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COUKSE OF SIX LECTURES

VAKIOUS FORCES OF MATTER

THEIR RELATIONS TO EACH OTHER

MICHAEL FARADAY, D.C.L. F.R.S.

FULLERIAN PBOFESSOE OE CHEMISTRY, ROYAL INSTITUTION

Delivered before a Juvenile Auditory at the Royal Institution
of Great Britain during the Christmas Holidays 0/1859-60

EDITED BY WILLIAM CROOKES, F.C.S.

WITH NUMEROUS ILLUSTRATIONS

LONDON AND GLASGOW

RICHARD GRIFFIN AND COMPANY

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PREFACE.

Which was first, Matter or Force ? If we think
on this question we shall find that we are
unable to conceive of matter without force, or
force without matter. When God created the
elements of which the earth is composed, He
created certain wondrous forces, which are set
free, and become evident when matter acts on
matter. All these forces, with many differences,
have much in common, and if one is set free
it will immediately endeavour to free its com-
panions. Thus heat will enable us to eliminate
light, electricity, magnetism, and chemical
action ; chemical action will educe light, elec-
tricity, and heat ; in this way we find that all
the forces in nature tend to form mutually
dependent systems, and as the motion of one
star affects another, so force in action liberates
and renders evident forces previously tranquil.
A 2

143053

IV PREFACE.

We say tranquil, and yet the word is almost
without meaning in the Cosmos; — where do
we find tranquillity? The sea, the seat of
animal, vegetable, and mineral changes, is at
war with the earth, and the air lends itself to
the strife. The globe, the scene of perpetual
intestine change, is as a mass, acting on, and
acted on, by the other planets of our system,
and the very system itself is changing its place
in space, under the influence of a known force
springing from an unknown centre.

For many years past the English public
have had the privilege of listening to the dis-
courses and speculations of Professor Faraday,
at the Eoyal Institution, on Matter and Forces,
and it is not too much to say that no lecturer
on Physical Science since the time of Sir
Humphry Davy has been listened to with
more delight. The pleasure which all derive
from the expositions of Faraday is of a some-
what different kind to that produced by any
other philosopher whose lectures we have ever

PREFACE. V

attended. It is partially derived from his ex-
treme dexterity as an operator, — with him we
have no chance of apologies for an unsuccessful
experiment, no hanging fire in the midst of a
series of brilliant demonstrations, producing
that depressing tendency akin to the pain felt
by an audience at a false note from a vocalist.
All is a sparkling stream of eloquence and
experimental illustration. We defy a chemist
who loves his science, no matter how often he
may himself have repeated an experiment,
to feel uninterested when seeing it done by
Faraday.

The present publication presents one or two
points of interest. In the first place, the
Lectures were especially intended for young
persons, and are therefore as free as possible
from technicalities; and in the second place
they are printed as they were spoken, verbatim
et literatim. A careful and skilful reporter
took them down, and the manuscript, as de-
ciphered from his notes, was subsequently most
carefully corrected by the Editor as regards

VI PREFACE,

any scientific points which were not clear to
the short-hand writer ; hence all that is differ-
ent arises solely from the impossibility, alas!
of conveying the manner as well as the matter
of the Lecturer.

The interest which was felt in those num-
bers of the Chemical News in which the
lectures appeared was so great that the re-
publication of them in a separate form was
considered to be almost a duty to those young
lovers of science to whom a purely chemical
journal with its inevitable technicalities would
be a sealed book. May the readers of these
lectures derive one tenth of the pleasure and
instruction from their perusal which they gave
to those who had the happiness of hearing
them !

W. C.

CONTENTS.

LECTURE I.

PAGE
THE FORCE OF GRAVITATION 1

LECTURE II.

GRAVITATION — COHESION 31

LECTURE III.

COHESION — CHEMICAL AFFINITY 58

LECTURE IV.

CHEMICAL AFFINITY — HEAT ...... til

LECTURE V.

MAGNETISM — ELECTRICITY 106

LECTURE VI.

THE CORRELATION OF THE PHYSICAL FORCES . .130

LIGHTHOUSE ILLUMINATION — THE ELECTRIC LIGHT . 155
NOTES 175

LECTURES

THE PHYSICAL FOKCES.

LECTUEE I.

THE FORCE OF GRAVITATION.

It grieves me much to think that I may have
been a cause of disturbance in your Christmas
arrangements (2), for nothing is more satisfactory
to my mind than to perform what I undertake ;
but such things are not always left in 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 thdfo a few words; and as a substi-
tute I will endeavour to make the illustrations
of the sense I try to express, as full as possible ;

2 GRAVITATION.

and if we find by the end of this lecture, that
we may be justified in continuing them, think-
ing 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
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 amongst 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
view these things with an almost entire ab-
sence of wonder to ourselves respecting the way
in which all this happens. So small, indeed, is
our wonder, that we are never taken by sur-
prise ; and I do think, that, to*a young person
of ten, fifteen, or twenty years of age, perhaps
the first sight of a cataract or a mountain would

VARIOUS POWERS OF MATTER. 3

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 anything wonderful in
it. These inquiries, which have occupied phi-
losophers 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 cer-
tain forces, or abilities to do things, or powers,
that are so common that nothing can be more
so : for nothing is commoner than the wonder-
ful powers by which we are enabled to stand
upright — they are essential to our existence
every moment.

B 2

4 GKAVITATION.

It is my purpose to-day to make you ac-
quainted 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 poiver 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 analyse 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 ; or, if I take now
this bit of shell-lac [a stick of shell-lac about
12 inches long and 1^ 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

WHAT IS FORCE.' 5

paper is immediately moved towards the shell-
lac, and by now drawing the latter away, the
paper falls over without having been touched
by anything. 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 wherewith 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 endeavour 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 embar-
rass 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 force; and you will
now be able to recognise 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
b 3

6 GRAVITATION.

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 illustrations of our subject; but
the mind of man is not confined like the
matter of his body, and thus he may and does
travel outwards, 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 see in a sort of map or picture how the
world is formed upon its surface. Then, when
we come to examine further, I refer you to this
sectional diagram of the geological strata of the

WHAT IS MATTER? 7

«arth, 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 ad-
vantage, 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 toge-
ther by means of forces in one great mass,
8000 miles in diameter, that the mind is over-
whelmed 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 enclosing it in a
bag) _they are all portions of matter with

B 4

8 GRAVITATION.

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 vapour
— 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
vapour 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 contains ; 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 in-
stance, 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 contained a half-pint glass
vessel], and the glass vessel is at present the
lighter of the two; but if I now take some

ATTRACTION OF THE EARTH. 9

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 ap-
plied 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 wTeigh it down. [The Lecturer
poured more water into the jar, which again
weut 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
towards 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 towards
the earth. Now here [exhibiting a small piece of
platinum (2)] is another thing which gravitates
towards 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

10 GRAVITATION.

water]. What a wonderful thing it is to see
that it requires so much water as that [a half-
pint vessel full] to fall towards the earth, com-
pared with the little mass of substance I have
here ! And again, if I take this metal [a bar of
aluminium (3) 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 downwards : — 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 or
property in the water. (4) It is now very hot,
it is steaming up, and I could perhaps light
phosphorus 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 copper (5)] which will il-

EVOLUTION OF HEAT FROM WATEK. 11

lustrate another kind of power. [The Lecturer
here poured some water over the white sulphate
of copper, which - immediately 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 some-
times (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 considera-
tion 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 mat-
ter 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

12 GRAVITATION.

a weight, because in it the exercise of that
power of pressing downwards is especially used
for the purposes of weighing ; and I have also
one of these littie 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 pos-
sesses of resisting that pressure ; — it may burst,
but we must try to avoid that. [During the
last few observations the Lecturer had suc-
ceeded 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
hundred weight, and you must conceive for
yourselves what a wonderful power there must
be to pull this weight downwards, 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.\\ 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

THE PENDULUM.

13

oscillates to and fro — that pendulum bob as it
is sometimes called. Observe, if I hold the
straight stick horizontally, as high as the posi-
tion of the balls 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 towards 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
properties in other respects — the time of its
vibration and so on — but concerning which we
will not now trouble ourselves.

Fig. l.

If a gold leaf, or piece of thread, or any
other substance, were hung where this ball is,

14 GRAVITATION.

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 of 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 towards
bodies which fall to it, but, that all these
bodies possess an attraction, every one towards
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 propor-
tion 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 to-
gether, I should not be able, by the most deli-
cate arrangement of apparatus, to make you, or

THE CENTRE OF GRAVITY. 15

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 towards 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 under-
stand 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 towards
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

16 GRAVITATION.

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(6), I get that which
you see — [the Lecturer here put several lumps
of marble into a glass jar, and poured water and
then acid over them ; the carbonic acid imme-
diately commenced 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 towards the
other, and all fall towards the earth, and what

GRAVITATION OF CARBONIC ACID.

17

I want you to see is that this steam gravitates.
I have here {Jig. 2) a large vessel placed upon

Fig. 2.

a balance, and the 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 no-
thing in but the invisible steam, or vapour, 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.
c

18 GRAVITATION.

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 gravitates. All these experiments show
you that, tried by the balance, tried by pouring
like water from one vessel to another, this
steam, or vapour, 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

CENTRE OP GRAVITY.

19

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 he 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 one
corner A (Jig, 3) and Mr. Anderson hold that
up in his hand before us, and I then take a

piece of thread and an ivory ball, and hang

C2

20 GRAVITATION.

that upon the awl, then the centre of gravity
of both the pasteboard 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 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 bradawl 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

CENTRE OF GRAVITY.

21

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
philosophy comes to our aid; and I am per-

Fig. 6.

fectly sure, without looking 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 lifting up the centre of gravity (a), and

c 3

22

GRAVITATION.

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 happen if I try to make the
figure stand upon a sharp point? You observe
I must get that point exactly under the centre

Fig. 7.

of gravity or it will fall over thus [endeavouring
unsuccessfully to balance it]; and this you see
is a difficult matter, I cannot make it stand
steadily; but if I embarrass this poor old lady

CENTKE OF GRAVITY.

23

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 philosophy by standing side-
ways.

I remember an experiment which puzzled me
very much when a boy. I read it in a conjur-
ing book, and this was how the problem was
put to us : " How," as the book said, " how to

Fig. 8.

hang a pail of water, by means of a stick, upon
the side of a table" (fig. 8). Now I have here a

c 4

24

GRAVITATION.

table, a piece of stick, and a pail, and the
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 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 misfor-
tune 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 conjuring books require.

CENTKE OF GRAVITY.

25

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

Fig. 10.

refer to your toy-books, and say you have seen
that before. Answer me rather, if I ask you,
have you understood it before ? It is an ex-
periment 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

26 GRAVITATION.

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 observation,
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 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

GRAVITATION IN MASSES. 27

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 towards
the earth, fall with the same velocity, whatever
be their size.

I cannot 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 sove-
reign 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

28 GRAVITATION.

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 undis-
turbed. Or I can try another kind of experi-
ment:— 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 to within a few
inches of the lecture table], and allow it then
to fall from the ceiling downwards (I will put
something beneath to catch it, supposing I
should be rnaladroif), 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 we let it loose we will

GRAVITATION IN VACUO. 29

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 ves-
sel, and you will perceive that the gold leaf
will fall as quickly in this vacuum as the coin

30 GRAVITATION.

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 consider them ; and although
I fear that I have not sufficiently pointed out
to you the more important circumstances con-
nected with this force of Gravitation, espe-
cially 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

31

LECTUEE II.

GRAVITATION. — COHESION.

Do me the favour to pay me as much attention
as you did at our last meeting, and I shall not
repent of that which I have proposed to under-
take. 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 your knowing some-
thing 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 gravi-
tation. I told you that when we brought these
two bodies [two equal-sized ivory balls sus-
pended by threads] near together, they attracted
each other, and that we might suppose that the

32 GRAVITATION.

whole power of this attraction was exerted be-
tween their respective centres of gravity ; and,
furthermore, you learned from me that if, in-
stead 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 at-
traction 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 [dropping
the ivory ball]. You sit there upright, and I
stand upright here, because we keep our centres
of gravity properly balanced with respect 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 towards our
feet, in a reversed position as compared with
us, and all by means of this power of gravita-
tion 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

LAW OF GRAVITATION. 33

balls and place it within an inch of the other,
they attract each other with a certain power.
If I hold it at a greater distance off, they at-
tract 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 gravitation, 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 under-
stand 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. 11) a lamp a, shining most
intensely upon this disc, b, c, d ; and this light

D

34

GRAVITATION.

acts as a sun by which I can get a 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

Fig. 11.

other one in size, and place it- midway between
the lamp and the screen ; now look at the 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 four instead of

LAW OF GRAVITATION. 35

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 quan-
tity 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 a, by the
side of the shadow, it can only receive one fourth,
of the proportion 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
brightest 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 cannot 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

D 2

36 GRAVITATION.

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 astrono-
mers 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 further. 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 frag-
ments of iron on the table]. There, I have
already told you that in all cases where bodies
fall, it is the 'particles that are attracted. You
may consider these then as separate particles
magnified, so as to be evident to your sight ;
they are loose from eachother — they all gravi-

MAGNETIC ATTRACTION.

37

tate — they all fall to the earth — for the force
of gravitation 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)

Fig. 12.

*

y

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

D 3

38 COHESION.

together in the form of an elliptical bridge, so
do the different particles of iron which constitute
this nail hold together 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 be-
tween the particles that it is held together 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 cf 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

TENACITY OF IRON. 39

which I have reduced to very fine powder, you
see that I can actually build them up into a
solid wall by pressure between two flat surfaces.
The power which I thus have of building 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 ?
Tlwt 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 parti-
cles. [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 par-

D 4

40 COHESION.

tides of the wire, just as in pantomimes they
sometimes suspend gentlemen and damsels.

How can we make this attraction of the par-
ticles a little more simple ? There are many
things which if brought together properly will
show this attraction. Here is a boy's experi-
ment (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 mak-
ing 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

COHESION OF ALUM PARTICLES. 41

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 alum crystallised 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 favours their coalescence) of
uniting together again and forming one mass.(7)
Now, having brought the alum into solution,
I will pour it into this glass basin, and you will,
to-morrow, find that those 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-mor-

42 COHESION.

row morning we shall have a great deal of the
alum crystallised 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, namelyf
warming it slowly and gradually, and in repeat-
ing 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 experiment 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 crystallisation 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
us beyond those of the attraction of gravitation !
See how it gives us great strength. The things

CHANGE IN CONDITION OF COHESION. 43

we deal with in building up the structures 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 wrhen I pour these two liquids
together — both colourless as water. [The
Lecturer here mixed together solutions of per-
chloride 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
colour. It was reddish-yellow at first, but it
has now become red. (8) I have previously pre-
pared 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. (9)]

44 COHESION.

i

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 colour 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 attrac-
tion 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 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 experi-
ment.] There it is. You see the red is not
put back, but brought back 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

PRINCE RUPERTS DROPS. 45

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 afterwards to look to and examine 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 has been
dropped into cold water [exhibiting a u Prince
Kupert's drop(10) {jig. 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 apiece of this. [The Lecturer nipped off a
small piece from the end of one of the Kupert'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

46

COHESION.

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 Kupert'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.]

Fig. 13.

Fig. 14.

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 (u) (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

COHESION IN CRYSTALLINE BODIES. 47

so make the least scratch upon the inside, the
whole bottle will break to pieces, — it cannot
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 to-
gether 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 col-
lection 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 (12)], and if I hammer this stone I
may batter it a great deal before I can break it

48

COHESION.

up. I may even bend it without breaking it ;
that is to say, I may bend it in one particular
direction without breaking it much, although I
feel in my hands that I am doing it some in-
jury. 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

Fig. 15.

Fig. 16.

Fiq. 17.

crystals; for there is some salt (Jig. 16) — you
know what common salt is (13) ; here is a piece
of this salt which by natural circumstances has
had its particles so brought together that they
have been allowed free opportunity of combin-
ing or coalescing, 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

CLEAVAGE OF CRYSTALS. 49

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 frag-
ments 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] (u), 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 beautifully 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 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

E

50 COHESION.

existing. You will be able to do it at once —
but if you try to cut it across the crystals you
cannot ; 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 light again. You see, we
cannot 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 cannot well find out how these forms
become so, and I 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 anything that
does not). All matter affects light more or
less by what we may consider as a kind of
attraction, and I have arranged •(%. 18) 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

REFRACTION OF LIGHT. 51

power, which matter possesses, of attracting a
ray of light. If Mr. Anderson pours some

Fig. 18.

water, gently and steadily, into the basin, the
water will attract the rays of light downwards,
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 see the silver and sealing-wax he
was answered by a general affirmative.] Now,
I suppose that everybody can see that they are
not at all disturbed, whilst 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

E 2

52

COHESION.

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 understand how glass attracts
light, and might then see how other substances,
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).

x\cN

Fig. 19.

¥

[The electric lamp was again lit, and the beam
of parallel rays of light, which it emitted, was

THE PRISMATIC SPECTRUM. 53

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 (un-
less 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 prismatic spectrum
about the theatre.] Here I have the rays
once more bent on to the screen, and you see
how wonderfully and beautifully that piece of
glass not only bends the light by virtue of its
attraction, but actually splits it up into diffe-
rent colours. 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
e a

54 COHESION.

are not uniform — which do not merely cohere,
but also have within them, in different parts,
different degrees 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
polarised, but about that you need not trouble
yourselves, it is only to make our illustration
more clear. Here, then, we have our polarised
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 analyser 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 polarised 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 the light. [The glass was removed, and
a piece of mica introduced.] Now, there is the
mica which we split up so curiously, into leaf

ACTION OF CALOSPAR ON LIGHT. 55

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 colours, 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

Fig. 20.

Ax

f

round, and produces these rings and that black
cross (Jig. 20). Look at those colours, are they

E 4

56 COHESION.

not most beautiful for you and for me ? (for I
enjoy these things as much as you do). In
what 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 polarised light, and I
will first of all show you that the 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 — ho wit makes the light come
through some parts and leaves it dark in others,
and all because we weaken the force of cohesion

EFFECT OF HEAT OR PRESSURE ON GLASS. 57

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 polarised 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 glass you put
a strain upon it sufficient to break it sometimes
— something like there was in the case of the
Prince Eupert'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.

53

LECTUEE III.

COHESION. — CHEMICAL AFFINITY.

We will first return for a few minutes to one of
the experiments made yesterday. You remem-
ber 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 multi-
tude of beautiful crystals. Here also are the
pieces of coke which 1 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 crystallise 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

COHESION WEAKENED BY HEAT. 59

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 learnt that
it makes calcareous spar, alum, and so forth,
crystallise 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. Anderson 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
Lecturer the iron rod, with one end red-hot,

60 COHESION.

which he showed could be easily twisted about
with a pair of pliers.] You see, I now find no
difficulty in bending this end about as I like ;
whereas I cannot 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 philosophers we call
it all water, even though it be in the form of
ice or steam). Why is this water hard ? [point-
ing 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]

SOLID, LIQUID, AND GASEOUS STATE. 61

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 cannot even hold together in the
liquid form, but escape as vapour. At the same
time you see I cannot 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 liquid, and a further

62 COHESION.

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
icebergs, while here, in a warmer climate, it has
its attraction of cohesion so much diminished
as to be liquid water. Well, in diminishing
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 cannot 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 pro-
ceed to make it liquid by any of the various
means we have at our command, it still must

ICE PRODUCED ARTIFICIALLY. 63

take the necessary quantity of heat from some-
thing, 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 alehouse ; 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 anything about the salt,
and they set the pot by the fire, 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

64

COHESION.

ice. Here you see the tin dish is frozen to the
board, I can even lift this little stool up by it.

This experiment cannot, I think, fail to im-
press upon your minds the fact, that whenever
a solid body loses some of that force of attraction
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 cor-
responding amount of heat is given out. I
have an experiment showing this to be the
case. Here {fig. 21) is a bulb, A, filled with air,

Fig. 21.

the tube from which dips into some coloured
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 coloured liquid which is now

HEAT EVOLVED DURING SOLIDIFICATION. 65

standing in the tube at c will travel for-
ward. Now we have discovered a means, by
great care and research into the properties of
various bodies, of preparing a solution of a salt(15)
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 coloured 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 circumstance that particles attract
each other. But you must not now suppose
that because they are liquid they have lost their
attraction 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

66 COHESION.

of fluid mercury, the particles of which have at-
traction 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 experi-
ment with soap-bubbles to take care to have
everything clean and soapy). I will now blow
a bubble, and that 1 may be able to talk and
blow a bubble too, I will take a plate with a

COHESION OF THE PAHTICLES OF LIQUIDS. 67

little of the soapsuds in it, and will jnst 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 tuft of it
into the neck of the alcohol bottle, and you see
when I turn it upside down, that it is perfectly

F2

68

COHESION.

well stoppered so far as the alcohol is concerned;
the air can pass through, but the alcohol cannot.
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 stop-
pered 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 stop-
pered). Now if it were not for the particles of

Fig. 22.

Fig. 23.

liquid cohering together, this alcohol would
run out, and if I had time I could have shown
you a vessel with the top, bottom and sides alto-
gether formed like a sieve, and yet it would
hold water owing to this 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

BULK OF BODIES IN VAPOROUS CONDITION. 69

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 ether (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 tempera-
ture, will become a solid, but if we apply heat
to it, it becomes vapour, 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 ex-
tent. You see it is very clear that as I apply
heat to the liquid I diminish its attraction of
cohesion — it is now boiling, and I will set fire
to the vapour, 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

1 3

70 COHESION.

from the spirit lamp is now being consumed,
not in making the ether any warmer, but in
converting it into vapour, and if I desired to
catch this vapour and condense it (as I could
without much difficulty), 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 [1700 cubic inches ; nearly
a cubic foot], so greatly is the attraction of co-
hesion 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 some-
times produced by this force of water. I want
you now to see another experiment which will
perhaps give you a better illustration of the bulk
occupied by a body when m the state of vapour.
Here is a substance which we call iodine, and I
am about to submit this solid body to the same

GASES AKE ALWAYS TRANSPARENT. 71

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 rilled with the violet vapour], and you
see the same kind of change produced. More-
over, it gives us the opportunity of observing
how beautiful is the violet-coloured vapour from
this black substance, or rather the mixture of
the vapour with air (for I would not wish you
to understand that this globe is entirely filled
with the vapour of iodine).

If I had taken mercury and converted it into
vapour (as I could easily do), I should have a
perfectly colourless vapour, for you must under-
stand this about vapours, that bodies in what
we call the vaporous, or the gaseous state, are
always perfectly transparent, never cloudy or
smoky ; they are, however, often coloured, and
we can frequently have coloured vapours or
gases produced by colourless particles themselves
mixing together, as in this case [the Lecturer
here inverted a glass cylinder full of binoxide of
nitrogen (16) over a cylinder of oxygen, when the
dark red vapour of hyponitrous acid was pro-
duced]. Here also you see a very excellent
illustration of the effect of a power of nature

F 4

72 CHEMICAL AFFINITY.

which we haye not as yet come to, but which
stands next on our list — Chemical Affinity.
And thus you see we can have a violet vapour
or an orange vapour, and different other kinds
of vapour, but they are always perfectly trans-
parent, or else they would cease to be vapours.
I am now going to lead you a step beyond
this consideration of the attraction of the par-
ticles for each other. You see we have come to
understand that, if we take water as an illustra-
tion, 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 ob-
taining 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 know-
ledge as we proceed. Now here (Jig. 24) is a

CONSTITUTION OF WATER.

73

portion of water in this little vessel c, and be-
sides 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

Fig. 24.

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 vapour,
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

74 CHEMICAL AFFINITY.

amount of this new power at our disposal.
Here you see it is [causing the ends of the
wires to touch] — that is the electric light 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 con-
nection (at A and b) you see the water boiling 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 vapour,
and if I take a little vessel (e), and fill it with
water, and put it in the trough over the end of
the tube (i>), there goes the vapour ascending
into the vessel. And yet that is not steam, for
you know that if steam is brought near cold
water, it would at once condense, and return
back again to water; this then cannot 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 com-
bustible, for if I take this small vessel to a
light, the vapour inside explodes in a manner
that steam could never do.

FORMATION OF WATER FROM ITS ELEMENTS. 75

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
vapour 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 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
vapour. Mr. Anderson will now pump all the
air out of this vessel (g), and when I have
screwed it on to the top of our jar of gas (f),
you will see upon opening the stopcocks (h'ih)
the water will jump up, showing that some of
the gas has passed into the glass vessel. I will
now shut these stopcocks, 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
Leyden 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

76 CHEMICAL AFFINITY.

there is no longer any gas remaining, if I place
it over the jar and open the stopcocks again, up
will go the gas, and we can have a second com-
bustion ; 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. An-
derson brought in another Leyden jar, which,
however, from some cause would not ignite the
gas. It was therefore recharged, when the ex-
plosion took place in the desired manner.] How
beautifully 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 condition 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

DECOMPOSITION OF WATEK. 77

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 con-

nection with the wires of the battery (at A b) I
shall get a similar decomposition of the water
at the two platinum plates. Now I will put
this little tube (o) over there, and that will col-
lect 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 ap-
paratus, the wires are a good way apart from each
other, and it now seems that each of them is capa-
ble 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

78 CHEMICAL AFFINITY.

collected in the other tube (o). Something is
coming out of the water there (at h) which
burns [setting 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 composed 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 ap-
paratus (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

PREPARATION OF OXYGEN. 79

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 dealing 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 with which
particles of different kinds attract each other.
Oxygen and hydrogen are particles of different
kinds, and it is their attraction to each other
which makes them chemically combine and
produce 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 substance 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 dis-
place the water, and fill the jars with gas.

80

CHEMICAL AFFINITY.

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 mate-
rially (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.

1

Hydrogen.

8
Oxygen.

9

Oxygen

. 88-9

Hydrogen .

. IM

Water .

. lOO'O

But oxygen is sixteen times the weight of
the other — eight times as heavy as the par-
ticles of hydrogen in the water ; and you there-
fore know that water is composed of nine parts
by weight — one of hydrogen and eight of oxy-
gen ; thus : —

PROPERTIES OF OXYGEN. 81

Hydrogen .

. 46*2 cubic inches

. = 1 grain.

Oxygen

. 231 „ „ .

. = 8 grains.

Water {steam)

. 69'3 „ „ .

. = 9 grains.

Now Mr. Anderson has prepared some oxygen,
and we will proceed to examine what is the
character of this gas. First of all you remem-
ber 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 running 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
a

82

CHEMICAL AFFINITY.

in a dazzling stream of fire, so powerfully does
the oxygen support combustion. Again, here is

Fig. 26.

another experiment which will serve to illus-
trate 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 scintil-
lations.] But if I now hold the flame over a
jar of oxygen [the experiment was repeated over

PROPERTIES OF OXYGEN.

83

a jar of oxygen, when the combustion of the
filings as they fell into the oxygen became almost
insupportably brilliant], you see how wonder-
fully different the effect is in the jar, because
there we have oxygen instead of common air.

O 2

84

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 at-
traction 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
reproduce 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 cha-
racter of oxygen, of which I have here two jars
(oxygen being those particles derived from the

PREPARATION OF HYDROGEN.

85

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 yes-
terday— it is so called because it helps to gene-
rate 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,

* £5cop, " water," and 7ewaw, " I generate."
G 3

86 CHEMICAL AFFINITY.

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 cer-
tain 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, not with a bright light, — you
will at all events hear it if you do not see it.
Now that is a body entirely different from oxy-
gen ; it is extremely light ; for although yester-
day you saw twice as much of this hydrogen
produced on the one side as on the other, by
the voltaic 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

PROPERTIES OF HYDROGEN. 87

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

Fig. 28.

the jar in which it was at first, and has poured
upwards into the other, and there we shall
find it.

You now understand that we can have par-
ticles 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 illustrate this. For instance, if 1 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

G 4

88

chemical Affinity.

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, 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

Fig. 29.
Hydrogen.

Flat in um.

Water

Q

are one from the other : and I will take as
illustrations these very common things, air,

BULK OF EQUAL WEIGHTS OF BODIES.

89

water, the heaviest body, platinum — and this
gas, and observe how they differ in this re-
spect ; for if I take a piece of platinum 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 illustra-
tion gives you a very good idea of the extra-
ordinary difference with regard to the gravity
of the articles having this enormous difference
in bulk. [The following tabular statement hav-
ing reference to this illustration appeared on
the diagram board.]

Hydrogen

1

Air

14-4

1

Water

11943

829

1

Platinum

256774

17831

21*5

Whenever oxygen and hydrogen unite to-
gether they produce water, and you have seen
the extraordinary difference between the bulk
and appearance of the water so produced and

90

CHEMICAL AFFINITY.

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 com-
bine 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 knowing it. Take a candle, for in-
stance, 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

Fig. 30.

a smoke — which presently evaporates. This
perhaps will serve to show it better. Mr. An-

EFFECTS OF CHEMICAL AFFINITY. 91

derson 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 exist-
ing in the wax of which that candle is formed.
And now, having brought you in the first
place to the consideration of chemical attrac-
tion, 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. (17) 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

92 CHEMICAL AFFINITY.

would follow. [A lighted match 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 (18), 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 travelling 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. (l9)
They are both colourless solutions, and in these
glasses you cannot 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

SOLID PRODUCED FROM TWO LIQUIDS. 93

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 stiffen-
ing, 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 manifestation 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 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

94 CHEMICAL AFFINITY.

are just like this unlighted 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 chemi-
cal 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 condi-
tion 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 anything be more beautiful than this com-
bustion 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,

COMBUSTION IN OXYGEN. 9 5

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 burning 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 burn-
ing very well in oxygen, and I will tell you the

96 CHEMICAL AFFINITY.

reason why those scissors and that lead do not
take fire whilst they are lying on the table.
Here the lead is in a lump, and the coating of
oxide remains on its surface, whilst 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 pyropliorus (20)],
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 enough 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,

COMBUSTION AND CHEMICAL AFFINITY. 97

— putting a varnish, as it were, upon its surface,
as we varnish a picture — absolutely forming
a substance which prevents the natural che-
mical affinity between the bodies from acting.

I must now take you a little further 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 phos-
phorus, 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 escaping
in that vapour, which will condense into a
H

98 CHEMICAL AFFINITY.

snow-like 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 of 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 (Jig. 30)
cannot 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 attrac-
tion 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 8 by weight of oxygen
to 1 by weight of hydrogen went to form water,

COMBUSTION IN DEFINITE PROPORTIONS. 99

I meant this, that neither of them would com-
bine in different proportions with the other, for
you cannot get 10 of hydrogen to combine with
6 of oxygen, or 10 of oxygen to combine with
6 of hydrogen — it must be 8 of oxygen and 1
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 some-
times 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 which 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 now with the way it is prepared ;
it is called gun-cotton. (21) See how that burns

H 2

100 CHEMICAL AFFINITY.

[setting fire to a piece] ; it is very different from
the other, because the oxygen that must be
present in its proper amount is put there before-
hand. And I have here some pieces of paper
which are prepared like the gun-cotton (22), and
imbued with bodies containing oxygen. Here
is some which has been soaked in nitrate of
strontia — you will see the beautiful red colour
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 inde-
pendent of the oxygen of the atmosphere. And
here we have some gunpowder 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 ex-
periment of putting this fuse under the water,
and you will see whether that water can extin-
guish it ; here it is burning out of the water,

LAWS OF CHEMICAL AFFINITY. 101

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 continue — 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 on. It is not so with gravi-
tation; 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 seerms to evolve heat and

H 3

102 HEAT.

light continuously, 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 chemi-
cal 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 recognise heat by its power
of liquefying solid bodies and vaporising 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 speak-
ing 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

VARIOUS METHODS OF OBTAINING HEAT. 103

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 tg' s '
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 affi-
nity 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 suddenly, 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
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 vapour, 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-vapour.

This, then, I think, will be sufficient, accom-

H 4

104 HEAT.

panied 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 ex-
pand. And here is a brass rod ( fig. 32) which

Fig. 32.

tL

goes through that hole and fits also accurately
into this gauge; but if I make it warm with

EXPANSION BY HEAT. 105

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 d,t all. Again; as soon as the heat
escapes from bodies they collapse ; see how the
air 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 per-
fect facility, so that we have a perfect proof of
this power of heat to contract and expand bodies.

106

LECTUKE V.

MAGNETISM — ELECTRICITY.

I wonder whether we shall be too deep to-day or
not. Kemember, that we spoke of the attraction
by gravitation of all bodies to all bodies by their
simple approach. Kemember, 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; be-
cause 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.

DIFFEEENT FORMS OF ATTRACTION. 107

To-day we come to a kind of attraction even
more curious than the last, namely, the attrac-
tion 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 sometimes 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 attrac-
tion of chemical affinity to 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 room slightly
moving the ball about, but there is no attrac-
tion. 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 by this
friction, and which I may take away as easily

108 ELECTRICITY.

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 towards 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 electricity (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

ELECTRICAL ATTRACTION AND REPULSION. 109

other ; but now what happens ? It does not
attract ; on the contrary, it very strongly repels,

Fig. 33.

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, 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 attrac-
tions,— they are actually two kinds of attraction

110

ELECTRICITY.

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 at-
traction— a dual attraction or force — one at-
tracting 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 excited shellac suspended in the stirrup] ;
it is rough, but delicate enough for my pur-
pose ; and suppose I take this other piece of
shellac, and take away the power, which I can

Fig. 34.

do by drawing it gently through the hand ; and
suppose I take a piece of flannel (%. 34) which

EYOLUTION OF ELECTRICITY. Ill

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 re-
peated]. Those two bodies therefore still con-
tain 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 of the nature of the force which we
call electricity. There is no 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).

112 ELECTRICITY.

Take a watch-glass (or your watch itself will do,
you only want something which shall have a

Fig. 35.

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

ELECTRICAL ATTRACTION. 113

which we may not use. Here are some indica-
tors {fig. 36). I bend round a strip of paper into

Fig. 36.

a hoop, and we have as good an indicator as can
be required ; see how it rolls along, travelling
after the sealing-wax. If I make them smaller,
of course we have them running faster, and
sometimes they are actually attracted up into
the air. Here also is a little collodion balloon.
It is so electrical that it will scarcely leave my
hand unless to go to the 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 anything 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 electri-
cal ; but our time forbids us to go further into
I

114 MAGNETISM.

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 gravita-
tion, 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 at-
traction, 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 sepa-
rate and dual attraction, and what is more, one
wrhich 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 arti-
ficial 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

MAGNETIC ATTKACTION. 115

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 sub-
stance 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 attrac-
tion resides. If I take this bar and balance it
carefully on a point, so that it will be 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
I 2

116 MAGNETISM.

examine it in a similar way, you see that one of
its ends repels the suspended magnet; the force

Fig. 37. Fig. 38.

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 coloured the ends differently
so that you may distinguish one from the other.
Now this end (s) of the magnet (fig. 37) attracts
the uneoloured of the little magnet. You see
it pulls it towards it with great power. And as
I carry it round, the uneoloured end still follows.
But now if I gradually bring the middle of the
bar magnet opposite the uneoloured end of the
needle, it has no effect upon it, either of attrac-
tion or repulsion, until, as I come to the oppo-

DOUBLE POWER OF THE MAGNET.

117

site extremity (n) you see that it is the coloured
end of the needle which is. pulled towards 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 under-
stand that it is to express this double power.

Now with this loadstone you may make mag-
nets artificially. Here is an artificial magnet
{fig. 39) in which both ends have been brought

Fig. 39.

together in order to increase the attraction.
This mass will lift that lump of iron, and what
is more, by placing this keeper, as it is called,

I 3

118 MAGNETISM.

on the top of the magnet, and taking hold of
the 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 pheno-
mena of attraction and repulsion, by bringing
another magnetised needle near to it.

I want you now to observe that although I
have shown you in these magnets that this
double power becomes evident principally at the
extremities, yet the whole of the magnet is con-
cerned 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.

DISTRIBUTION OF POWER IN MAGNETS. 119

falling the whole of the mass is acted upon by
the force of gravitation. 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 oppo-
site 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 cannot,
because it is half way between the two ends.
But now, if I break out that piece (n. s.) and
then examine it — see how strongly one end {n)
pulls at this end {sfig. 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 repulsion, but that the power is
only rendered evident at the end of the mass.
You will 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 attractive and the other is re-
pulsive. Now, is not this power a most wonder-
ful thing? And very strange, the means of
i 4

120

ELECTRICITY.

taking it from one substance and bringing it to
other matters. I cannot make a piece of iron
or anything else heavier or lighter than it is ;
its cohesive power it must and does have ; but,
as you have seen by these experiments, 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

Fig 41.

machine arranged for the purpose of rubbing
glass with silk, and for obtaining the power
called electricity ; and the moment the handle

TRANSFERENCE OF ELECTRICITY. 121

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 conductors (b b), and I
want you to see the manner in which that elec-
tricity 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 awTay you shall see by an experi-
ment of this sort. If I hold this cylinder of
brass by the glass handle and touch the con-
ductor with it I take away a little of the electri-
city. 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 elec-
tricity 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 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 C}dinder to a jet
from which gas was issuing ; the spark was seen

122 ELECTKICITT.

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
afterwards. 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 transfer 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
fire. 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. Ob-
serve how long the heat stops in this ball. I
might touch it with the wire, or with 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

CONDUCTION OF ELECTRICITY AND HEAT. 123

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 transmitted is extraordinary, and most es-
sential 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 arrangement 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

Fig. 42.

(this is one way of obtaining the power of heat)
and place it under that little chimney, the flame
will strike against the bar of copper and keep it

124 ELECTRICITY.

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 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 wood is, and burn
the paper with which I have covered it; but
where the metal is beneath, the heat is con-
ducted 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

CONDUCTION OF ELECTKICITY. 125

piece of phosphorus 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 travelling 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 cylinder. 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 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

126 ELECTRICITY.

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 elec-
tricity passes ! — it has flashed instantaneously
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
machine,^. 41]. Because it conducts electri-
city. And why do I have these columns 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

ELECTRICAL REPULSION.

127

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

Fig. 43.

which it has endowed these strips of paper, each
spreading outwards to the ceiling and sides of

123 ELECTRICITY.

the room. The outside 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 transfer-
able power of electricity. 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 gun-
powder [the connection was made and the gun-
powder 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, therefore, I were to stand on this
stool, and receive the electricity through this
conductor, I could give it to anything that I

CONDUCTION OF ELECTRICITY. 129

touched. [The Lecturer stood upon the in-
sulating stool, and placed himself in connection
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 finger 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 elec-
tricity into comparison with those which we have
previously examined, and to-morrow we shall be
able to go further into the consideration of these
transferable powers.

130

LECTUEE 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

HEAT AND CHEMICAL AFFINITY. 131

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, be-
cause 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
chemical 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

K 2

132 CORRELATION OF THE PHYSICAL FORCES.

rather striking when seen for the first time. I
have here some iron filings and gunpowder, and
will mix them carefully together, with as little
rough handling as possible ; now we will com-
pare 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 applied a lighted
match to it, when a sudden flash showed how
large a proportion of gunpowder had escaped

CHEMICAL AFFINITY AND ELECTRICITY. 133

combustion 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 re-
tort ; 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 mer-
cury is spread over that zinc, it does not take
away its power of decomposing the water, but
modifies it most curiously. See how that mix-
ture 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

X 3

134 CORRELATION OF THE PHYSICAL FORCES.

and beautiful manner. Here are some pieces
of zinc covered with mercury, exactly in the
same way as the zinc in that retort is covered ;
and if I put this plate into sulphuric acid I get
no gas, but this most extraordinary thing occurs,
that if I introduce along with the zinc another
metal which is not so combustible, then I re-
produce all the action. I am now going to put
to the amalgamated 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 coming 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,

TRANSFERENCE OF CHEMICAL AFFINITY. 135

days, months, or even years, and no action
would take place. But by putting them both
together and allowing them to touch {fig. 44),

Fig. 44.

Fig. 45.

you see what a coating of copper there is im-
mediately 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 plati-
num 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 mo-
ment the zinc was introduced and touched the

K 4

136 CORRELATION OF THE PHYSICAL FORCES.

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.

Let us now examine these phenomena a little
more closely. Here is a drawing {Jig. 45) in
which I have represented a vessel containing
the acid liquid and the slips of zinc and plati-
num or copper, and I have shown them touchiDg
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 and
platinum, zinc and platinum, zinc and platinum,

TRANSFERENCE OF CHEMICAL AFFINITY. 137

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 platinum, 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 powerful
then in existence. You see there are in this

138 CORRELATION OF THE PHYSICAL FORCES.

glass jar two leaves of gold, which I can cause
to move to and fro by this rack work. I will

Fig. 46.

Fig. 47.

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

POLAR ATTRACTION OF THE BATTERY. 139

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 to-
gether. Now they are attracting 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 pheno-
mena.

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 having a machine which we are obliged to
turn for a long time together, wre have here a

140 CORRELATION OF THE PHYSICAL FORCES.

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 showing
you the kind of light produced by the actual
combustion in 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 com-
bustion 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

THE ELECTRIC LIGHT. 141

burning in those cells, and that this is the light
of that burning [bringing the two poles in con-
tact 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 purposes, 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 mo-
ment we require it. Now here (fig. 47) is a
vessel containing two charcoal points, and I
bring it forward as an illustration of the won-
derful 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 cannot
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

142 CORRELATION OF THE PHYSICAL FORCES.

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 cir-
cumstances, chemical affinity would not act.
We may absolutely take these two charcoal
poles down under water, 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 combus-
tible, and I am going to take one of them, a
small platinum wire, and suspend it between

Fig. 48.

these two rods which are connected with the
battery, and when contact is made at the bat-

HEAT, ELECTRICITY, AND CHEMICAL ACTION. 143

tery see what heat we get (Jig. 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 showing 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 con-
nected with one end of the battery, and lower
the other charcoal pole on to it. See how bril-
liantly it burns (Jig. 49). Here is a piece of
iron on the charcoal, see what a combustion is
going on; and we might go on in this way
burning almost everything 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

144 CORRELATION OF THE PHYSICAL FORCES.

source, or the way in which it travels, we still
shall find it to be chemical action. Here is a

Fig. 49.

coloured liquid which can show by its change
of colour 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 combus-
tion 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 colour will be entirely destroyed. See,
it is colourless 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 che-
mical action. There is also another curious

CHEMICAL ACTION OF THE BATTERY. 145

thing to be noticed now we are dealing with the
chemistry of electricity, which is that the chemi-
cal power which destroys the colour is only due to
the action on one side. I will pour some more
of this sulphindigotic acid (23) into a flat dish
and will then make a porous dyke of sand sepa-
rating the two portions of fluid into two parts
(fig. 50), and now we shall be*able to see whether

Fig. 50.

there is any difference in the two ends of the
battery, and which it is that possess 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
must not imagine, that because you cannot per-
ceive any action none has taken place.

Here we have another instance of chemical

L

146 CORRELATION OF THE PHYSICAL FORCES.

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 them-
selves 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 80001. or 9000Z.
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

MAGNETISM AND CHEMICAL AFFINITY. 14 7

as this. And there is also another beautiful re-
sult dependent upon chemical affinity in that fine
lead-tree (24), 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 manner far more important than the power-
ful one you see here, because in nature these
minute actions are going on for ever, and are of
great and wonderful importance in the precipi-
tation 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
for ever in small degrees, accumulating more
and more of the results.

I have here given you all the illustrations
that time will permit me to show you of
chemical affinity producing electricity, and
electricity again becoming chemical affinity.
Let that suffice for the present ; and let us now
go a little deeper into the subject of this chemical
force, or this electricity — which shall I name first
— the one producing the other in a variety of
ways. These forces are also wonderful 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

L 2

148 CORRELATION OF THE PHYSICAL FORCES.

since I was born, that the discovery of the
relations of these two forces of electricity and
chemical affinity to produce magnetism have
become known. Philosophers had been sus-
pecting 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 realise and establish
never again to be lost, and upon them we found
new expectations of further discoveries, and so
go on pursuing, realising, 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 the two
ends of the battery. I^t is copper wire only, and

Fig. 51.

is therefore not magnetic of itself. We will
examine this wire with our magnetic needle

RELATION OF MAGNETISM TO ELECTRICITY. 149

(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 cir-
cumstances. 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 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 bat-
tery 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 phenomena as I had with the bar
magnet, — one end attracting and the other
repelling. Is not this then curious to see that
we can construct a magnet of copper? Further-

L 3

150 CORRELATION OF THE PHYSICAL FORCES.

more, 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 attracted {fig. 52), but as soon as I break

Fig. 52.

contact the power is all gone and they fall.
What then can be a better or a stronger proof

ELECTRO-MAGNETISM. 1 51

than this of the relation of the powers of mag-
netism and electricity ? Again, here is a little
piece of iron which is not yet magnetised. It
will not at present take up any one of these
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 elec-
tricity,— 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 you saw the
other day, of building up a bridge of iron nails ;
the contact 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 attraction with
which we have endowed these portions of iron ?

L 4

152 CORRELATION OF THE PHYSICAL FORCES.

Here again is a fine illustration of this strong
power of magnetism. It is a magnet of the

Fig. 53.

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 mag-
net ; 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 (Jig. 54) ; and if I take
such a piece as this cylinder, and place it on,
woe be to me if I get my finger between ; I can
roll it over, but if I try to pull it off, I might

ELECTKO-MAGNETISM.

153

lift up the whole magnet, but I have no power
to overcome the magnetic power which is here

Fig. 54.

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 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
of 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

154 CORRELATION OF THE PHYSICAL FORCES,

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 honoured me with their presence during
these lectures. 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 amongst 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 everything " ?

155

LECTUEE

LIGHTHOUSE ILLUMINATION — THE ELECTRIC
LIGHT.

[Delivered before the Royal Institution on Friday, 9th March,
I860.]

There is no part of my life which gives me
more delight than my connection with the
Trinity House. The occupation of nations
joined together to guide the mariner over the
sea, to all a point of great interest, is infinitely
more so to those who are concerned in the ope-
rations which they carry into effect, and it cer-
tainly has astonished me since I have been
connected with the Trinity House to see how
beautifully and how wonderfully shines forth
amongst nations at large the desire to do good ;
and you will not regret having come here
to-night, if you follow me in the various
attempts which have been made to carry out
the great object of guiding in safety all people
across the dark and dreary waste of waters. It

156 LIGHTHOUSE ILLUMINATION.

is wonderful to think how eagerly efforts at
improvement are made by the various public
bodies — the Trinity House in this country, and
commissions in France and other nations ; and
whilst the improvements progress we come to
the knowledge of such curious difficulties and
such odd modes of getting over those difficulties
as are not easy to be conceived. I must ask
you this evening to follow me from the simplest
possible method of giving a sign by means of a
light to persons at a distance, to the modes at
which we have arrived in the present day ; and
to consider the difficulties which arise when
carrying out these improvements to a practical
result, and the extraordinary care which those
who have to judge on these points must take
in order to guard against the too hasty adoption
of some fancied improvement, thus, as has
happened in some few cases, doing harm instead
of good.

If I try to make you understand these things
partly by old models, and partly by those which
we have here, it is only that I may the better
be enabled to illustrate that which I look for-
ward to as the higher mode of lighting, by
means of the electric lamp and the lime light.

There is nothing more simple than a candle

REFLECTION OF LIGHT.

157

being set down in a cottage window to guide a
husband to his home, but when we want to
make a similar guide on a large scale, not
merely over a river or over a moor, but over
large expanses of sea, how can we then make
the signal using only a candle ? I have shown
in this diagram (fig. 55) what we may imagine

Fig. 55.

to be the rays of a candle or any other source
of light emanating from the centre of a sphere
in all directions round to infinite distances.
After this simple kind of light had been used
for some time, it being found to be liable to be
obscured by fogs, or distance, or other circum-
stance, there arose the attempt to make larger

158 LIGHTHOUSE ILLUMINATION.

lights by means of fires ; and after that there
was introduced a very important refinement in
the mode of dealing with the light, namely the
principle of reflection ; — for understand this
(which is not known by all, and not known by
many who should know it), that when we take
a source of light, a single candle, for instance,
giving off any quantity of light, we can by no
means increase that light : we can make ar-
rangements around and about the light, as you
see here, but we can by no means increase the
quantity of light. The utmost I can do is to
direct the light which the lamp gives me by
taking a certain portion of the rays going off on
one side and reflecting them on to the course of
the rays which issue in the opposite direction.
First of all, let us consider how we may gather
in the rays of light which pass off from this
candle. You will easily see that if I could take
the half rays on the one side and could send
them by any contrivance over to the other side,
I should gain an advantage in light on the side
to which I directed them. This is effected in
a beautiful manner by the parabolic mirror, by
means of which I gather all that portion of the
rays which are included in it ; upwards, down-

KEFRACTION OF LIGHT. 159

wards, sideways, anywhere within its sphere of
action ; they are all picked up and sent forward.
You thus see what a beautiful and important
invention is that of the parabolic reflector for
throwing forward the rays of light.

Before I go further into the subject of reflec-
tion let me point out a further mode of dealing
with the direction of the light. For instance,
here is a candle, and I can employ the principle
of refraction to bend and direct the rays of
light, and if I want to increase the light in any
one direction, I must either take a reflector or
use the principle of refraction. I will place
this lens {fig. 56) in front of the candle and you

Fig. 56.

will easily see that by its means I can throw on
to that sheet of paper a great light, that is to
say, that instead of the light being thrown all
about, it is refracted and concentrated on to
that paper ; so here I have another means of
bending the light and sending it in one direc-

160 LIGHTHOUSE ILLUMINATION.

tion ; and you see above a still better arrange-
ment for the same purpose, — one which comes
up to the maximum, I may say, of the ability
of directing light by this means. You are aware
that without that arrangement of glass the light
would be dispersed in all directions, but the lens
being there, all the light which passes through
it is thrown into parallel beams and cast hori-
zontally along. There is consequently no loss
of light, the beam goes forward of the same
dimensions, and will consequently continue to
go forward for five or ten miles, or so long as the
imperfection of the atmosphere does not absorb
it ; and see ! What a glorious power that is, to
be able to convert what was just now darkness
on that paper into brilliant light.

Whenever we have refraction of this sort we
are liable to an evil consequent upon the ne-
cessary imperfections in the form of the lens ;
and Dr. Tyndall will take this lens, and will
show you even in this small and perfect appara-
tus what is the evil of spherical aberration with
which we have to fight. This can be illustrated
by means of the electric lamp ; if you look at
the screen, you will see produced, by means of
this lens, a figure of the coal points. This

POLYZONAL LENSES. 161

image is produced by the rays which pass
through the middle of the lens, a piece of card
with a hole in the centre being placed in front ;
but if, keeping the rest of the apparatus in the
same position, I change this card for another
piece which will only allow the rays to pass
through the edge of the lens, you observe how
inferior the image will be. In order to get it
distinct I have to bring the screen much nearer
the lamp ; and so if I take the card away alto-
gether, and allow the light to pass through all
parts of the lens, we cannot get a perfect image,
because the different parts of the lens are not
able to act together. This spherical aberration
is, therefore, what we try to avoid by building
up compound lenses in the manner here shown
(fig. 58). Look at this beautiful apparatus, is it
not a most charming piece of workmanship?
Buffon first and Fresnel afterwards, built up
these kind of lenses, ring within ring, each at
its proper adjustment, to compensate for the
effects of spherical aberration ; the ring round
that centre lens is ground so as to obviate what
would otherwise give rise to spherical aberration,
and the next ring, being corrected in the same
manner, you will perceive, if you look at the

M

162

LIGHTHOUSE ILLUMINATION.

disc of light thrown by the apparatus upstairs,
that there is nothing like the amount of aberra-
tion that there would have been if it had been
one great bull's-eye. Here is one of Fresnel's
lamps of the fourth order so constructed (Jig. 57):

Fig. 57.

observe the fine effect obtained by these differ-
ent lenses as you see them revolve before you,
and understand that all this upper part is made
to form part of the lens, each prism throwing
its rays to increase the effect, and, although you
may think it is imperfect because if you happen

FRESNEL's CONSTRUCTION OF LAMPS. 163

to sit below or above the horizontal line, you
perceive but little if any of the light, yet you
must bear in mind that we want the rays to go
in a straight line to the horizon. So that all
that building up of rings of glass is for the
purpose of producing one fine and glorious lens
of a large size, to send the rays all in one direc-
tion. Here is another apparatus used to pull
the rays down to a horizontal sheet of light, so
that the mariner may see it as a constant and
uniform fixed light ; the former lamp is a re-
volving one, and the light is seen only at certain
times as the lenses move round, and these are
the points which make them valuable in their
application.

There are various orders and sizes of lights in
lighthouses to shine for twenty or thirty miles
over the sea, and to give indications according
to the purposes for which they are required;
but suppose we want more effect than is pro-
duced by these means, how are we to get more
light ? Here comes the difficulty. We cannot
get more light, because we are limited by the
condition of the burner. In any of these cases^
if he spreading of the ray, or divergence as it is
called, is not restrained, it soon fails from weak*
>x 2

164 LIGHTHOUSE ILLUMINATION.

ness, and if it does not diverge at all, it makes
the light so small, that perhaps only one in
a hundred can see it at the same time. The
South Foreland lighthouse is, I think, 300 or 400
feet above the level of the sea, and therefore it
is necessary to have a certain divergence of the
beam of light in order that it may shine along
the sea to the horizon. I have drawn here two
wedges, one has an angle of 15°, and shows you
the manner in which the light opens out from
this reflector seen at the distance of half a mile
or more, the other wedge has an angle of 6°,
which is the beautiful angle of Fresnel. When
the angle is less than 6°, the mariner is not
quite sure that he will see the light — he may
be beneath or above it; and in practice it is
found that we cannot have a larger angle than
15°, or a less one than 6°. In order, therefore, to
get more light, we must have more combustion,
more cotton, more oil ; ' but already there are in
that lamp four wicks put in concentric rings,
one within the other, and we cannot increase
them much more, owing to the divergence
which would be caused by an increase in the
size of the light — the more the divergence, the
more the light is diffused and lost. We are,

THE ELECTRIC LIGHT. 165

therefore, restrained by the condition of the light
and the apparatus to a certain sized lamp. At
Teignmouth, some of the revolving lights have
ten lamps and reflectors, all throwing their light
forward at once. But even with ten lamps and
reflectors we do not get sufficient light, and we
want, therefore, a means of getting a light more
intense than a candle in the space of a candle —
not merely an accumulation of candle upon
candle, but a concentration into the space of a
candle, of a greater amount of light, and it is here
that the electric light comes to be of so much
value.

Let me now show you what are the proper-
ties of that light which make it useful for light-
house illumination, and which has been brought
to a practical condition by the energy and con-
stancy of Professor Holmes. I will first of all
show you the image of the charcoal points on
the screen, and draw your attention to the spot
where the light is produced. There are the
coal points. The two carbons are brought
within a certain distance ; the electricity is
being urged across by the voltaic battery, and
the coal points are brought into an intense
state of ignition. You will observe that the

M 3

166 THE ELECTKIC LIGHT.

light is essentially given by the carbons ; you
see that one is much more luminous than the
other, and that is the end which principally
forms the spark, the other does not shine so
much, and there is a space between the two
which, although not very luminous, is most
important to the production of the light. Dr.
Tyndall will help me in showing you that a
blast of wind will blow out that light; the
electric light can in fact be blown out easier
than a candle. We have the power of getting
our light where we please ; if I cause the elec-
tricity to pass between carbon and mercury I
get a most intense and beautiful light, most of
it being given off from the portion of the mer-
cury between the liquid and the solid pole. I
can show you that the light is sometimes pro-
duced by the vapour between the two poles,
better if I take silver than when I use mercury.
Here is the carbon pole, there is the silver, and
there is the beautiful green light which comes
from the intervening portions. Now that light
is more easily blown out than the common
lamp, the slightest puff of wind being sufficient
to extinguish it, as you will see if Dr. Tyndall
breathes upon it.

ADVANTAGES OF THE ELECTRIC LIGHT. 167

You see, therefore, how we are able, by using
this electric spark, to get, first of all, the light
into a very small space. That oil lamp
has a burner 3| inches in diameter ; compare
the size of the flame with the space occupied by
this electric light. Next compare the intensity
of this light with any other; if I take this
candle and place it by the side, I actually seem
to put out the candle. We are thus able to
get a light which, while it surpasses all others
in brilliancy, is at the same time not too large,
for I might put this light into an apparatus not
larger than a hat, and yet I could count upon
the rays being useful. Moreover, when such
large burners are used in a lantern, we have to
consider whether the bars of the window do not
interfere to throw a shadow or otherwise ; but
with this light there will be no difficulty of that
sort, as a single small speculum no larger than
a hat will send it in any direction we please ;
and it is wonderful what advantages, by reason
of its small bulk, we have in the consideration
of the different kinds of apparatus required, re-
flecting or refracting, irrespective of other reasons
for using the electric light. And it is these
kind of things which make us decide most

M 4

168

THE ELECTRIC LIGHT.

earnestly and carefully in favour of the electric
light.

I am going to show you the effect that will
take place with that large lens when we throw
the oil lamp out of action, and put the electric
light into use. It is astonishing to find how
little the eye can compare the relative inten-
sities of two lights ; look at that screen and try
to recollect the amount of light thrown upon it
from the 3| inch lamp of Fresnel, and now,
when we shift the lens sideways, look at the
glorious light arising from that small carbon
point {fig. 58) ; see how beautifully it shines in

tM

wm>

Fig. 58.

the focus of that lens and throws the rays for-
ward. At present the electric light is put at
just the same distance as the oil light, and
therefore, being in the focus of the lens, we

THE MAGNETO ELECTRIC MACHINE. 169

iiave parallel rays which are thrown forward in
a perfectly straight line, as you will see by com-
paring the size of the lens with that of the
light thrown on the screen. You will now see
how far we can affect this beam of light by in-
creasing or diminishing the distance of the
lamp. We are able by a small adjustment to
get a beam of a large or small angle, and observe
what power I have now over it ; for if I want to
increase the degrees of divergence, I am limited
by the power of light in the case of the oil lamp,
but with the electric light, I can make it spread
over any width of the horizon by this simple
adjustment. These then are some of the reasons
which make it desirable to employ the electric
light.

By means of a magnet, and of motion, we
can get the same kind of electricity as I have
here from the battery ; and under the authority
of the Trinity House, Professor Holmes has
been occupied in introducing the magneto-
electric light in the lighthouse at the South
Foreland ; for the voltaic battery has been tried
under every conceivable circumstance, and I
take the liberty of saying it has hitherto proved
a decided failure. Here, however, is an in-

170 THE ELECTRIC LIGHT.

strument wrought only by mechanical motion.
The moment we give motion to this soft iron
in front of the magnet, we get a spark. It is
true in this apparatus it is very small, but it is
sufficient for you to judge of its character. It

Fig. 59.

is the magneto-electric light, and an instrument
has been constructed as there shown (fig. 59)
which represents a number of magnets placed
radially upon a wheel — three wheels of magnets

THE MAGNETO-ELECTKIC LIGHT. 171

and two sets of helices. When the machine,
which is worked by a two-horse power engine,
is properly set in motion, and the different cur-
rents are all brought together, and thrown by Pro-
fessor Holmes up into the lantern, we have a
light equal to the one we have been using this
evening. For the last six months the South
Foreland has been shining by means of this
electric light — beyond all comparison better
than its former light. It has shone into France,
and has been seen there and taken notice of by
the authorities, who work with beautiful accord
with us in all these matters. Never for once
during six months has it failed in doing its
duty ; — never once, more than was expected
by the inventor. It has shone forth with its
own peculiar character, and this even with the
old apparatus — for as yet no attempt has been
made to construct special reflectors or refractors
for it, because it is not yet established. I will
not tell you that the problem of employing the
magneto-electric spark for lighthouse illumina-
tion is quite solved yet, although I desire it
should be established most earnestly (for I
regard this magnetic spark as one of my own
offspring). The thing is not yet decidedly ac-

172 THE ELECTRIC LIGHT.

complished, and what the considerations of ex-
pense and other matters may be, I cannot tell.
I am only here to tell you as a philosopher, how
far the results have been carried, but I do hope
that the authorities will find it a proper thing
to carry out in full. If it cannot be introduced
at all the lighthouses, if it can only be used at
one, why really it will be an honour to the
nation which can originate such an improvement
as this, — one which must of necessity be fol-
lowed by other nations.

You may ask, what is the use of this bright
light ? It would not be useful to us were it
not for the constant changes which are taking
place in the atmosphere, which is never pure.
Even when we can see the stars clearly on a
bright night it is not a pure atmosphere. The
light of a lighthouse, more than any other, is
liable to be dimmed by vapours and fogs, and
where we most want this great power, is not in
the finest condition of the atmosphere, but
when the mariner is in danger, when the sleet
and rain are falling, and the fogs arise, and the
winds are blowing, and he is nearing coasts
where the water is shallow and abounds with
rocks — then is his time of danger, when he

WHEN OF MOST VALUE. 173

most wants this light. I am going to show you
how, by means of a little steam, I can com-
pletely obscure this glorious sun, this electric
light which you see. The cloud now obscuring
the light on the screen is only such a cloud as
you see when sitting in a train on a fine summer's
day ; you may observe that the vapour, passing
out of the funnel, casts as deep a shadow on
the ground as the black funnel; the very sun
itself is extinguished by the steam from the
funnel, so that it cannot give any light; and
the sun itself if set in the lighthouse would not
be able to penetrate such a vapour.

Now the haze of this cloud of steam is just
what we have to overcome, and the electric
light is as soon, proportionally, extinguished
by an obstruction of this kind as any other
light. If we take two lights, one four times the
intensity of the other, and we extinguish half
of one by a vapour, we extinguish half of the
other, and that is a fact which cannot be set
aside by any arrangement. But then we fall
back upon the amount of light which the
electric spark does give us in aid of the power of
penetrating the fog, for the light of the electric
spark shines so far at times, that even before it

174 THE ELECTRIC LIGHT.

has arisen above the horizon twenty five miles
off, it can be seen. This intense light has, there-
fore, that power which we can take advantage of,
— of bearing a great deal of obstruction before
it is entirely obscured by fogs or otherwise.

Taking care that we do not lead our author-
ities into error by the advice given, we hope
that we shall soon be able to recommend the
Trinity House, from what has passed, to esta-
blish either one or more good electric lights in
this country.

175

NOTES.

LECTUEE I.

(*) Page 1. The opening lecture was twice post-
poned on account of Dr. Faraday's illness.

(2) Page 9. Platinum, with one exception the heaviest
body known, is 21 J times heavier than water.

(3) Page 10. Aluminium is 2j times heavier than
water.

(4) Pages 10 and 11. Power or property in water.
This power — the heat by which the water is kept in & fluid
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.

(5) Page 10. Anhydrous sulphate of copper: sulphate
of copper deprived of its water of crystallisation. To
obtain it the blue sulphate is calcined in an earthen
crucible.

(6) Page 16. Add a little liquid to the marble and de-
compose it. Marble is composed of carbonic acid and lime,

176 NOTES.

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 colour-
less 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 che-
mist, afterwards found that it could be solidified.

LECTURE II.

(7) Page 41. Crystallisation 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 so-
lution to stand a few minutes and then pour it off from
the dirt and undissolved alum.

(8) Page 43. 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 little
more of the iodide of potassium is added a pale red pre-
cipitate is formed, which, on the further addition of the
iodide, changes into the brilliant scarlet biniodide of mer-
cury. If too much iodide of potassium is added, the
scarlet precipitate disappears, and a colourless solution is
left.

(9) Page 43. Paper coated with scarlet biniodide of
mercury. In order to fix the biniodide on paper, it must

NOTES. 177

be mixed with a little 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.

(10) Page 45. (i 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 of curiosity, and
were considered u a kind of miracle in nature. "

(n) Page 46. Thick glass vessels. They are called
Proofs or Bologna phials.

(12) Page 47. Mica. A silicate of alumina and mag-
nesia. It has a bright metallic lustre, hence its name,
from micoj to shine.

(13) Page 48. Common salt or chloride of sodium
crystallises in the form of solid cubes, which aggregated
together, form a mass, which may be broken up into the
separate cubes.

(14) Page 49. Iceland or calc spar. Native carbonate
of lime in its primitive crystalline form.

LECTURE IIL

*

(15) Page 65. 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.

(16) Page 71. Binoxide of nitrogen and hyponitrous
acid. Binoxide of nitrogen is formed when nitric acid
and a little water are added to some copper turnings.

178 NOTES.

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 com-
posed of two parts oxygen and one part of nitrogen;
hyponitrous acid is composed of one part of nitrogen and
three parts of oxygen.

LECTUEE IV.

(17) Page 91. Chlorate of Potash and Sutyhiiret of An-
timony. Great care must be taken in mixing these sub-
stances, as the mixture is dangerously explosive. They
must be powdered separately and mixed together with
a feather on a sheet of paper, or by passing them several
times through a small sieve.

(18) Page 92. The mixture of chlorate of potash and
sugar does not require the same precautions. 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 witli
a glass rod dipped in oil of vitriol.

(19) Page 92. Two salts dissolved in water. Sulphate
of soda and chloride of calcium. The solutions must be
saturated for the experiment to succeed well.

(20) Page 96. Lead pyrophorous. This is tartrate of
lead which has been heated in a glass tube to dull red-
ness as long as vapours are emitted. As soon as they
cease to be evolved the end of the tube is sealed, and it
is allowed to cool.

(21) Page .99. Gkm cotton is made by immersing cot-
ton-wool in a mixture of sulphuric acid and the strongest
nitric acid, or of sulphuric acid and nitrate of potash.

NOTES. 179

(22) Page 100. Paper prepared like gun cotton. 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 afterwards 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 immersion in
a warm solution of these salts. (See Chemical News
Vol. I. page 36.)

LECTURE VI.

(23) Page 145., Sulphindigotic acid. A mixture of
one part of indigo and fifteen parts of concentrated oil of
vitriol. It is bleached on the side at which hydrogen gas is
evolved in consequence of the liberated hydrogen with-
drawing oxygen from the indigo, thereby forming a colour-
less deoxidised indigo. In making the experiment, only
enough of the sulphindigotic acid must be added to give
the water a decided blue colour.

(24) Page 147. 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 crystallise
around the divergent wires, and form a beautiful object.

THE END.

LONDON

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COMPEISING

Applied Science

Geology and Physical

Jfotany

Geography

Chemistry-
General Science

Mathematics
Physical Science
Zoology

1860

INDEX.

Applied Science —

General Science —

Aitken's Medicine

Anderson's Applied Chemistry . . .
.Barlow's Manufactures

. S
. 3
. 3

Mental Science . .

6
6
6

Napier's Metallurgy of the Bihle

Olmstead's Noah

Book of Trades

1

Palace of the Great King

7

4

Paley's Natural Theology

Senior's Political Economy

6

Bronner's Chemistry of Food

. 4

7

Davy's Agricultural Chemistry . . .

. 4

Smedley's Occult Sciences

7

. 4
. 5
. 5

Geology & Physical Geography

—

Hunt's Manual of Photography . . .
„ Practice of Photography. . .

Imray's Practical Mechanics

. 5

Ansted's Inanimate Creation

3

„ Steam Engine

5

,, Geology

3

Martin's Photography

. 6

Bryce's Cyclopaedia

„ Physical Geography

Griffin's Crystallography

4

. 6
. 6

4
5

„ „ Receipts

„ Electro-Metallurgy

. 6

Hitchcock's Religion and Geology. .
Kitto's Physical Geography

5

Phillips's Metallurgy

. 7

5

„ Gold Mining

. 7

Phillips's Geology

7

„ Records of Mining

. 7

Schoedler's Geology

8

Rankine's Applied Mechanics

. 7

Tennent's Mineralogy

8

„ Civil Engineering

. 7

Wittich's Physical Geography

8

„ Steam Engine

Scoffern's Artificial Light

. V

. 7

Mathematics—

Spooner's Veterinary Art

Thomson's Medicine

. 7

Airy's Trigonometry

3

. 8

Brougham s Tracts

4

Young's Navigation

. 8

Copland's Arithmetic

4

Jardine's Geometry

5

Botany —

Peacock's Arithmetic

7

Balfour's Botany

. 3
. 4

. 8
. 7

8
8
8
8
8

„ Trigonometry

Young's Arithmetic

„ Algebra

„ Geometry

Schoedler's Botany

Smith's Botany

Chemistry —

Physical Science—

Anderson's Applied Chemistry . . .

. 3

Bakewell's Electricity

3

Bronner's Chemistry of Food

. 4

Breen's Astronomy

4

Davy's Agricultural Chemistry . . .

. 4

Faraday's Physical Forces

4

Griffin's Chemical Recreations . . .

. b

Herschel on Light

5

„ Radical Chemistry

. 5

„ on Sound

5

Napier's Chemistry of Dveing

. 6

Lowe's Meteorology

5

Phillips's Chemistry of the Metals
Schoedler's Elementary Chemistrj

. 7

Mitchell's Statics

6

f 8

„ Astronomy

6

Scoffern's Elementary Chemistry .

„ Inorganic Bodies

„ Artificial Light

. 7

Nicnol's Physical Sciences

6

. 7

Schoedler's Astronomy

„ N atural Philosophy

8

. 7

8

Thomson's Dictionary of Chemistr

V 8

„ Physical Science

8

Thomson's Electricity

8

General Science—

Young's Dynamics

8

Book of Nature

. 3

. 4

. 4

4

„ Nautical Astronomy

Zoology—

Baird's Natural History

8
3

Brougham's Natural Theology . . .
Circle of the Sciences

Dick's Christian Philosopher

Fleming's Moral Philosophy

. 4

Broderip's Zoological Recreations. .

4

Greek and Roman Science

5

Bushnan's Physiology

4

Harry's Vacation

Hitchcock's Religious Truth

. 7
. 5

4
5

Knight's Natural History

Hughes's Reading Lessons

Keddie's Scientific Anecdotes

. 1

Latham's Ethnology

5

6

Owen's Skeleton and Teeth

6

Maurice's Moral Philosophy

. 6

Schoedler's Zoology

8

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