SURFACE-FILMS. 23 
The condition necessary for the support of solids in this 
way is that, since the water-air surface must necessarily 
meet the solid at the definite angle of contact characteristic 
for that solid and liquid, the slope of the solid at the point 
of insertion must be such that at the same time the water- 
air surface pulls more or less upwards, 7.e., it must dip 
towards the solid. If the solid is a horizontal cylinder or a 
sphere, its surface has every degree of slope, and con- 
sequently there must always be some area on its surface 
where a water-air surface can be inserted at its proper angle 
of contact and at the same time dip towards the solid. If 
the angle be acute, this area will be entirely above the 
equator, and the dip of the water will be slight, but if it 
be obtuse, this area will extend both above and below the 
equator, and the dip of the water may be considerable or 
even vertical. As therefore it is only when the angle of 
contact is obtuse and the solid is not ‘ moistened’ by water 
that the weight-carrying power can be notable, it may be 
taken as certain that this is the case with the animals which 
are supported by the tension of a water surface. Whether 
this is secured by ‘ greasing’ the solids concerned has not 
been ascertained. 
While talking of biological subjects, let me mention 
incidentally that I have examined ‘ cuckoo-spit,’ the protec- 
tive froth with which the ‘ frothing hopper,’ Aphrophoria 
spumaria, surrounds itself, and find that the substance 
which confers ‘ frothability ? on the water is a mucin-like 
protein. } 
Time does not permit even a brief discussion of the 
more complex subjects of prime biological importance con- 
cerned with surfaces, chief of which may be reckoned that 
of the ‘ plasma-haut,’ the pellicle or membrane with which 
every living cell surrounds itself and by means of which it 
is defended from many undesirable losses and intrusions. It 
B 
