THERMODYNAMICAL SYSTEM OF GIBBS 145 



Substances which exhibit the phenomenon of peptisation, i.e., 

 when a large mass of a substance spontaneously breaks up into 

 small particles, may be examples of such behavior. How- 

 ever in such a case large masses of the substance in the given 

 medium would be inherently unstable and there would be no 

 advantage in substituting (197) for (195). 



It is evident that (197) cannot be regarded as a necessary 

 condition of equilibrium, for (195) may be satisfied and the 

 system will therefore be in a state of equilibrium even when 

 (197) is unsatisfied. Cases of this kind are met with in super- 

 heated liquids, supersaturated solutions, etc. In the case of a 

 supersaturated solution of a given substance (197) is negative, 

 but we must suppose that on account of capillary forces etc. the 

 separation of an infinitely small quantity would give rise to 

 positive (or zero) value in (195). It is however difficult to 

 distinguish between effects of this kind and "passive resist- 

 ances" to change. Gibbs remarks that "such an equilibrium 

 will, however, be practically unstable. By this is meant that, 

 although, strictly speaking, an infinitely small disturbance or 

 change may not be sufficient to destroy the equilibrium, yet a 

 very small change in the initial state, perhaps a circumstance 

 which entirely escapes our powers of perception, will be sufficient 

 to do so. The presence of a small portion of the substance for 

 which the condition [53] does not hold true, is sufficient to 

 produce this result, when this substance forms a variable com- 

 ponent of the original homogeneous masses. In other cases, 

 when, if the new substances are formed at all, different kinds 

 must be formed simultaneously, the initial presence of the 

 different kinds, and that in immediate proximity, may be 

 necessary." 



25. Generalized Statement of the Conditions of Equilibrium. 

 The conditions of equilibrium of the parts initially present, and 

 with respect to the formation of new parts, may be summed up as 

 follows. Since for any homogeneous mass, by (48), the equation 



€ — trj -\- pv — Himi — /LI2W2 ... — MnW„ = 0, (198) 



holds when mi, m^, . . .mn refer to the ultimate components of the 

 mass, the condition of equilibrium between the original parts 



