GREATER PROBLEMS OF BIOLOGY—THOMPSON, 387 
Now, the state, including the shape or form, of a portion of matter 
-is the resultant of a number of forces which represent or symbolize 
the manifestations of various kinds of energy; and it is obvious, 
accordingly, that a great part of physical science must be under- 
stood or taken for granted as the necessary preliminary to the dis- 
cussion on which we are engaged. 
I am not going to attempt to deal with or even to enumerate 
all the physical forces or the properties of matter with which the 
pursuit of this subject would oblige us to deal—with gravity, pres- 
sure, cohesion, friction, viscosity, elasticity, diffusion, and all the 
rest of the physical factors that have a bearing on our problem. I 
propose only to take one or two illustrations from the subject. of 
surface tension, which subject has already so largely engaged the 
attention of the physiologists. Nor will I even attempt to sketch 
the general nature of the phenomenon, but will only state a few of 
its physical manifestations or laws. Of these the most. essential 
facts for us are as follows: Surface tension is manifested only in 
fluid or semifluid bodies, and only at the surface of these, though 
we may have to interpret surface in a liberal sense in cases where 
the interior of the mass is other than homogeneous. Secondly, a 
fluid may, according to the nature of the substance with which it is 
in contact, or, more strictly speaking, according to the distribution 
of energy in the system to which it belongs, tend either to spread 
itself out in a film or, conversely, to contract into a drop, striving 
in the latter case to reduce its surface to a minimal area. Thirdly, 
when three substances are in contact and subject to surface tension, 
as when water surrounds a drop of protoplasm in contact with a 
solid, then at any and every point of contact certain definite angles 
of equilibrium are set up and maintained between the three bodies, 
which angles are proportionate to the magnitudes of the surface ten- 
sions existing between the three. Fourthly, a fluid film can only 
remain in equilibrium when its curvature is everywhere constant. 
Fifthly, the only surfaces of revolution which meet this condition 
are six in number, of which the plane, the sphere, the cylinder, and 
the so-called unduloid and catenoid are important for us. Sixthly, 
the cylinder can not remain in free equilibrium if prolonged beyond 
a length equal to its own circumference, but, passing through the 
unduloid, tends to break up into spheres, though this limitation may 
be counteracted or relaxed, for instance, by viscosity. Finally, we 
have the curious fact that in a complex system of films, such as a 
homogeneous froth of bubbles, three partition walls and no more 
always meet at a crest, at equal angles, as, for instance, in the very 
simple case of a layer of uniform hexagonal cells; and (in a solid 
system) the crests, which may be straight or curved, always meet, 
also at equal angles, four by four, in a common point. From these 
