WATER-SURFACE PLANTS AND ANIMALS. 221
by surface-tension as, in one way or another, it enters into the
lives of a considerable number of others. Most, if not all, of
the ways in which they are affected, however, are illustrated by
the forms now under review.
In view of the fact that there are many to whom the subject
of surface-tension, or capillarity as it is sometimes called, is
probably only vaguely familiar, it will, I trust, not be out of
place if, before proceeding to an examination of the various
ways in which water-surface plants and animals are affected
thereby, a few remarks are made about this characteristic
property of the surface of water. That the surface of water,
and the surface of all other liquids for that matter, is in a state
of tension can be readily appreciated on theoretical grounds.
The molecules at the surface, owing to their position in contact
with the air, evidently must be free from the attraction of other
water molecules on their upper sides which they would ex-
perience were they in the interior. It follows that, not only are
they being attracted more strongly towards the interior, but
also that the effect of their attractions to one another laterally
must be increased. The net result is, therefore, that the surface
molecules act as if they were part of a stretched elastic membrane
constantly endeavouring to contract. It is to this excessively
thin layer of surface molecules that the term surface-film is
given and it will be seen that the term does not imply a definite
structure, but simply the special physical condition of those
particular molecules which happen at any moment to be at the
surface of the water. Experimentally there are numerous
ways by which the existence of a surface-film in a state of tension
can be shown. A few of these, having a bearing upon the
subject in hand, may be referred to as follows :—
1. Resistance to the passage of solid bodies both from above
and from below. The simplest experiment in this connection,
and one probably known to all, is the very old one of floating
a needle upon water. Although the needle has a specific gravity
so much greater than water, it can, with care, be floated upon
the surface, showing that there must be some resistance to its
free passage. The success of the experiment, it will be found,
depends upon keeping the needle dry on its upper surface, and
this can, of course, be much assisted by rubbing the needle
with some greasy material. This resistance of the surface-film