etted, I think you will see, without any explanation from me, that
they should be pressed together, and this is made evident by experiment.
Two other beads which have been dipped in paraffin wax so that they are
neither of them wetted by water float up to one another again when
separated as though they attracted each other just as the clean glass
beads did.

[Illustration: Fig. 11.]

If you again consider these two cases, you will see that a plate that is
wetted tends to move towards the higher level of the liquid, whereas one
that is not wetted tends to move towards the lower level, that is if the
level of the liquid on the two sides is made different by capillary
action. Now suppose one plate wetted and the other not wetted, then, as
the diagram imperfectly shows, the level of the liquid between the
plates _where it meets_ the non-wetted plate is higher than that
outside, while where it meets the wetted plate it is lower than that
outside; so each plate tends to go away from the other, as you can see
now that I have one paraffined and one clean ball floating in the same
water. They appear to repel one another.

You may also notice that the surface of the liquid near a wetted plate
is curved, with the hollow of the curve upwards, while near a non-wetted
plate the reverse is the case. That this curvature of the surface is of
the first importance I can show you by a very simple experiment, which
you can repeat at home as easily as the last that I have shown. I have a
clean glass bead floating in water in a clean glass vessel, which is
not quite full. The bead always goes to the side of the vessel. It is
impossible to make it remain in the middle, it always gets to one side
or the other directly. I shall now gradually add water until the level
of the water is rather higher than that of the edge of the vessel. The
surface is then rounded near the vessel, while it is hollow near the
bead, and now the bead sails away towards the centre, and can by no
possibility be made to stop near either side. With a paraffined bead the
reverse is the case, as you would expect. Instead of a paraffined bead
you may use a common needle, which you will find will float on water in
a tumbler, if placed upon it very gently. If the tumbler is not quite
full the needle will always go away from the edge, but if rather
over-filled it will work up to one side, and then possibly roll over the
edge; any bubbles, on the other hand, which were adhering