s section.

The idea of surface-tension introduced by Segner had a most important
effect on the subsequent development of the theory. We may regard it as
a physical fact established by experiment in the same way as the laws of
the elasticity of solid bodies. We may investigate the forces which act
between finite portions of a liquid in the same way as we investigate
the forces which act between finite portions of a solid. The experiments
on solids lead to certain laws of elasticity expressed in terms of
coefficients, the values of which can be determined only by experiments
on each particular substance. Various attempts have also been made to
deduce these laws from particular hypotheses as to the action between
the molecules of the elastic substance. We may therefore regard the
theory of elasticity as consisting of two parts. The first part
establishes the laws of the elasticity of a finite portion of the solid
subjected to a homogeneous strain, and deduces from these laws the
equations of the equilibrium and motion of a body subjected to any
forces and displacements. The second part endeavours to deduce the facts
of the elasticity of a finite portion of the substance from hypotheses
as to the motion of its constituent molecules and the forces acting
between them. In like manner we may by experiment ascertain the general
fact that the surface of a liquid is in a state of tension similar to
that of a membrane stretched equally in all directions, and prove that
this tension depends only on the nature and temperature of the liquid
and not on its form, and from this as a secondary physical principle we
may deduce all the phenomena of capillary action. This is one step of
the investigation. The next step is to deduce this surface-tension from
a hypothesis as to the molecular constitution of the liquid and of the
bodies that surround it. The scientific importance of this step is to be
measured by the degree of insight which it affords or promises into the
molecular constitution of real bodies by the suggestion of experiments
by which we may discriminate between rival molecular theories.

In 1756 J.G. Leidenfrost (_De aquae communis nonnullis qualitatibus
tractatus_, Duisburg) showed that a soap-bubble tends to contract, so
that if the tube with which it was blown is left open the bubble will
diminish in size and will expel through the tube the air which it
contains. He attributed this force, however, not to any general proper