REPRESENTATION BY ZETA FUNCTION 321 



At a temperature a little higher than T{b) the f-curve inter- 

 sects the line {H){X) (Fig. 18). We may now draw the 

 lines h'{H) and x'(X) which touch the f-curve in the points h' 

 and x' (not shown). Hence point h' is the f-point of a liquid h, 

 saturated with respect to H and x' that of a liquid saturated 

 with respect to X. Thus at this temperature the systems 



L(h) + solid H, L{x) + solid X, (30) 



exist. It appears from the position of these points of contact 

 in Fig. 18 that h'{H) and (H){X) are situated above x'{X). 

 Therefore the first one of the systems (30) is metastable, the 

 second one stable. From this it follows that at T > T(h) the 

 saturation curve of H is metastable, that of X stable. 



Fig. 18 



If we take T < T(h), the f-curve lies above (H)(X) (Fig. 18). 

 If we now also imagine the tangents h'(H) and x'(X) drawn, 

 then we see that h'(H) and {H){X) now lie below x'{X). From 

 this follows: at 7^ < T(b) the saturation curve of H is 

 stable, but that of X metastable; also solid H + solid X (Field 

 II) is a stable system. We can now make the following deduc- 

 tions from Fig. 16 as regards Fig. 17. Two saturation curves, 

 namely those of H and X, must go through point h of Fig. 17. 

 Towards higher temperatures that of H is metastable and that 

 of X stable, whilst towards lower temperatures the reverse holds 

 good. 



In Fig. 15, at r = T(b), reaction (28) occurs, so that T{b) is 

 the common melting point or the eutectic temperature of H 

 and X. In Fig. 17, at r = T{h) reaction (29) occurs. Then 

 T(b) is, as appears also from Fig. 17, the highest temperature at 

 which solid // can exist, or the temperature at which solid H 

 decomposes with formation of a liquid and separation of solid X. 



