necessary that I should any further insist upon the fact
that the outside of a liquid acts as if it were a perfectly elastic skin
stretched with a certain definite force.

Suppose now that you take a small quantity of water, say as much as
would go into a nut-shell, and suddenly let it go, what will happen? Of
course it will fall down and be dashed against the ground. Or again,
suppose you take the same quantity of water and lay it carefully upon a
cake of paraffin wax dusted over with lycopodium which it does not wet,
what will happen? Here again the weight of the drop--that which makes it
fall if not held--will squeeze it against the paraffin and make it
spread out into a flat cake. What would happen if the weight of the drop
or the force pulling it downwards could be prevented from acting? In
such a case the drop would only feel the effect of the elastic skin,
which would try to pull it into such a form as to make the surface as
small as possible. It would in fact rapidly become a perfectly round
ball, because in no other way can so small a surface be obtained. If,
instead of taking so much water, we were to take a drop about as large
as a pin's head, then the weight which tends to squeeze it out or make
it fall would be far less, while the skin would be just as strong, and
would in reality have a greater moulding power, though why I cannot now
explain. We should therefore expect that by taking a sufficiently small
quantity of water the moulding power of the skin would ultimately be
able almost entirely to counteract the weight of the drop, so that very
small drops should appear like perfect little balls. If you have found
any difficulty in following this argument, a very simple illustration
will make it clear. Many of you probably know how by folding paper to
make this little thing which I hold in my hand (Fig. 15). It is called a
cat-box, because of its power of dispelling cats when it is filled with
water and well thrown. This one, large enough to hold about half a pint,
is made out of a small piece of the _Times_ newspaper. You may fill it
with water and carry it about and throw it with your full power, and the
strength of the paper skin is sufficient to hold it together until it
hits anything, when of course it bursts and the water comes out. On the
other hand, the large one made out of a whole sheet of the _Times_ is
barely able to withstand the weight of the water that it will hold. It
is only just strong enoug