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entropy, if the masses of fluid were different. It is evident, therefore, that it is equilibrium with respect to generic phases, and not with respect to specific, with which we have to do in the evaluation of entropy, and therefore, that we must use the average of $\mathrm {H}$ or of $\eta _{\rm {gen}}$, and not that of $\eta$, as the equivalent of entropy, except in the thermodynamics of bodies in which the number of molecules of the various kinds is constant.