222
THE ESSEX NATURALIST.
to the passage of bodies from above is more easily demonstrated
by a piece of dry wire gauze, which can almost be thrown on
to the surface without breaking through. By means of the little
contrivance known as Mensbrugghe's float, the resistance to the
passage of bodies from below can also be proved. The float
consists of a light ring attached by wires to a cork weighted so
that the cork will just float with the ring in a horizontal position
well out of the water. Although the whole apparatus is there-
fore somewhat lighter than water, it will be found that if
allowed to rise from a completely submerged position, the ring
will not break through the surface-film.
2. Capillary phenomena in fine tubes and narrow spaces.
When glass tubes of small but various diameters are held upright
in water, it is found that the water in them docs not remain at
the same level as outside, but rises above that level and to the
greatest height in the tube with the smallest bore. The surface
of the water in the tubes is also seen not to be flat but concave,
i.e. curving upwards all round as if trying to ascend to a still
higher level where in contact with the glass. If the tubes are
placed in mercury, the opposite effect is produced, the mercury
being depressed in the tubes below the general level and to the
greatest extent in the tube with the finest bore. The surface of
the mercury is moreover convex, i.e. curving downwards instead
of upwards as in the case of water. These opposite effects
depend upon whether the material of which the tubes are com-
posed is "wettable" or not by the liquid. If the glass tubes
were coated inside with a thin film of, say, paraffin wax, which
is not wettable by water, then the water would not rise, but
would be depressed as in the case of mercury, and its surface
would be convex. These effects are usually known as capillary
phenomena, being typically seen in fine hair-like or capillary
tubes, but exactly the same effects are produced in any narrow
spaces such as those between closely set rods, imbricated plates,
etc. It follows from the foregoing that, if any object is covered
with closely-set water-repellent hairs, scales, etc., water, so far
from being sucked into the fine spaces between them by capillary
action, is actually prevented from entering, except, of course,
to the extent due to hydrostatic pressure, which depends upon
the depth to which the object is sunk below the surface.
3. Adhesion of plates. If water is allowed to enter between