soap-film or bubble is really elastic, like a piece of stretched
india-rubber.

A soap-bubble consisting, as it does, of a thin layer of liquid, which
must have of course both an inside and an outside surface or skin, must
be elastic, and this is easily shown in many ways. Perhaps the easiest
way is to tie a thread across a ring rather loosely, and then to dip the
ring into soap water. On taking it out there is a film stretched over
the ring, in which the thread moves about quite freely, as you can see
upon the screen. But if I break the film on one side, then immediately
the thread is pulled by the film on the other side as far as it can go,
and it is now tight (Fig. 19). You will also notice that it is part of a
perfect circle, because that form makes the space on one side as great,
and therefore on the other side, where the film is, as small, as
possible. Or again, in this second ring the thread is double for a short
distance in the middle. If I break the film between the threads they are
at once pulled apart, and are pulled into a perfect circle (Fig. 20),
because that is the form which makes the space within it as great as
possible, and therefore leaves the space outside it as small as
possible. You will also notice, that though the circle will not allow
itself to be pulled out of shape, yet it can move about in the ring
quite freely, because such a movement does not make any difference to
the space outside it.

[Illustration: Fig. 19.]

[Illustration: Fig. 20.]

[Illustration: Fig. 21.]

[Illustration: Fig. 22.]

I have now blown a bubble upon a ring of wire. I shall hang a small ring
upon it, and to show more clearly what is happening, I shall blow a
little smoke into the bubble. Now that I have broken the film inside the
lower ring, you will see the smoke being driven out and the ring lifted
up, both of which show the elastic nature of the film. Or again, I have
blown a bubble on the end of a wide pipe; on holding the open end of the
pipe to a candle flame, the outrushing air blows out the flame at once,
which shows that the soap-bubble is acting like an elastic bag (Fig.
21). You now see that, owing to the elastic skin of a soap-bubble, the
air inside is under pressure and will get out if it can. Which would you
think would squeeze the air inside it most, a large or a small bubble?
We will find out by trying, and then see if we can tell why. You now
see two pipes each with a tap. These are joined togeth