— a 
Boned is called the surface of tension. 
J. W. Gibbs— Equilibrium of Heterogeneous Substances. 458 
6&=t dnf6+o b3s+ 4, dmi+ u, dm}+ ete. (23) 
in which s denotes the area of the surface considered, / the tem- 
perature, 4;, f, etc. the potentials for the various components 
in the adjacent masses. It may be, however, that some of the 
components are found only at the surface of discontinuity, in 
which case the letter # with the suffix relating to such a sub- 
Stance denotes, as the equation shows, the rate of increase of 
energy at the surface per unit of the substance added, when the 
entropy, the area of the surface, and the quantities of the other 
components are unchanged. The quantity o we may regard as 
defined by the equation itself, or by the following, which is 
obtained by integration : 
= t B+ 8+ py mi+ fl, m+ ete. (24) 
i o(a+e)=p'—p", (85) 
where p’, p’” denote the pressures in the two masses, and Cy Co 
the principal curvatures of the surface. Since this equation 
1a8 the same form as if a tension equal to @ resided at the sur- 
face, the quantity @ is called (as is usual) the superficial tension, 
d the dividing surface in the particular position above men- 
