174 



BUTLER 



ART. D 



system. Suppose that at a pressure p and a temperature t', 

 the f-x curve for homogeneous phases has the form AB (Fig. 9), 

 with a double tangent PQ. Homogeneous phases between P 



Fig. 9 



and Q are unstable with respect to discontinuous changes. 

 Between R and S, the ^-curve is convex upwards, i.e., 



{d^^/dx%, t < 0, 



and these phases are unstable with respect to continuous 

 changes. Between P and R, and between Q and S the f-curve 

 is still concave upwards, i.e., 



and these phases, though unstable with regard to discontinuous 

 changes are stable with regard to continuous changes. The 

 points R and S, for which 



d'^/dx' = 0, 



