REPRESENTATION BY ZETA FUNCTION 317 



(i) Point (H) is situated below {W){X) (as in Fig. 14). It is 

 clear that only the points of (W){H) and of {H){X) represent 

 stable states, so that both the systems (22) are stable whilst 

 (21) is metastable. From this it follows that the solid sub- 

 stances W and X cannot exist next to each other in stable 

 equilibrium, and that the reaction 



solid W + sohd X -^ solid H (24) 



will tend to occur. 



(ii) Point (H) is situated above {W)iX). It is clear that now 

 only the points of (W){X) represent stable states; in other 

 words, system (21) is stable, whilst both the systems (22) are 

 metastable. Thus the compound H is now metastable and 

 tends to separate into its components according to the reaction 



solid W + solid X ^ solid H. . (25) 



(iii) Point {H) is situated on the line {W){X). We have now 

 again the special case that three points of a line represent stable 

 phases (compare also {W)e'{X) in Fig. 8 and z'u's in Fig. 11). 

 It is clear that all the systems (21), (22) and (23) are now stable 

 and that the reaction 



solid W + solid X :f± solid H (26) 



can occur. The direction of the reaction on addition of heat 

 will depend on whether the compound is endothermic or 

 exothermic. It depends on the temperature and the pressure 

 which of the three cases mentioned above will occur. In the 

 considerations that follow we shall suppose that (H) always lies 

 below (W){X). 



18. In Fig. 13 the point H' of the f-curve is the f-point of a 

 liquid which has the same composition as the solid compound, 

 i.e., H' is the f-point of liquid H. Denoting the melting-point 

 of H (under the pressure p) by T(H), then T < T{H). If we 

 draw the two tangents z'{H) and u'{H) we see that they repre- 

 sent more stable systems than the points on the part z'H'u' of 

 the f-curve. From this follows: liquids between z and u (Fig. 

 13) are supersaturated; those between z and H separate into 



