T along the indifferent line

Consider again a system of f phases, c components in which r chemical reactions can occur, but we shall not limit ourselves, as in the previous paragraph, to a monovariant system. 

We may now write down a set of equations like (29.62) in which (f) + r terms appear on the left hand side, and the rest on the right. To simplify the notation we write ip — (j)r - I  

Along the line of indifference, equation (29.75) thus gives 

Comparison of (29.77) with (29.63) shows immediately the close similarity between the indifferent line of a polyvariant system, and the equilibrium line of a monovariant system. This similarity is perhaps less surprising when we remember that all equilibrium states of a monovariant system are indifferent states. Thus (29.63) can be regarded as a particular case of (29.77), for in this case, since w = l, 

Since in a system of this kind the indifferent line is the line of uniform composition, w =wl, and 

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