WATER-SURFACE PLANTS AND ANIMALS. 223
two plates of glass not too widely separated, it is found that
they will, if free to do so, approach and then adhere to one
another and that the thinner the layer of water between them
the stronger becomes the adhesion.
4. Support of a weight against gravity. This, of course, is
shown in the first experiment of floating a needle, but there
are various facts in connection with this matter that require
special notice. If the floating needle be looked at carefully,
it will be seen that along its sides the water curves down from
the general level so that the needle is really lying in a depression.
Now, whenever such depressions are produced in the surface
by whatever means, it can be shown that they exert an upward
pull upon the body against which they are formed, just as if the
curved surface-film were in fact an elastic membrane trying to
get back to its normal position. Such capillary depressions are
formed whenever the water-surface is in contact with water-
repellent, i.e. non-wettable, bodies such as those consisting of
or coated with oily and waxy substances, and the strength of
the upward pull is approximately three grains per linear inch
of contact.
5. Attraction and repulsion of capillary curves. One
peculiarity of the downward capillary curves forming the de-
pressions in the surface just referred to, is that they attract one
another. This is equally true of upward capillary curves, but
the two kinds of curves repel one another. It follows, there-
fore, that whenever there are on the surface of water any
small bodies not very far apart, against whose sides capillary
curves have been formed, the little objects will, if the curves are
either all upward or all downward, move towards one another
and huddle together in a compact mass ; but if both kinds of
curves are present on each, repulsion will take place between
the dissimilar curves and only those points will be in contact
where similar curves exist. In this case the little bodies will
space themselves out into various patterns, depending upon the
way in which the capillary curves come into play.
6. Movements due to local alteration of surface-tension. The
force of surface-tension varies very much in different liquids,
but it can also be altered in the same liquid in many ways. Water
for example has a very high surface tension, compared with all
other liquids, except mercury, but it varies slightly with the