to the glass
before will, the instant that the water is above the edge of the glass,
shoot away from the edge in the most sudden and surprising manner. This
sudden change can be most easily seen by nearly filling the glass with
water, and then gradually dipping in and taking out a cork, which will
cause the level to slowly change.

So far I have given you no idea what force is exerted by this elastic
skin of water. Measurements made with narrow tubes, with drops, and in
other ways, all show that it is almost exactly equal to the weight of
three and a quarter grains to the inch. We have, moreover, not yet seen
whether other liquids act in the same way, and if so whether in other
cases the strength of the elastic skin is the same.

You now see a second tube identical with that from which drops of water
were formed, but in this case the liquid is alcohol. Now that drops are
forming, you see at once that while alcohol makes drops which have a
definite size and shape when they fall away, the alcohol drops are not
by any means so large as the drops of water which are falling by their
side. Two possible reasons might be given to explain this. Either
alcohol is a heavier liquid than water, which would account for the
smaller drop if the skin in each liquid had the same strength, or else
if alcohol is not heavier than water its skin must be weaker than the
skin of water. As a matter of fact alcohol is a lighter liquid than
water, and so still more must the skin of alcohol be weaker than that of
water.

[Illustration: Fig. 12.]

We can easily put this to the test of experiment. In the game that is
called the tug-of-war you know well enough which side is the strongest;
it is the side which pulls the other over the line. Let us then make
alcohol and water play the same game. In order that you may see the
water, it is coloured blue. It is lying as a shallow layer on the bottom
of this white dish. At the present time the skin of the water is pulling
equally in all directions, and so nothing happens; but if I pour a few
drops of alcohol into the middle, then at the line which separates the
alcohol from the water we have alcohol on one side pulling in, while we
have water on the other side pulling out, and you see the result. The
water is victorious; it rushes away in all directions, carrying a
quantity of the alcohol away with it, and leaves the bottom of the dish
dry (Fig. 13).

[Illustration: Fig. 13.]

This difference in t