ar 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