h to allow of its being filled and carried, and
then it may be dropped from a height, but you cannot throw it. In the
same way the weaker skin of a liquid will not make a large quantity take
the shape of a ball, but it will mould a minute drop so perfectly that
you cannot tell by looking at it that it is not perfectly round every
way. This is most easily seen with quicksilver. A large quantity rolls
about like a flat cake, but the very small drops obtained by throwing
some violently on the table and so breaking it up appear perfectly
round. You can see the same difference in the beads of gold now upon the
screen (Fig. 16). They are now solid, but they were melted and then
allowed to cool without being disturbed. Though the large bead is
flattened by its weight, the small one appears perfectly round. Finally,
you may see the same thing with water if you dust a little lycopodium on
the table. Then water falling will roll itself up into perfect little
balls. You may even see the same thing on a dusty day if you water the
road with a water-pot.

[Illustration: Fig. 15.]

[Illustration: Fig. 16.]

If it were not for the weight of liquids, that is the force with which
they are pulled down towards the earth, large drops would be as
perfectly round as small ones. This was first beautifully shown by
Plateau, the blind experimentalist, who placed one liquid inside another
which is equally heavy, and with which it does not mix. Alcohol is
lighter than oil, while water is heavier, but a suitable mixture of
alcohol and water is just as heavy as oil, and so oil does not either
tend to rise or to fall when immersed in such a mixture. I have in front
of the lantern a glass box containing alcohol and water, and by means of
a tube I shall slowly allow oil to flow in. You see that as I remove the
tube it becomes a perfect ball as large as a walnut. There are now two
or three of these balls of oil all perfectly round. I want you to notice
that when I hit them on one side the large balls recover their shape
slowly, while the small ones become round again much more quickly. There
is a very beautiful effect which can be produced with this apparatus,
and though it is not necessary to refer to it, it is well worth while
now that the apparatus is set up to show it to you. In the middle of the
box there is an axle with a disc upon it to which I can make the oil
adhere. Now if I slowly turn the wire and disc the oil will turn also.
As I gradual