ble 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