ly increase the speed the oil tends to fly away in all
directions, but the elastic skin retains it. The result is that the ball
becomes flattened at its poles like the earth itself. On increasing the
speed, the tendency of the oil to get away is at last too much for the
elastic skin, and a ring breaks away (Fig. 17), which almost
immediately contracts again on to the rest of the ball as the speed
falls. If I turn it sufficiently fast the ring breaks up into a series
of balls which you now see. One cannot help being reminded of the
heavenly bodies by this beautiful experiment of Plateau's, for you see a
central body and a series of balls of different sizes all travelling
round in the same direction (Fig. 18); but the forces which are acting
in the two cases are totally distinct, and what you see has nothing
whatever to do with the sun and the planets.

[Illustration: Fig. 17.]

[Illustration: Fig. 18.]

We have thus seen that a large ball of liquid can be moulded by the
elasticity of its skin if the disturbing effect of its weight is
neutralized, as in the last experiment. This disturbing effect is
practically of no account in the case of a soap-bubble, because it is so
thin that it hardly weighs anything. You all know, of course, that a
soap-bubble is perfectly round, and now you know why; it is because the
elastic film, trying to become as small as it can, must take the form
which has the smallest surface for its content, and that form is the
sphere. I want you to notice here, as with the oil, that a large bubble
oscillates much more slowly than a small one when knocked out of shape
with a bat covered with baize or wool.

The chief result that I have endeavoured to make clear to-day is this.
The outside of a liquid acts as if it were an elastic skin, which will,
as far as it is able, so mould the liquid within it that it shall be as
small as possible. Generally the weight of liquids, especially when
there is a large quantity, is too much for the feebly elastic skin, and
its power may not be noticed. The disturbing effect of weight is got rid
of by immersing one liquid in another which is equally heavy with which
it does not mix, and it is hardly noticed when very small drops are
examined, or when a bubble is blown, for in these cases the weight is
almost nothing, while the elastic power of the skin is just as great as
ever.

LECTURE II.

I did not in the last lecture by any direct experiment show that a