEOGRAPHICAL EVOLUTION 333

ridges have probably always existed in some form, and as a corollary
we may infer that the present deep ocean basins likewise date from
the remotest geological antiquity.

§ 2. Crystalline. — While the greater part of the framework of the
land has been slowly built up of sedimentary materials, it is abun-
dantly varied by the occurrence of crystalline masses, many of which
have been injected in a molten condition into rents underground, or
have been poured out in lava streams at the surface.

Without entering at all into geological detail, it will be enough for
the present purpose to recognise the characters and origin of two
great types of crystalline material which have been called respectively
the Eruptive and Metamorphic.

(a) Eruptive. — As the name denotes, eruptive or igneous rocks
have been ejected from the heated interior of the earth. In a modern
volcano lava ascends the central funnel, and issuing from the lip of
the crater or from lateral fissures pours down the slopes of the cone
in sheets of melted rock. The upper surface of the lava column
within the volcano is kept in constant ebullition by the rise of steam
through its mass. Every now and then a vast body of steam rushes
out with a terrific explosion, scattering the melted lava into impal-
pable dust, and filling the air with ashes and stones, which descend
in showers upon the surrounding country. At the surface, therefore,
igneous rocks appear, partly as masses of congealed lava, and partly
as more or less consolidated sheets of dust and stones. But beneath
the surface there must be a downward prolongation of the lava
column, which no doubt sends out veins into rents of the subter-
ranean rocks. We can suppose that the general aspect of the lava
which consolidates at some depth will differ from that which solidi-
fies above ground.

As a result of the revolutions which the crust of the earth has
undergone, the roots of many ancient volcanoes have been laid bare.
We have been, as it were, admitted into the secrets of these sub-
terranean laboratories of nature, and have learned much regarding
the mechanism of volcanic action which we could never have dis-
covered from any modern volcano. Thus, while on the one hand we
meet with beds of lava and consolidated volcanic ashes which were
undoubtedly erupted at the surface of the ground in ancient periods.

334 GEIKIE

and were subsequently buried deep beneath sedimentary accumula-
tions now removed, on the other hand we find masses of igneous
rock which certainly never came near the surface, but must have
been arrested in their ascent from below while still at a great depth,
and have been laid bare to the light after the removal of the pile of
rock under which they originally lay.

By noting these and other characters, geologists have learnt that,
besides the regions of still active volcanoes, there are few large areas
of the earth's surface where proofs of former volcanic action or of
the protrusion of igneous rocks may not be found. The crust of the
earth, crumpled and fissured, has been, so to speak, perforated and
cemented together by molten matter driven up from below.

{b) Metamorphic. — The sedimentary rocks of the land have
undergone many changes since their formation, some of which are
still far from being satisfactorily accounted for. One of these changes
is expressed by the term Metamorphism, and the rocks which have
undergone this process are called Metatnorphic. It seems to have
taken place under widely varied conditions, being sometimes con-
fined to small local tracts, at other times extending across a large
portion of the continent. It consists in the rearrangement of the com-
ponent materials of rocks, and notably in their recrystallisation along
particular lines or laminae. It is usually associated with evidence of
great pressure; the rocks in which it occurs having been corrugated
and crumpled, not only in vast folds, which extend across whole
mountains, but even in such minute puckerings as can only be
observed with the microscope. It shows itself more particularly
among the older geological formations, or those which have been
once deeply buried under more recent masses of rock, and have been
exposed as the result of the removal of these overlying accumulations.
The original characters of the sandstones, shales, grits, conglomer-
ates, and limestones, of which, no doubt, these metamorphic masses
once consisted, have been more or less effaced, and have given place
to that peculiar crystalline laminated or foliated structure so dis-
tinctively a result of metamorphism.

An attentive examination of a metamorphic region shows that
here and there the alteration and recrystallisation have proceeded so
far that the rocks graduate into granites and other so-called igneous
rocks. A series of specimens may be collected showing unaltered or

GEOGRAPHICAL EVOLUTION 335

at least quite recognisable sedimentary rocks at the one end, and
thoroughly crystalline igneous rocks at the other. Thus the remark-
able fact is brought home to the mind that ordinary sandstones,
shales, and other sedimentary materials may, in the course of ages,
be converted by underground changes into crystalline granite. The
framework of the land, besides being knit together by masses of
igneous rock intruded from below, has been strengthened by the
welding and crystallisation of its lowest rocks. It is these rocks
which rise along the central crests of mountain chains, where, after
the lapse of ages, they have been uncovered and laid bare, to be
bleached and shattered by frost and storm.

ii. — The Architecture of the Land.

Let us now proceed to consider how these materials, sedimentary
and crystalline, have been put together, so as to constitute the solid
land of the globe.

It requires but a cursory examination to observe that the sedi-
mentary masses have not been huddled together at random; that,
on the contrary, they have been laid down in sheets one over the
other. An arrangement of this kind at once betokens a chronological
sequence. The rocks cannot all have been formed simultaneously.
Those at the bottom must have been laid down before those at the
top. A truism of this kind seems hardly to require formal statement.
Yet it lies at the very foundation of any attempt to trace the geo-
logical history of a country. Did the rocks everywhere lie undis-
turbed one above another as they were originally laid down, their
clear order of succession would carry with it its own evident interpre-
tation. But such have been the changes that have arisen, partly from
the operation of forces from below, partly from that of forces acting
on the surface, that the true order of a series of rocks is not always
so easily determined. By starting, however, from where the succes-
sion is normal and unbroken, the geologist can advance with confi-
dence into regions where it has been completely interrupted; where
the rocks have been shattered, crumpled, and even inverted.

The clue which guides us through these labyrinths is a very simple
one. It is afforded by the remains of once living plants and animals
which have been preserved in the rocky framework of the land
Each well-marked series of sedimentary accumulations contains its

336 GEIKIE

own characteristic plants, corals, crustaceans, shells, fishes, or other
organic remains. By these it can be identified and traced from
country to country across a whole continent. When, therefore, the
true order of superposition of the rocks has been ascertained by ob-
serving how they lie upon each other, the succession of their fossils
is at the same time fixed. In this way the sedimentary part of the
earth's crust has been classified into different formations, each char-
acterised by its distinct assemblage of organic remains. In the most
recent formations, most of these remains are identical with still living
species of plants and animals; but as we descend in the series and
come into progressively older deposits the proportion of existing
species diminishes until at last all the species of fossils are found to
be extinct. Still lower and older rocks reveal types and assemblages
of organisms which depart farther and farther from the existing
order.

By noting the fossil contents of a formation, therefore, even in a
district where the rocks have been so disturbed that their sequence
is otherwise untraceable, the geologist can confidently assign their
relative position to each of the fractured masses. He knows, for
instance, using for our present purpose the letters of the alphabet to
denote the sequence of the formations, that a mass of limestone con-
taining fossils typical of the formation B must be younger than
another mass of rock containing the fossils of A. A series of strata
full of the fossils of H resting immediately on others charged with
those of C, must evidently be separated from these by a great gap,
elsewhere filled in by the intervening formations D, E, F, G. Nay,
should the rocks in the upper part of a mountain be replete with the
fossils proper to D, while those in the lower slopes showed only the
fossils of E, F, and G, it could be demonstrated that the materials
of the mountain had actually been turned upside down, for, as
proved by its organic remains, the oldest and therefore lowest forma-
tion had come to lie at the top, and the youngest, and therefore high-
est, at the bottom.

Of absolute chronology in such questions science can as yet give
no measure. How many millions of years each formation may have
required for its production, and how far back in time may be the era
of any given group of fossils, are problems to which no answer, other

GEOGRAPHICAL EVOLUTION 337

than a mere guess, can be returned. But this is a matter of far less
moment than the relative chronology, which can usually be accu-
rately fixed for each country, and on which all attempts to trace back
the history of the land must be based.

While, then, it is true that most of the materials of the solid land
have been laid down at successive periods under the sea, and that the
relative dates of their deposition can be determined, it is no less
certain that the formation of these materials has not proceeded unin-
terruptedly, and that they have not finally been raised into land by
a single movement. The mere fact that they are of marine origin
shows, of course, that the land owes its origin to some kind of
terrestrial disturbance. But when the sedimentary formations are
examined in detail, they present a most wonderful chronicle of long-
continued, oft-repeated, and exceedingly complex movements of the
crust of the globe. They show that the history of every country has
been long and eventful; that, in short, hardly any portion of the
land has reached its present condition, save after a protracted series
of geological revolutions.

One of the most obvious and not the least striking features in the
architecture of the land is the frequency with which the rocks,
though originally horizontal, or approximately so, have been tilted
up at various angles, or even placed on end. At first it might be sup-
posed that these disturbed positions have been assumed at random,
according to the capricious operations of subterranean forces. They
seem to follow no order, and to defy any attempt to reduce them to
system. Yet a closer scrutiny serves to establish a real connection
among them. They are found, for the most part, to belong to great,
though fractured, curves, into which the crust of the earth has been
folded. In low countries far removed from any great mountain
range, the rocks often present scarcely a trace of disturbance, or if
they have been affected, it is chiefly by having been thrown into
gentle undulations. As we approach the higher grounds, however,
they manifest increasing signs of commotion. Their undulations be-
come more frequent and steeper, until, entering within the moun-
tain region, we find the rocks curved, crumpled, fractured, inverted,
tossed over each other into yawning gulf and towering crest, like
billows arrested at the height of a furious storm.

338 GEIKIE

Yet even in the midst of such apparent chaos it is not impossible
to trace the fundamental law and order by which it is underlaid.
The prime fact to be noted is the universal plication and crumpling
of rocks which were at first nearly horizontal. From the gentle
undulations of the strata beneath the plains to their violent con-
tortion and inversion among the mountains, there is that insensible
gradation which connects the whole of these disturbances as parts
of one common process. They cannot be accounted for by any mere
local movements, though such movements no doubt took place
abundantly. The existence of a mountain chain is not to be explained
by a special upheaval or series of upheavals caused by an expansive
force acting from below. Manifestly the elevation is only one phase
of a vast terrestrial movement which has extended over whole con-
tinents, and has affected plains as well as high grounds.

The only cause which, so far as our present knowledge goes, could
have produced such widespread changes is a general contraction of
the earth's mass. There can be no doubt that at one time our planet
existed in a gaseous, then in a liquid condition. Since these early
periods it has continued to lose heat, and consequently to contract
and to grow more and more solid, until, as the physicists insist, it has
now become practically as rigid as a globe of glass or of steel. But
in the course of the contraction, after the solid external crust was
formed, the inner hot nucleus has lost heat more rapidly than the
crust, and has tended to shrink inward from it. As a consequence
of this internal movement, the outer solid shell has sunk down upon
the retreating nucleus. In so doing, it has of course had to accom-
modate itself to a diminished area, and this it could only accomplish
by undergoing plication and fracture. Though the analogy is not
a very exact one, we may liken our globe to a shrivelled apple. The
skin of the apple does not contract equally. As the internal moisture
passes off, and the bulk of the fruit is reduced, the once smooth
exterior becomes here and there corrugated and dimpled.

Without entering into this difficult problem in physical geology, it
may suffice if we carry with us the idea that our globe must once
have had a greater diameter than it now possesses, and that the
crumpling of its outer layers, whether due to mere contraction or, as
has been suggested, to the escape also of subterranean vapours,

GEOGRAPHICAL EVOLUTION 339

aflfords evidence of this diminution. A little reflection suffices to
show us that, even without any knowledge of the actual history of
the contraction, we might anticipate that the effects would neither
be continuous nor everywhere uniform. The solid crust would not,
we may be sure, subside as fast as the mass inside. It would, for a
time at least, cohere and support itself, until at last, gravitation prov-
ing too much for its strength, it would sink down. And the areas
and amount of descent would be greatly regulated by the varying
thickness and structure of the crust. Subsidence would not take place
everywhere; for, as a consequence of the narrower space into which
the crust sank, some regions would necessarily be pushed up. These
conditions appear to have been fulfilled in the past history of the
earth. There is evidence that the terrestrial disturbance has been
renewed again and again, after long pauses, and that, while the
ocean basins have on the whole been the great areas of depression,
the continents have been the lines of uprise or relief, where the rocks
were crumpled and pushed out of the way. Paradoxical, therefore,
as the statement may appear, it is nevertheless strictly true, that the
solid land, considered with reference to the earth's surface as a
whole, is the consequence of subsidence rather than of upheaval.

Grasping, then, this conception of the real character of the move-
ments to which the earth owes its present surface configuration, we
are furnished with fresh light for exploring the ancient history and
growth of the solid land. The great continental ridges seem to lie
nearly on the site of the earliest lines of relief from the strain of
contraction. They were forced up between the subsiding oceanic
basins at a very early period of geological history. In each succeeding
epoch of movement they were naturally used over again, and received
an additional push upward. Hence we see the meaning of the evi-
dence supplied by the sedimentary rocks as to shallow seas and
proximity of land. These rocks could not have been otherwise pro-
duced. They were derived from the waste of the land, and were
deposited near the land. For it must be borne in mind that every
mass of land as soon as it appeared above water was at once attacked
by the ceaseless erosion of moving water and atmospheric influences,
and immediately began to furnish materials for the construction of
future lands to be afterwards raised out of the sea.

340 GEIKIE

Each great period of contraction elevated anew the much-worn
land, and at the same time brought the consolidated marine sedi-
ments above water as parts of a new terrestrial surface. Again a
long interval would ensue, marked perhaps by a slow subsidence
both of the land and sea bottom. Meanwhile the surface of the land
was channelled and lowered, and its detritus was spread over the
sea floor, until another era of disturbance raised it once more with
a portion of the surrounding ocean bed. These successive upward
and downward movements explain why the sedimentary formations
do not occur as a continuous series, but often lie each upon the
upturned and worn edges of its predecessors.

Returning now to the chronological sequence indicated by the
organic remains preserved among the sedimentary rocks, we see
how it may be possible to determine the relative order of the suc-
cessive upheavals of a continent. If, for example, a group of rocks,
which as before may be called A, were found to have been upturned
and covered over by undisturbed beds C, the disturbance could be
affirmed to have occurred at some part of the epoch represented else-
where by the missing series B. If, again, the group C were observed
to have been subsequently tilted, and to pass under gently-inclined
or horizontal strata E, a second period of disturbance would be
proved to have occurred between the time of C and E.

I have referred to the unceasing destruction of its surface which
the land undergoes from the time when it emerges out of the sea.
As a rule, our conceptions of the rate of this degradation are exceed-
ingly vague. Yet they may easily be made more definite by a con-
sideration of present changes on the surface of the land. Every
river carries yearly to the sea an immense amount of sand and mud.
But this amount is capable of measurement. It represents, of course,
the extent to which the general level of the surface of the river's
drainage basin is annually lowered. According to such measure-
ments and computations as have been already made, it appears that
somewhere about -grcnnr of a foot is every year removed from the
surface of its drainage basin by a large river. This seems a small
fraction, yet by the power of mere addition it soon mounts up to
a large total. Taking the mean level of Europe to be 600 feet, its
surface, if everywhere worn away at what seems to be the present

GEOGRAPHICAL EVOLUTION 34 1

mean normal rate, would be entirely reduced to the sea-level in
little more than three and a half millions of years.

But of course the waste is not uniform over the whole surface. It
is greatest on the slopes and valleys, least on the more level grounds.
A few years ago, in making some estimates of the ratios between the
rates of waste on these areas, I assumed that the tracts of more rapid
erosion occupy only one-ninth of the whole surface affected, and
that in these the rate of destruction is nine times greater than on the
more level spaces. Taking these proportions, and granting that a.o'^ao
of a foot is the actual ascertained amount of loss from the whole
surface, we learn by a simple arithmetical process that t^ of an
inch is carried away from the plains and tablelands in seventy-five
years, while the same amount is worn out of the valleys in eight and
a half years. One foot must be removed from the former in 10,800
years, and from the latter in 1,200 years. Hence, at the present rate
of erosion, a valley 1,000 feet deep may be excavated in 1,200,000
years — by no means a very long period in the conception of most
geologists.

I do not offer these figures as more than tentative results. They
are based, however, not on mere guesses, but on data which, though
they may be corrected by subsequent inquiry, are the best at present
available, and are probably not far from the truth. They are of value
in enabling us more vividly to realise how the prodigious waste of
the land, proved by the existence of such enormous masses of sedi-
mentary rock, went quietly on age after age, until results were
achieved which seem at first scarcely possible to so slow and gentle
an agency.

It is during this quiet process of decay and removal that all the
distinctive minor features of the land are wrought out. When first
elevated from the sea, the land doubtless presents on the whole a
comparatively featureless surface. It may be likened to a block of
marble raised out of the quarry — rough and rude in outline, massive
in solidity and strength, but giving no indication of the grace into
which it will grow under the hand of the sculptor. What art effects
upon the marble block, nature accomplishes upon the surface of the
land. Her tools are many and varied — air, frost, rain, springs, tor-
rents, rivers, avalanches, glaciers, and the sea — each producing its

342 GEIKIE

own characteristic traces in the sculpture. With these implements,
out of the huge bulk of the land she cuts the valleys and ravines,
scoops the lake-basins, hews with bold hand the colossal outlines of
the mountains, carves out peak and crag, crest and cliff, chisels the
courses of the torrents, splinters the sides of the precipices, spreads
out the alluvium of the rivers, and piles up the moraines of the
glaciers. Patiently and unceasingly has this great earth-sculptor sat
at her task since the land first rose above the sea, washing down into
the ocean the debris of her labour, to form the materials for the
framework of future countries; and there will she remain at work so
long as mountains stand, and rain falls, and rivers flow.

II. The Growth of the European Continent.

Passing now from the general principles with which we have
hitherto been dealing, we may seek an illustration of their applica-
tion to the actual history of a large mass of land. For this purpose
let me ask your attention to some of the more salient features in the
gradual growth of Europe. This continent has not the simplicity of
structure elsewhere recognisable; but without entering into detail or
following a continuous sequence of events, our present purpose will
be served by a few broad outlines of the condition of the European
area at successive geological periods.

It is the fate of continents, no less than of the human communities
that inhabit them, to have their first origin shrouded in obscurity.
When the curtain of darkness begins to rise from our primeval
Europe, it reveals to us a scene marvellously unlike that of the
existing continent. The land then lay chiefly to the north and north-
west, probably extending as far as the edge of the great submarine
plateau by which the European ridge is prolonged under the Atlantic
for 230 miles to the west of Ireland. Worn fragments of that land
exist in Finland, Scandinavia, and the northwest of Scotland, and
there are traces of what seem to have been some detached islands in
Central Europe, notably in Bohemia and Bavaria. Its original height
and extent can of course never be known; but some idea of them
may be formed by considering the bulk of solid rock which was
formed out of the waste of that land. I find that if we take merely
one portion of the detritus washed from its surface and laid down in

GEOGRAPHICAL EVOLUTION 343

the sea — viz. that which is comprised in what is termed the Silurian
system — and if we assume that it spreads over 60,000 square miles of
Britain with an average thickness of 16,000 feet, or 3 miles, which is
probably under the truth, then we obtain the enormous mass of
180,000 cubic miles. The magnitude of this pile of material may be
better realised if we reflect that it would form a mountain ridge
three times as long as the Alps, or from the North Cape to Marseilles
(1800), with a breadth of more than 33 miles, and an average height
of 16,000 feet — that is, higher than the summit of Mont Blanc. All
this vast pile of sedimentary rock was worn from the slopes and
shores of the primeval northern land. Yet it represents but a small
fraction of the material so removed, for the sea of that ancient time
spread over nearly the whole of Europe eastwards into Asia, and
everywhere received a tribute of sand and mud from the adjoining
shores.

There is perhaps no mass of rock so striking in its general aspect
as that of which this northern embryo of Europe consisted. It lacks
the variety of composition, structure, colour, and form, which dis-
tinguishes rocks of more modern growth; but in dignity of massive
strength it stands altogether unrivalled. From the headlands of the
Hebrides to the far fjords of arctic Norway it rises up grim and
defiant of the elements. Its veins of quartz, felspar, and hornblende
project from every boss and crag like the twisted and knotted sinews
of a magnificent torso. Well does the old gneiss of the north deserve
to have been made the foundation-stone of a continent.

What was the character of the vegetation that clothed this earliest
prototype of Europe is a question to which at present no definite
answer is possible. We know, however, that the shallow sea which
spread from the Atlantic southward and eastward over most of
Europe was tenanted by an abundant and characteristic series of
invertebrate animals — trilobites, graptolites, cystideans, brachiopods,
and cephalopods, strangely unlike, on the whole, to anything living
in our waters now, but which then migrated freely along the shores
of the arctic land between what are now America and Europe.

The floor of this shallow sea continued to sink, until over Britain,
at least, it had gone down several miles. Yet the water remained
shallow because the amount of sediment constantly poured into it

344 GEIKIE

from the northwest filled it up about as fast as the bottom subsided.
This slow subterranean movement was varied by uprisings here and
there, and notably by the outburst at successive periods of a great
group of active submarine volcanoes over Wales, the Lake district,
and the south of Ireland; but at the close of the Silurian period a
vast series of disturbances took place, as the consequence of which
the first rough outlines of the European continent were blocked out.
The floor of the sea was raised into long ridges of land, among
which were some on the site of the Alps, the Spanish peninsula, and
the hills of the west and north of Britain. The thick mass of marine
sediment was crumpled up, and here and there even converted into
hard crystalline rock. Large enclosed basins, gradually cut off from
the sea, like the modern Caspian and Sea of Aral, extended from
beyond the west of Ireland across to Scandinavia and even into the
west of Russia. These lakes abounded in bone-covered fishes of
strange and now long-extinct types, while the land around was
clothed with a club moss and reed-like vegetation — Psilophyton,
Sigillaria, Calamite, etc. — the oldest terrestrial flora of which any
abundant records have yet been found in Europe. The sea, dotted
with numerous islands, appears to have covered most of the heart
of the continent.

A curious fact deserves to be noticed here. During the convul-
sions by which the sediments of the Silurian sea floor were crumpled
up, crystallised, and elevated into land, the area of Russia seems to
have remained nearly unaffected. Not only so, but the same immu-
nity from violent disturbance has prevailed over that vast territory
during all subsequent geological periods. The Ural Mountains on
the east have again and again served as a line of relief, and have been
from time to time ridged up anew. The German domains on the
west have likewise suffered extreme convulsion. But the wide inter-
vening plateau of Russia has apparently always maintained its flat-
ness either as sea-bottom or as terrestrial plains. As I have already
remarked, there has been a remarkable persistence alike in exposure
to and immunity from terrestrial disturbance. Areas that lay along
lines of weakness have suffered repeatedly in successive geological
revolutions, while tracts outside of these regions of convulsion have

GEOGRAPHICAL EVOLUTION 345

simply moved gently up or down without material plication or
fracture.

By the time of the coal growths, the aspect of the European area
had still further changed. It then consisted of a series of low ridges
or islands in the midst of a shallow sea or of wide salt-water lagoons.
A group of islands occupied the site of some of the existing high
grounds of Britain. A long, irregular ridge ran across what is now
France from Brittany to the Mediterranean. The Spanish peninsula
stood as a detached island. The future Alps rose as a long, low
ridge, to the north of the eastern edge of which lay another insular
space, where now we find the high grounds of Bavaria and Bohemia.
The shallow waters that wound among these scattered patches of
land were gradually silted up. Many of them became marshes,
crowded with a most luxuriant cryptogamic vegetation, specially of
lycopods and ferns, while the dry grounds waved green with conifer-
ous trees. By a slow intermittent subsidence, islet after islet sank
beneath the verdant swamps. Each fresh depression submerged
the rank jungles and buried them under sand and mud, where they
were eventually compressed into coal. To this united co-operation
of dense vegetable growth, accumulation of sediment, and slow sub-
terranean movement, Europe owes her coal-fields.

All this time the chief area of high ground in Europe appears still
to have lain to the north and northwest. The old gnarled gneiss of
that region, though constantly worn down and furnishing materials
towards each new formation, yet rose up as land. It no doubt
received successive elevations during the periods of disturbance,
which more or less compensated for the constant loss from its surface.

The next scene we may contemplate brings before us a series of
salt lakes, covering the centre of the continent from the north of
Ireland to the heart of Poland. These basins were formed by the
gradual cutting off of portions of the sea which had spread over the
region. Their waters were red and bitter, and singularly unfavour-
able to life. On the low intervening ridges a coniferous and cyca-
daceous vegetation grew, sometimes in quantity sufficient to supply
materials for the formation of coal seams. The largest of these salt
lakes stretched from the edge of the old plateau of Central France

346 GEIKIE

along the base of the Alpine ridge to the high grounds of Bohemia,
and included the basin of the Rhine from Bale down to the ridge
beyond Mayence, which has been subsequently cut through by the
river into the picturesque gorge between Bingen and the Siebenge-
birge. This lake was filled up with red sand and mud, limestone,
and beds of rock salt. Where the eastern Alps now rise the opener
waters were the scene of a long-continued growth of dolomite, out
of which in later ages the famous dolomite mountains of the Tyrol
were carved.

These salt lakes of the Triassic period seem to have been every-
where quietly effaced by a widespread depression, which allowed the
water of the main ocean once more to overspread the greater part of
Europe. This slow subsidence went on so long as to admit of the
accumulation of masses of limestone, shale, and sandstone, several
thousand feet in thickness, and probably to bring most of the insular
tracts of Central Europe under water. To this period, termed by
geologists the Jurassic, we can trace back the origin of a large part
of the rock now forming the surface of the continent, from the low
plains of Central England up to the crests of the northern Alps, while
in the Mediterranean basin, rocks of the same age cover a large area
of the plateau of Spain, and form the central mass of the chain of
the Apennines. It is interesting to know that the northwest of
Britain continued still to rise as land in spite of all the geographical
changes which had taken place to the south and east. We can trace
even yet the shores of the Jurassic sea along the skirts of the moun-
tains of Skye and Ross-shire.

The next long era, termed the Cretaceous, was likewise more
remarkable for slow accumulation of rock under the sea than for
the formation of new land. During that time the Atlantic sent its
waters across the whole of Europe and into Asia. But they were
probably nowhere more than a few hundred feet deep over the site
of our continent, even at their deepest part. Upon their bottom there
gathered a vast mass of calcareous mud, composed in great part of
foraminifera, corals, echinoderms, and molluscs. Our English chalk
which ranges across the north of France, Belgium, Denmark, and
the North of Germany, represents a portion of the deposits of that
sea floor, probably accumulated in a northern, somewhat isolated

GEOGRAPHICAL EVOLUTION 347

basin, while the massive hippurite Hmestone of Southern Europe
represents the deposits of the opener ocean. Some of the island spaces
which had remained for a vast period above water, and had by their
degradation supplied materials for the sediment of successive geo-
logical formations, now went down beneath the Cretaceous sea. The
ancient high-grounds of Bohemia, the Alps, the Pyrenees, and the
Spanish tableland were either entirely submerged, or at least had
their area very considerably reduced. The submergence likewise
affected the northwest of Britain; the western highlands of Scotland
lay more than 1,000 feet below their present level.

When we turn to the succeeding geological period, that of the
Eocene, the proofs of widespread submergence are still more striking.
A large part of the Old World seems to have sunk down; for we find
that one wide sea extended across the whole of Central Europe and
Asia. It was at the close of this period of extreme depression that
those subterranean movements began to which the present con-
figuration of Europe is mainly due. The Pyrenees, Alps, Apennines,
Carpathians, the Caucasus, and the heights of Asia Minor mark, as it
were, the crests of the vast earth-waves into which the solid frame-
work of Europe was then thrown. So enormous was the contortion
that, as may be seen along the northern Alps, the rocks for thousands
of feet were completely inverted, this inversion being accompanied
by the most colossal folding and twisting. The massive sedimentary
formations were crumpled up, and doubled over each other, as we
might fold a pile of cloth. In the midst of these commotions the
west of Europe remained undisturbed. It is strange to reflect that
the soft clays and sands under London are as old as some of the
hardened rocks which have been upheaved into such picturesque
peaks along the northern flanks of the Alps.

After the completion of these vast terrestrial disturbances, the
outlines of Europe began distinctly to shape themselves into their
present form. The Alps rose as a great mountain range, flanked on
the north by a vast lake which covered all the present lowlands of
Switzerland, and stretched northward across a part of the Jura
Mountains, and eastward into Germany. The size of this fresh-water
basin may be inferred from the fact that one portion only of the
sand and gravel that accumulated in it even now measures 6,000 feet

348 GEIKIE

in thickness. The surrounding land was densely clothed with a
vegetation indicative o£ a much warmer climate than Europe now
can boast. Palms o£ American types, as well as date palms, huge
Californian pines (Sequoia), laurels, cypresses, and evergreen oaks,
with many other evergreen trees, gave a distinctive character to the
vegetation. Among the trees too were planes, poplars, maples, wil-
lows, oaks, and other ancestors of our living woods and forests;
numerous ferns grew in the underwood, while clematis and vine
wound themselves among the branches. The waters were haunted
by huge pachyderms, such as the dinotherium and hippopotamus;
while the rhinoceros and mastodon roamed through the woodlands.

A marked feature of this period in Europe was the abundance
and activity of its volcanoes. In Hungary, Rhineland, and Central
France, numerous vents opened and poured out their streams of
lava and showers of ashes. From the south of Antrim, also, through
the west coast of Scotland, the Faroe Islands, and Iceland, even far
into arctic Greenland, a vast series of fissure-eruptions poured forth
successive floods of basalt, fragments of which now form the exten-
sive volcanic plateaux of these regions.

The mild climate indicated by the vegetation in the deposits of the
Swiss lake prevailed even into Polar latitudes, for the remains of
numerous evergreen shrubs, oaks, maples, walnuts, hazels, and many
other trees have been found in the far north of Greenland, and even
within 8° 15' of the pole. The sea still occupied much of the low-
lands of Europe. Thus it ran as a strait between the Bay of Biscay
and the Mediterranean, cutting off the Pyrenees and Spain from the
rest of the continent. It swept round the north of France, covering
the rich fields of Touraine and the wide flats of the Netherlands.
It rolled far up the plains of the Danube, and stretched thence east-
ward across the south of Russia into Asia.

By this time some of the species of shells which still people the
European seas had appeared. So long have they been natives of our
area that they have witnessed the rise of a great part of the continent.
Some of the most stupendous changes which they have seen have
taken place in the basin of the Mediterranean, where, at a com-
paratively recent geological period, parts of the sea floor were up-
heaved to a height of 3,00a feet. It was then that the breadth of the

GEOGRAPHICAL EVOLUTION 349

Italian peninsula was increased by the belt of lower hills that flanks
the range of the Apennines. Then, too, Vesuvius and Etna began
their eruptions. Among these later geographical events also we must
place the gradual isolation of the Sea of Aral, the Caspian, and the
Black Sea from the rest of the ocean, which is believed to have once
spread from the arctic regions down the west of Asia, along the
base of the Ural Mountains into the southeast of Europe.

The last scene in this long history is one of the most unexpected
of all. Europe, having nearly its present height and outlines, is
found swathed deep in snow and ice. Scandinavia and Finland are
one vast sheet of ice, that creeps down from the watershed into the
Atlantic on the one side, and into the basin of the Baltic on the other.
All the high grounds of Britain are similarly buried. The bed of the
North Sea as well as of the Baltic is in great measure choked with
ice. The Alps, the Pyrenees, the Carpathians, and the Caucasus send
down vast glaciers into the plains at their base. Northern plants find
their way south even to the Pyrenees, while the reindeer, musk-ox,
lemming, and their arctic companions, roam far and wide over
France.

As a result of the prolonged passage of solid masses of ice over
them, the rocks on the surface of the continent, when once more
laid bare to the sun, present a worn, flowing outline. They have
been hollowed into basins, ground smooth, and polished. Long
mounds and wide sheets of clay, gravel, and sand have been left
over the low grounds, and the hollows between them are filled with
innumerable tarns and lakes. Crowds of boulders have been perched
on the sides of the hills and dropped over the plains. With the advent
of a milder temperature the arctic vegetation gradually disappeared
from the plains. Driven up step by step before the advancing flora
from more genial climates, it retired into the mountains, and there
to this day continues to maintain itself. The present Alpine flora
of the Pyrenees, the Alps, Britain, and Scandinavia, is thus a living
record of the ice age. The reindeer and his friends have long since
been forced to return to their northern homes.

After this long succession of physical revolutions, man appears as
a denizen of the Europe thus prepared for him. The earliest records
of his presence reveal him as a fisher and hunter, with rude flint-

350 GEIKIE

pointed spear and harpoon. And doubdess for many a dim century
such was his condition. He made no more impress on external
nature than one of the beasts which he chased. But in course of time,
as civilisation grew, he asserted his claim to be one of the geographi-
cal forces of the globe. Not content with gathering the fruits and
capturing the animals which he found needful for his wants, he
gradually entered into a contest with nature to subdue the earth and
to possess it. Nowhere has this warfare been fought out so vigorously
as on the surface of Europe. On the one hand, wide dark regions of
ancient forest have given place to smiling cornfields. Peat and moor
have made way for pasture and tillage. On the other hand, by the
clearance of woodlands the rainfall has been so diminished that
drought and barrenness have spread where verdure and luxuriance
once prevailed. Rivers have been straitened and made to keep their
channels, the sea has been barred back from its former shores. For
many generations the surface of the continent has been covered with
roads, villages and towns, bridges, aqueducts and canals, to which this
century has added a multitudinous network of railways, with their
embankments and tunnels. In short, wherever man has lived, the
ground beneath him bears witness to his presence. It is slowly cov-
ered with a stratum either wholly formed by him or due in great
measure to his operations. The soil under old cities has been increased
to a depth of many feet by the rubbish of his buildings; the level of
the streets of modern Rome stands high above that of the pavements
of the Csesars, and that again above the roadways of the early
Republic. Over cultivated fields his potsherds are turned up in
abundance by the plough. The loam has risen within the walls of
his graveyards as generation after generation has mouldered into
dust.

It must be owned that man, in much of his struggle with the
world around him, has fought blindly for his own ultimate interests.
His contest, successful for the moment, has too often led to sure and
sad disaster. Stripping forests from hill and mountain, he has gained
his immediate object in the possession of their abundant stores of
timber; but he has laid open the slopes to be parched by drought, or
swept bare by rain. Countries once rich in beauty, and plenteous
in all what was needful for his support, are now burnt and barren,

GEOGRAPHICAL EVOLUTION 35 1

or almost denuded of their soil. Gradually he has been taught, by his
own bitter experience, that while his aim still is to subdue the earth,
he can attain it, not by setting nature and her laws at defiance, but
by enlisting them in his service. He has learnt at last to be the
minister and interpreter of nature, and he finds in her a ready and
unrepining slave.

In fine, looking back across the long cycles of change through
which the land has been shaped into its present form, let us realise
that these geographical revolutions are not events wholly of the
dim past, but that they are still in progress. So slow and measured
has been their march, that even from the earliest times of human
history they seem hardly to have advanced at all. But none the less
are they surely and steadily transpiring around us. In the fall of
rain and the flow of rivers, in the bubble of springs and the silence
of frost, in the quiet creep of glaciers and the tumultuous rush of
ocean waves, in the tremor of the earthquake and the outburst of
the volcano, we may recognise the same play of terrestrial forces by
which the framework of the continents has been step by step evolved.
In this light the familiar phenomena of our daily experience acquire
an historical interest and dignity. Through them we are enabled to
bring the remote past vividly before us, and to look forward hope-
fully to that great future in which, in the physical not less than in
the moral world, man is to be a fellow-worker with God.