( 167 ) 



All this becomes perfect Ij clear when we consider the spacial 

 representation, fig. 4, to the right of the plane v x v t v 3 v A . 

 Sa is the triplepoint of the component A 



cSa is the vaponr-pressnre-line of solid A 



SaKa „ „ „ ,, liquid A terminating in the 



critical point Ka ■ 



aSs is the vapour-pressnre-line of solid B and 



SbKb „ ,y „ „ liquid B terminating in the 



critical point Kb • 



SaS'a is the melting-point-line of A and S'bS'b the melting-point- 

 line of B. 



Sa^Sb/Sa is the three-phase-region discussed before, and the 

 ö,£-section drawn between the triple-points Sa and Sb shows that 

 Sa<J Sb is the vapour line, SaISb the liquid line, and Sa/Sb the 

 mixed-crystal line. 



On the two-phase-region Sa(/SbISa lie the vapour and liquid 

 phases which are in equilibrium with the mixed-crystals, and on 

 the two-phase-region SaI Sb/Sa are found the liquid and the mixed- 

 crystal phases which can coexist with the vapour. 



The two-sheet surface for mixed-crystal and liquid rests on this 



latter two-phase-region, which surface will in general be very steep. 



It has been assumed in the spacial figure that as is actually the 



dp 

 case as a rule, — of the melting-point lines of the components is 

 dt 



at first positive, which causes the two-sheet surface mentioned to 

 run to higher temperatures with increasing pressure, which is here 

 represented in an exaggerated manner. 



A consequence of this situation is this that, as has been indicated 

 in the section, at a temperature lying between the triple-point tem- 

 peratures of A and B, the region for liquid and mixed crystal ceases 

 to exist above a certain pressure, r, so that the three regions of 

 two-phase equilibria, mixed-crystal-vapour, liquid-vapour, and mixed- 

 cry stal-liquid are limited on all sides. 



dp 

 It is evident that when — is negative for both melting-point lines, 



point r will not lie on the melting-point line of the component A, 

 but on that of B. 



In the case that the two components pass into the fluid-crystalline 

 state before melting, the spacial figure of such a system is represented 

 by the whole of figure 4 for the simplest case. 



