the regular geometrical solids, very beautiful figures will be found
upon them after they have been dipped in soap-water. In the case of the
triangular prism these surfaces are all flat, and at the edges where
these planes meet one another there are always three meeting each other
at equal angles (Fig. 50). This, owing to the fact that the frame is
three-sided, is not surprising. After looking at this three-sided frame
with three films meeting down the central line, you might expect that
with a four-sided or square frame there would be four films meeting each
other in a line down the middle. But it is a curious thing that it does
not matter how irregular the frame may be, or how complicated a mass of
froth may be, there can never be more than three films meeting in an
edge, or more than four edges, or six films, meeting in a point.
Moreover the films and edges can only meet one another at equal angles.
If for a moment by any accident four films do meet in the same edge, or
if the angles are not exactly equal, then the form, whatever it may be,
is unstable; it cannot last, but the films slide over one another and
never rest until they have settled down into a position in which the
conditions of stability are fulfilled. This may be illustrated by a very
simple experiment which you can easily try at home, and which you can
now see projected upon the screen. There are two pieces of window-glass
about half an inch apart, which form the sides of a sort of box into
which some soap and water have been poured. On blowing through a pipe
which is immersed in the water, a great number of bubbles are formed
between the plates. If the bubbles are all large enough to reach across
from one plate to the other, you will at once see that there are nowhere
more than three films meeting one another, and where they meet the
angles are all equal. The curvature of the bubbles makes it difficult to
see at first that the angles really are all alike, but if you only look
at a very short piece close to where they meet, and so avoid being
bewildered by the curvature, you will see that what I have said is true.
You will also see, if you are quick, that when the bubbles are blown,
sometimes four for a moment do meet, but that then the films at once
slide over one another and settle down into their only possible position
of rest (Fig. 51).
The air inside a bubble is generally under pressure, which is produced
by its elasticity and curvature. If the bubble would let the air pass
through it from one side to the other of course it would soon shut up,
as it did when a ring was hung upon one, and the film within the ring
was broken. But there are no holes in a bubble, and so you would expect
that a gas like air could not pass through to the other side.
Nevertheless it is a fact that gases can slowly get through to the other
side, and in the case of certain vapours the process is far more rapid
than any one would think possible.
[Illustration: Fig. 51.]
[Illustration: Fig. 52.]
Ether produces a vapour which is very heavy, and which also burns very
easily. This vapour can get to the other side of a bubble almost at
once. I shall pour a little ether upon blotting-paper in this bell jar,
and fill the jar with its heavy vapour. You can see that the jar is
filled with something, not by looking at it, for it appears empty, but
by looking at its shadow on the screen. Now I tilt it gently to one
side, and you see something pouring out of it, which is the vapour of
ether. It is easy to show that this is heavy; it is only necessary to
drop into the jar a bubble, and so soon as the bubble meets the heavy
vapour it stops falling and remains floating upon the surface as a cork
does upon water (Fig. 52). Now let me test the bubble and see whether
any of the vapour has passed to the inside. I pick it up out of the jar
with a wire ring and carry it to a light, and at once there is a burst
of flame. But this is not sufficient to show that the ether vapour has
passed to the inside, because it might have condensed in sufficient
quantity upon the bubble to make it inflammable. You remember that when
I poured some of this vapour upon water in the first lecture, sufficient
condensed to so weaken the water-skin that the frame of wire could get
through to the other side. However, I can see whether this is the true
explanation or not by blowing a bubble on a wide pipe, and holding it in
the vapour for a moment. Now on removing it you notice that the bubble
hangs like a heavy drop; it has lost the perfect roundness that it had
at first, and this looks as if the vapour had found its way in, but this
is made certain by bringing a light to the mouth of the tube, when the
vapour, forced out by the elasticity of the bubble, catches fire and
burns with a flame five or six inches long (Fig. 53). You might also
have noticed that when the bubble was removed, the vapour inside it
began to pass out again and fell away in a heavy stream, but this you
could only see by looking at the shadow upon the screen.
[Illustration: Fig. 53]
You may have noticed when I made the drops of oil in the mixture of
alcohol and water, that when they were brought together they did not at
once unite; they pressed against one another and pushed each other away
if allowed, just as the water-drops did in the fountain of which I
showed you a photograph. You also may have noticed that the drops of
water in the paraffin mixture bounced against one another, or if filled
with the paraffin, formed bubbles in which often other small drops, both
of water and paraffin, remained floating.
In all these cases there was a thin film of something between the drops
which they were unable to squeeze out, namely, water, paraffin, or air,
as the case might be. Will two soap-bubbles also when knocked together
be unable to squeeze out the air between them? This you can try at home
just as well as I can here, but I will perform the experiment at once. I
have blown a pair of bubbles, and now when I hit them together they
remain distinct and separate (Fig. 54).
[Illustration: Fig. 54.]
I shall next place a bubble on a ring, which it is just too large to get
through. In my hand I hold a ring, on which I have a flat film, made by
placing a bubble upon it and breaking it on one side. If I gently press
the bubble with the flat film, I can push it through the ring to the
other side (Fig. 55), and yet the two have not really touched one
another at all. The bubble can be pushed backwards and forwards in this
way many times.
[Illustration: Fig. 55.]
I have now blown a bubble and hung it below a ring. To this bubble I can
hang another ring of thin wire, which pulls it a little out of shape.
Since the pressure inside is less than that corresponding to a complete
sphere, and since it is greater than that outside, and this we can tell
by looking at the caps, the curve is part of one of those represented by
the dotted lines in C or E, Fig. 31. However, without considering the
curve any more, I shall push the end of the pipe inside, and blow
another bubble there, and let it go. It falls gently until it rests
upon the outer bubble; not at the bottom, because the heavy ring keeps
that part out of reach, but along a circular line higher up (Fig. 56). I
can now drain away the heavy drops of liquid from below the bubbles with
a pipe, and leave them clean and smooth all over. I can now pull the
lower ring down, squeezing the inner bubble into a shape like an egg
(Fig. 57), or swing it round and round, and then with a little care peel
away the ring from off the bubble, and leave them both perfectly round
every way (Fig. 58). I can draw out the air from the outer bubble till
you can hardly see between them, and then blow in, and the harder I
blow, the more is it evident that the two bubbles are not touching at
all; the inner one is now spinning round and round in the very centre
of the large bubble, and finally, on breaking the outer one the inner
floats away, none the worse for its very unusual treatment.
[Illustration: Fig. 56.]
[Illustration: Fig. 57.]
[Illustration: Fig. 58.]
There is a pretty variation of the last experiment, which, however,
requires that a little green dye called fluorescine, or better, uranine,
should be dissolved in a separate dish of the soap-water. Then you can
blow the outer bubble with clean soap-water, and the inner one with the
coloured water. Then if you look at the two bubbles by ordinary light,
you will hardly notice any difference; but if you allow sunlight, or
electric light from an arc lamp, to shine upon them, the inner one will
appear a brilliant green, while the outer one will remain clear as
before. They will not mix at all, showing that though the inner one is
apparently resting against the outer one, there is in reality a thin
cushion of air between.
Now you know that coal-gas is lighter than air, and so a soap-bubble
blown with gas, when let go, floats up to the ceiling at once. I shall
blow a bubble on a ring with coal-gas. It is soon evident that it is
pulling upwards. I shall go on feeding it with gas, and I want you to
notice the very beautiful shapes that it takes (Fig. 59, but imagine the
globe inside removed). These are all exactly the curves that a
water-drop assumes when hanging from a pipe, except that they are the
other way up. The strength of the skin is now barely able to withstand
the pull, and now the bubble breaks away just as the drop of water did.
[Illustration: Fig. 59.]
I shall next place a bubble blown with air upon a ring, and blow inside
it a bubble blown with a mixture of air and gas. It of course floats up
and rests against the top of the outer bubble (Fig. 60). Now I shall let
a little gas into the outer one, until the surrounding gas is about as
heavy as the inner bubble. It now no longer rests against the top, but
floats about in the centre of the large bubble (Fig. 61), just as the
drop of oil did in the mixture of alcohol and water. You can see that
the inner bubble is really lighter than air, because if I break the
outer one, the inner one rises rapidly to the ceiling.
[Illustration: Fig. 60.]
Instead of blowing the first bubble on a heavy fixed ring, I shall now
blow one on a light ring, made of very thin wire. This bubble contains
only air. If I blow inside this a bubble with coal-gas, then the
gas-bubble will try and rise, and will press against the top of the
outer one with such force as to make it carry up the wire ring and a
yard of cotton, and some paper to which the cotton is tied (Fig. 62);
and all this time, though it is the inner one only which tends to rise,
the two bubbles are not really touching one another at all.
[Illustration: Fig. 61.]
[Illustration: Fig. 62.]
I have now blown an air-bubble on the fixed ring, and pushed up inside
it a wire with a ring on the end. I shall now blow another air-bubble
on this inner ring. The next bubble that I shall blow is one containing
gas, and this is inside the other two, and when let go it rests against
the top of the second bubble. I next make the second bubble a little
lighter by blowing a little gas into it, and then make the outer one
larger with air. I can now peel off the inner ring and take it away,
leaving the two inner bubbles free, inside the outer one (Fig. 63). And
now the multiple reflections of the brilliant colours of the different
bubbles from one to the other, set off by the beautiful forms which the
bubbles themselves assume, give to the whole a degree of symmetry and
splendour which you may go far to see equalled in any other way. I have
only to blow a fourth bubble in _real_ contact with the outer bubble and
the ring, to enable it to peel off and float away with the other two
inside.
[Illustration: Fig. 63.]
We have seen that bubbles and drops behave in very much the same way.
Let us see if electricity will produce the same effect that it did on
drops. You remember that a piece of electrified sealing-wax prevented a
fountain of water from scattering, because where two drops met, instead
of bouncing, they joined together. Now there are on these two rings
bubbles which are just resting against one another, but not really
touching (Fig. 64). The instant that I take out the sealing-wax you see
they join together and become one (Fig. 65). Two soap-bubbles,
therefore, enable us to detect electricity, even when present in minute
quantity, just as two water fountains did.
[Illustration: Fig. 64.]
[Illustration: Fig. 65.]
We can use a pair of bubbles to prove the truth of one of the
well-known actions of electricity. Inside an electrical conductor it is
impossible to feel any influence of electricity outside, however much
there may be, or however near you go to the surface. Let us, therefore,
take the two bubbles shown in Fig. 56, and bring an electrified stick of
sealing-wax near. The outer bubble is a conductor; there is, therefore,
no electrical action inside, and this you can see because, though the
sealing-wax is so near the bubble that it pulls it all to one side, and
though the inner one is so close to the outer one that you cannot see
between them, yet the two bubbles remain separate. Had there been the
slightest electrical influence inside, even to a depth of a
hundred-thousandth of an inch, the two bubbles would have instantly come
together.
[Illustration: Fig. 66.]
There is one more experiment which I must show, and this will be the
last; it is a combination of the last two, and it beautifully shows the
difference between an inside and an outside bubble. I have now a plain
bubble resting against the side of the pair that I have just been using.
The instant that I take out the sealing-wax the two outer bubbles join,
while the inner one unharmed and the heavy ring slide down to the bottom
of the now single outer bubble (Fig. 66).
And now that our time has drawn to a close I must ask you whether that
admiration and wonder which we all feel when we play with soap-bubbles
has been destroyed by these lectures; or whether now that you know more
about them it is not increased. I hope you will all agree with me that
the actions upon which such common and every-day phenomena as drops and
bubbles depend, actions which have occupied the attention of the
greatest philosophers from the time of Newton to the present day, are
not so trivial as to be unworthy of the attention of ordinary people
like ourselves.
PRACTICAL HINTS.
I hope that the following practical hints may be found useful by those
who wish themselves to successfully perform the experiments already
described.
_Drop with India-rubber Surface._
A sheet of thin india-rubber, about the thickness of that used in
air-balls, as it appears _before_ they have been blown out, must be
stretched over a ring of wood or metal eighteen inches in diameter, and
securely wired round the edge. The wire will hold the india-rubber
better if the edge is grooved. This does not succeed if tried on a
smaller scale. This experiment was shown by Sir W. Thomson at the Royal
Institution.
_Jumping Frame._
This is easily made by taking a light glass globe about two inches in
diameter, such, for instance, as a silvered ball used to ornament a
Christmas-tree or the bulb of a pipette, which is what I used. Pass
through the open necks of the bulb a piece of wire about one-twentieth
of an inch in diameter, and fix it permanently and water-tight upon the
wire by working into the necks melted sealing-wax. An inch or two above
the globe, fasten a flat frame of thin wire by soldering, or if this is
too difficult, by tying and sealing-wax. A lump of lead must then be
fastened or hung on to the lower end, and gradually scraped away until
the wire frame will just be unable to force its way through the surface
of the water. None of the dimensions or materials mentioned are of
importance.
_Paraffined Sieve._
Obtain a piece of copper wire gauze with about twenty wires to the inch,
and cut out from it a round piece about eight inches in diameter. Lay it
on a round block, of such a size that it projects about one inch all
round. Then gently go round and round with the hands pressing the edge
down and keeping it flat above, until the sides are evenly turned down
all round. This is quite easy, because the wires can allow of the kind
of distortion necessary. Then wind round the turned-up edge a few turns
of thick wire to make the sides stiff. This ought to be soldered in
position, but probably careful wiring will be good enough.
Melt some paraffin wax or one or two paraffin candles of the best
quality in a clean flat dish, not over the fire, which would be
dangerous, but on a hot plate. When melted and clear like water, dip the
sieve in, and when all is hot quickly take it out and knock it once or
twice on the table to shake the paraffin out of the holes. Leave upside
down until cold, and then be careful not to scratch or rub off the
paraffin. This had best be done in a place where a mess is of no
consequence.
There is no difficulty in filling it or in setting it to float upon
water.
_Narrow Tubes and Capillarity._
Get some quill-glass tube from a chemist, that is, tube about the size
of a pen. If it is more than, say, a foot long, cut off a piece by
first making a firm scratch in one place with a three-cornered file,
when it will break at the place easily. To make very narrow tube from
this, hold it near the ends in the two hands very lightly, so that the
middle part is high up in the brightest part of an ordinary bright and
flat gas flame. Keep it turning until at last it becomes so soft that it
is difficult to hold it straight. It can then be bent into any shape,
but if it is wanted to be drawn out it must be held still longer until
the black smoke upon it begins to crack and peel up. Then quickly take
it out of the flame, and pull the two ends apart, when a long narrow
tube will be formed between. This can be made finer or coarser by
regulating the heat and the manner in which it is pulled out. No
directions will tell any one so much as a very little practice. For
drawing out tubes the flame of a Bunsen burner or of a blow-pipe is more
convenient; but for bending tubes nothing is so good as the flat gas
flame. Do not clean off smoke till the tubes are cold, and do not hurry
their cooling by wetting or blowing upon them. In the country where gas
is not to be had, the flame of a large spirit-lamp can be made to do,
but it is not so good as a gas-flame. The narrower these tubes are, the
higher will clean water be observed to rise in them. To colour the
water, paints from a colour-box must not be used. They are not liquid,
and will clog the very fine tubes. Some dye that will quite dissolve (as
sugar does) must be used. An aniline dye, called soluble blue, does very
well. A little vinegar added may make the colour last better.
_Capillarity between Plates._
Two plates of flat glass, say three to five inches square, are required.
Provided they are quite clean and well wetted there is no difficulty. A
little soap and hot water will probably be sufficient to clean them.
_Tears of Wine._
These are best seen at dessert in a glass about half filled with port. A
mixture of from two to three parts of water, and one part of spirits of
wine containing a very little rosaniline (a red aniline dye), to give it
a nice colour, may be used, if port is not available. A piece of the
dye about as large as a mustard-seed will be enough for a large
wine-glass. The sides of the glass should be wetted with the wine.
_Cat-Boxes._
Every school-boy knows how to make these. They are not the boxes made by
cutting slits in paper. They are simply made by folding, and are then
blown out like the "frog," which is also made of folded paper.
_Liquid Beads._
Instead of melting gold, water rolled on to a table thickly dusted with
lycopodium, or other fine dust, or quicksilver rolled or thrown upon a
smooth table, will show the difference in the shape of large and small
beads perfectly. A magnifying-glass will make the difference more
evident. In using quicksilver, be careful that none of it falls on gold
or silver coins, or jewellery, or plate, or on the ornamental gilding on
book-covers. It will do serious damage.
_Plateau's Experiment._
To perform this with very great perfection requires much care and
trouble. It is easy to succeed up to a certain point. Pour into a clean
bottle about a table-spoonful of salad-oil, and pour upon it a mixture
of nine parts by volume spirits of wine (not methylated spirits), and
seven parts of water. Shake up and leave for a day if necessary, when it
will be found that the oil has settled together by itself. Fill a
tumbler with the same mixture of spirit and water, and then with a fine
glass pipe, dipping about half-way down, slowly introduce a very little
water. This will make the liquid below a little heavier. Dip into the
oil a pipe and take out a little by closing the upper end with the
finger, and carefully drop this into the tumbler. If it goes to the
bottom, a little more water is required in the lower half of the
tumbler. If by chance it will not sink at all, a little more spirit is
wanted in the upper half. At last the oil will just float in the middle
of the mixture. More can then be added, taking care to prevent it from
touching the sides. If the liquid below is ever so little heavier, and
the liquid above ever so little lighter than oil, the drop of oil
perhaps as large as a halfpenny will be almost perfectly round. It will
not appear round if seen through the glass, because the glass magnifies
it sideways, but not up and down, as may be seen by holding a coin in
the liquid just above it. To see the drop in its true shape the vessel
must either be a globe, or one side must be made of flat glass.
Spinning the oil so as to throw off a ring is not material, but if the
reader can contrive to fix a disc about the size of a threepenny-piece
upon a straight wire, and spin it round without shaking it, then he will
see the ring break off, and either return if the rotation is quickly
stopped, or else break up into three or four perfect little balls. The
disc should be wetted with oil before being dipped into the mixture of
spirit and water.
_A Good Mixture for Soap-Bubbles._
Common yellow soap is far better than most of the fancy soaps, which
generally contain a little soap and a lot of rubbish. Castille soap is
very good, and this may be obtained from any chemist.
Bubbles blown with soap and water alone do not last long enough for many
of the experiments described, though they may sometimes be made to
succeed. Plateau added glycerine, which greatly improves the lasting
quality. The glycerine should be pure; common glycerine is not good, but
Price's answers perfectly. The water should be pure distilled water, but
if this is not available, clean rain-water will do. Do not choose the
first that runs from a roof after a spell of dry weather, but wait till
it has rained for some time, the water that then runs off is very good,
especially if the roof is blue slate or glass. If fresh rain-water is
not to be had, the softest water should be employed that can be
obtained. Instead of Castille soap, Plateau found that a pure soap
prepared from olive-oil is still better. This is called oleate of soda.
It should be obtained freshly prepared from a manufacturing chemist.
Old, dry stuff that has been kept a long time is not so good. I have
always used a modification of Plateau's formula, which Professors
Reinold and Rucker found to answer so well. They used less glycerine
than Plateau. It is best made as follows. Fill a clean stoppered bottle
three-quarters full of water. Add one-fortieth part of its weight of
oleate of soda, which will probably float on the water. Leave it for a
day, when the oleate of soda will be dissolved. Nearly fill up the
bottle with Price's glycerine and shake well, or pour it into another
clean bottle and back again several times. Leave the bottle, stoppered
of course, for about a week in a dark place. Then with a syphon, that
is, a bent glass tube which will reach to the bottom inside and still
further outside, draw off the clear liquid from the scum which will have
collected at the top. Add one or two drops of strong liquid ammonia to
every pint of the liquid. Then carefully keep it in a stoppered bottle
in a dark place. Do not get out this stock bottle every time a bubble is
to be blown, but have a small working bottle. Never put any back into
the stock. In making the liquid _do not warm or filter it_. Either will
spoil it. Never leave the stoppers out of the bottles or allow the
liquid to be exposed to the air more than is necessary. This liquid is
still perfectly good after two years' keeping. I have given these
directions very fully, not because I feel sure that all the details are
essential, but because it exactly describes the way I happen to make it,
and because I have never found any other solution so good. Castille
soap, Price's glycerine, and rain-water will almost certainly answer
every purpose, and the same proportions will probably be found to work
well.
_Rings for Bubbles._