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
undertake. It will be impossible for us to consider the Laws of
Nature, and what they effect, unless we now and then give our sole
attention, so as to obtain a clear idea upon the subject. Give me now
that attention, and then, I trust, we shall not part without your
knowing something about those Laws, and the manner in which they act.
You recollect, upon the last occasion, I explained that all bodies
attracted each other, and that this power we called _gravitation_. I
told you that when we brought these two bodies [two equal sized ivory
balls suspended by threads] near together, they attracted each other,
and that we might suppose that the whole power of this attraction was
exerted between their respective centres of gravity; and furthermore,
you learned from me, that if, instead of a small ball, I took a larger
one, like _that_ [changing one of the balls for a much larger one],
there was much more of this attraction exerted; or, if I made this
ball larger and larger, until, if it were possible, it became as large
as the Earth itself--or, I might take the Earth itself as the large
ball--that _then_ the attraction would become so powerful as to cause
them to rush together in this manner [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 gravitation to the centre of the earth.
I must not, however, leave the subject of gravitation, without telling
you something about its laws and regularity; and first, as regards
its power with respect to the distance that bodies are apart. If I
take one of these balls and place it within an inch of the other, they
attract each other with a certain power. If I hold it at a greater
distance off, they attract with less power; and if I hold it at a
greater distance still, their attraction is still less. Now this fact
is of the greatest consequence; for, knowing this law, philosophers
have discovered 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 understand what this
law is. They say (and they are right) that two bodies attract each
other _inversely as the square of the distance_--a sad jumble of words
until you understand them; but I think we shall soon comprehend what
this law is, and what is the meaning of the “inverse square of the
distance.”
[Illustration: Fig. 11.]
I have here (fig. 11) a lamp A, shining most intensely upon this disc,
B, C, D; and this light acts as a sun by which I can get a shadow from
this little screen, B F (merely a square piece of card), which, as
you know, when I place it close to the large screen, just shadows as
much of it as is exactly equal to its own size. But now let me take
this card E, which is equal to the other one in size, and place it
midway between the lamp and the screen: now look at the 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 _two_, which is the _square_ of
the distance; and, if I put the screen at one-third of the distance
from the lamp, the shadow on the large screen would be _nine_ times
the size. Again, if I hold this screen _here_, at B F, a certain
amount of light falls on it; and if I hold it nearer the lamp at E,
_more_ light shines upon it. And you see at once how much--exactly
the quantity which I have shut off from the part of this screen, B D,
now in shadow; moreover, you see that if I put a single screen here,
at G, by the side of the shadow, it can only receive _one-fourth_ of
the 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 at another, you get the
inverse accordingly. So it is as regards the attraction of these two
balls--they attract according to the square of the distance, inversely.
I want you to try and remember these words, and then you will be able
to go into all the calculations of astronomers as to the planets and
other bodies, and tell why they move so fast, and why they go _round_
the sun without falling into it, and be prepared to enter upon many
other interesting inquiries of the like nature.
Let us now leave this subject which I have written upon the board
under the word FORCE--GRAVITATION--and go a step further. All bodies
attract each other at sensible distances. I shewed you the electric
attraction on the last occasion (though I did not call it so); that
attracts at a distance: and in order to make our progress a little
more gradual, suppose I take a few iron particles [dropping some small
fragments of iron on the table]. There, I have already told you that in
all cases where bodies fall, it is the _particles_ that are attracted.
You may consider these then as separate particles magnified, so as
to be evident to your sight; they are loose from each other--they all
gravitate--they all fall to the earth--for the force of 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.
[Illustration: Fig. 12.]
Here I have an arch of iron filings (fig. 12) regularly built up like
an iron bridge, because I have put them within a sphere of action which
will cause them to attract each other. See!--I could let a mouse run
through it, and yet if I try to do the same thing with them _here_
[on the table], they do not attract each other at all. It is _that_
[the magnet] which makes them hold together. Now, just as these iron
particles hold together in the form of an elliptical bridge, so do the
different particles of iron which constitute this nail hold 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 _between_ 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 shew you an experiment to prove that this attraction
does still exist in those particles, for here is a piece of glass (for
what was true of the flint and the bar of iron is true of the piece of
glass, and is true of every other solid--they are all held together in
the lump by the attraction between their parts), and I can shew you the
attraction between its separate particles; for if I take these portions
of glass, which I have reduced to very fine powder, you see that I
can actually build them up into a solid wall by pressure 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? _That_ bar of iron one inch
square has such power of attraction in its particles--giving to it such
strength--that it will hold up twenty tons weight before the little set
of particles in the small space, equal to one division across which it
can be pulled apart, will separate. In this manner suspension bridges
and chains are held together by the attraction of their particles; and
I am going to make an experiment which will shew how strong is this
attraction of the particles. [The Lecturer here placed his foot on a
loop of wire fastened to a support above, and swung with his whole
weight resting upon it for some moments.] You see while hanging here
all my weight is supported by these little particles of the wire, just
as in pantomimes they sometimes suspend gentlemen and damsels.
How can we make this attraction of the particles a little more simple?
There are many things which if brought together properly will shew this
attraction. Here is a boy’s experiment (and I like a boy’s experiment).
Get a tobacco-pipe, fill it with lead, melt it, and then pour it out
upon a stone, and thus get a clean piece of lead (this is a better plan
than scraping it--scraping alters the condition of the surface of the
lead). I have here some pieces of lead which I melted this morning for
the sake of making them clean. Now these pieces of lead hang together
by the attraction of their particles; and if I press these two separate
pieces close together, so as to bring their particles within the
sphere of attraction, you will see how soon they become one. I have
merely to give them a good squeeze, and draw the upper piece slightly
round at the same time, and here they are as one, and all the bending
and twisting I can give them will not separate them again: I have
joined the lead together, not with solder, but simply by means of the
attraction of the particles.
This, however, is not the best way of bringing those particles
together--we have many better plans than that; and I will shew 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 shew 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-morrow 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, namely, warming it slowly and gradually;
and in repeating this experiment, do as I do--pour the liquid out
gently, leaving all the dirt behind in the basin: and remember that
the more carefully and quietly you make this 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 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 when
I pour these two liquids together--both colourless as water. [The
Lecturer here mixed together solutions of perchloride of mercury and
iodide of potassium, when a yellow precipitate of biniodide of mercury
fell down, which almost immediately became crimson red.] Now, there
is a substance which is very beautiful, but see how it is changing
colour. It was reddish-yellow at first, but it has now become red.[8] I
have previously prepared a little of this red substance, which you see
formed in the liquid, and have put some of it upon paper. [Exhibiting
several sheets of paper coated with scarlet biniodide of mercury.[9]]
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 attraction of cohesion is in a
certain degree changed; for I will take this red body, and apply heat
to it (you may perhaps see a little 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 experiment.] 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 when I tell you that this piece of common charcoal
is just the same thing, only differently calesced, 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 “Prince Rupert’s
drop”.[10] (fig. 13)]; and if I take one of these little tear-like
pieces and break off ever so little from the point, the whole will at
once burst and fall to pieces. I will now break off a piece of this.
[The Lecturer nipped off a small piece from the end of one of the
Rupert’s drops, whereupon the whole immediately fell to pieces.] There!
you see the solid glass has suddenly become powder--and more than that,
it has knocked a hole in the glass vessel in which it was held. I can
shew the effect better in this bottle of water; and it is very likely
the whole bottle will go. [A 6-oz. vial was filled with water, and a
Rupert’s drop placed in it, with the point of the tail just projecting
out; upon breaking the tip off, the drop burst, and the shock being
transmitted through the water to the sides of the bottle, shattered the
latter to pieces.]
Here is another form of the same kind of experiment. I have here
some more glass which has not been annealed [showing some thick glass
vessels[11] (fig. 14)], and if I take one of these glass vessels and
drop a piece of pounded glass into it (or I will take some of these
small pieces of rock crystal--they have the advantage of being harder
than glass), and so make the least scratch upon the inside, the whole
bottle will break to pieces,--it 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.
[Illustration: Fig. 13. and Fig. 14.]
Now, I have shewn you these things for the purpose of bringing your
minds to see that bodies are not merely held together by this power
of cohesion, but that they are held together in very curious ways. And
suppose I take some things that are held together by this force, and
examine them more minutely. I will first take a bit of glass, and if
I give it a blow with a hammer, I shall just break it to pieces. You
saw how it was in the case of the flint when I broke the piece off; a
piece of a similar kind would come off, just as you would expect; and
if I were to break it up still more, it would be as you have seen,
simply a collection of small particles of no definite shape or form.
But supposing I take some other thing, this stone for instance (fig.
15) [taking a piece of mica[12]], and if I hammer this stone, I may
batter it a great deal before I can break it up. I may even bend it
without breaking it; that is to say, I may bend it in _one particular
direction_ without breaking it much, although I feel in my hands that
I am doing it some injury. But now, if I take it by the edges, I find
that it breaks up into leaf after leaf in a most extraordinary manner.
Why should it break up like that? Not because all stones do, or all
crystals; for there is some salt (fig. 16)--you know what common salt
is[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 combining 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 exact
surfaces, beautiful and glittering as diamonds [breaking it by gentle
blows with a hammer]; there is a square prism which I may break up into
a square cube. You see these fragments are all square--one side may be
longer than the other, but they will only split up so as to form square
or oblong pieces with cubical sides. Now, I go a little further, and
I find another stone (fig. 17) [Iceland, or calc-spar][14], 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 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.
[Illustration: Fig. 15.]
[Illustration: Fig. 16. and Fig. 17.]
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 (fig. 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 power, which matter
possesses, of attracting a ray of light. If Mr. Anderson pours some
water, gently and steadily, into the basin, the water will attract the
rays of light 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 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.
[Illustration: Fig. 18. and Fig. 19.]
I have shewn 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 shew you how it may be bent by a
piece of glass (fig. 19). [The electric lamp was again lit, and the
beam of parallel rays of light which it emitted was bent about and
decomposed by means of the prism.] Now, here you see, if I send the
light through this piece of plain glass, A, it goes straight through,
without being bent, unless the glass be held obliquely, and then the
phenomenon becomes more complicated; but if I take this piece of
glass, B [a prism], you see it will shew 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 different 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 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 shewed 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 shew 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 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.
[Illustration: Fig. 20.]
Now we will see how calcareous spar acts upon this light,--that stone
which split up into rhombs, and of which you are each of you going
to take a little piece home. [The mica was removed, and a piece of
calc-spar introduced at A.] See how that turns the light round and
round, and produces these rings and that black cross (fig. 20). Look
at those colours--are they 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 shew 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 shew 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 shew the position of the strain--in these
parts the force of cohesion is being exerted in a different degree to
what it is in the other parts, and hence it allows the light to pass
through. How beautiful that is--how it makes the light come through
some parts, and leaves it dark in others, and all because we weaken
the force of cohesion between particle and particle. Whether you have
this mechanical power of straining, or whether we take other means, we
get the same result; and, indeed, I will shew 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 Rupert’s
drops. [The glass was warmed in the spirit-lamp, and again placed
across the ray of light.] Now you see how beautifully the light goes
through those parts which are hot, making dark and light lines just
as the crystal did, and all because of the alteration I have effected
in its internal condition; for these dark and light parts are a proof
of the presence of forces acting and dragging in different directions
within the solid mass.