224 THE ESSEX NATURALIST.
temperature and can be very much decreased by other liquids and
more or less soluble solids such as oil, ox-gall, alcohol, camphor,
&c., and also by emanations from such volatile substances as
ether. When a local reduction of surface-tension takes place, the
rest of the surface-film appears to contract, carrying with it any
floating particles, and in this way movements can be set up which
may be very rapid and also definite, in direction, if the cause of
the reduction of tension be applied continuously from one point
in the moving body.1
We are now in a position to consider some of the ways in
which water-surface plants and animals are affected by surface-
tension and the various methods which they have adopted for
utilising it. The chief of these may be dealt with under the
heads of support, keeping dry, and distribution, but there is
also a number of other less frequently used methods to which
reference will be made later.
As regards the support which the surface-film is capable of
affording, it would seem that none of the plants have used this to
their advantage. All the floating plants owe their position at the
surface to their own buoyancy, even the surface forms which are
always submerged being slightly lighter than water, at least during
their periods of active growth. It is just possible that some of
the Alga; (Euglena, Chromulina, &c.), when forming what appears
to be a dry scum on the surface, and the Bacteria when in the
zoogloea condition, may be exceptions to the general rule, but it
is not certain at present that this is the case.
On the other hand the number of animals exploiting the force
of surface-tension for support is very considerable and they do
this in a variety of ways. Of the forms which live on the surface,
three types may be distinguished, namely, those which walk and
run on the surface, those which not only walk, but also leap about
on the surface and those which swim at the surface. To the first
type belong the water-measurers and pond-skaters (Hydrometra,
Gerris, &c). These insects hold themselves well above the water
and only touch the surface with the extremities of their legs. Their
whole weight is therefore borne by the surface-film and it is very
1 Various experiments in illustration of the foregoing peculiarities of surface-tension were made,
including the floating of needles, wire-gauze, wax models weighted so as to be slightly heavier
than water and match-sticks (similarly weighted) tipped with vaseline ; Menshrugghe's float ;
water between glass plates ; lozenges of paper with upturned ends arranging themselves in
star-like patterns ; paper boat with piece of camphor at stern showing continuous movement
in definite direction due to local reduction of surface-tension, etc.