452 J. W. Gibbs—Hquilibrium of Heterogeneous Substances. 
equilibrium (2). The condition of equilibrium which relates 
to the dissolving of the solid at a surface where it meets a fluid 
may be expressed by the equation 
Ps See AL be Et (22) 
nt 
Ky 
The fundamental equations which have been described above 
of strain, or by an equation between ¢ [see (3)] as determined 
for a given quantity of the solid, the temperature, and the 
quantities which express the state of strain. 
Capillarity.—The solution of the problems which precede 
may be regarded as a first approximation, in which the peculiar 
state of thermodynamic equilibrium about the surfaces of dis- 
continuity is neglected. To take account of the condition of 
things at these surfaces, the following method is used. Let us 
suppose that two homogeneous fluid masses are separated by 8 
surface of discontinuity, i. e., by a very thin non-homogeneous 
film. Now we may imagine a state of things in which each of 
the homogeneous masses extends without variation of the densi- 
ties of its several components, or of the densities of energy and 
entropy, quite up to a geometrical surface (to be called the divid- 
