102 



In the point of intersection /), of the vapour line a^c^ with the 

 line FF' the correlated liquid has tlie composition D. As of this 

 four-phase equilibrium 7'' -|- i»^' -|- liquid />-{- vapour Z^i the three 

 phases F,F' and Z), are situated on a straight line, a reaction occurs 

 in which the liquid takes no part. According to the situation of 

 the three points F, F' and D^, in regard to each other this reaction 

 is : F-^ F' ^ (r (fig. 1) or F':^F ^ G (fig. 2) or F-^ F -\- G 

 (fig. 3). 



The poiiit /)j is, therefore a point of the congruent or incongruent 

 sublimation curve of the complex F -\- F' . As, however, this vapour 

 Z)i may also be in equilibrium with a liquid D, the point D^ is 

 the maximum sublimation point of the complex F -\- F' . 



Hence, we find : the point of intersection S of the line FF' with 

 the liquidum curve of the four-phase equilibrium F -\- F' -\- L -\- G 

 represents the minimum melting point (congruent or incongruent) ; 

 the point of intersection D^ of this line with the vapour curve of 

 this four-phase equilibrium represents the maximum sublimation point 

 (congruent or incongruentj of the complex F -\- F' . 



Let us now ascertain which reaction can occur in the four-phase 

 equilibrium F -\- F' -\- L -\- G if Uie liquid is represented bj a point 

 in the proximity of -S. We will call that side of the line i^i^^', where 

 the vapour S^ conjugated with the liquid S is found, the vapour 

 side, the other side oï FF' the liquidum side. We distinguish three cases. 



1. In the point S the i-eaction F -\- F' "^ L takes place. We 

 imagine in fig. 1 or 2, on the part of the liquidum curve situated 

 at the vapour side of FF' , a liquid L in the vicinity of the point 

 S ; the correlated vapour (r will then be represented by a point of 

 the vapour curve situated in the vicinity of >SV As the point Z now 

 lies within the triangle FF'G the reaction F -\- F' -\- G :^ L will 

 occur. If we take a liquid L situated in the vicinity of S at the 

 liquidum side of FF', the lines FF' and LG then intersect each 

 other in a point between F and F' and also situated between L 

 and G so that the reaction F -\- F' "^ L -\- G takes place. 



It is evident that this goes on independently of the situation of 

 the point D,. 



2. In the point S the reaction F' "^ F -\- L occurs. If we take 

 in fig. 3 a liquid L in the vicinity of S and at the vapour side of 

 the line FF' , and a vapour G in the vicinity of >S'i, the lines FL 

 and F' G will intersect each other so that the reaction F' ■■{- G ^ F -\- L 

 occurs. If we take a liquid L in the vicinity of -S at the liquidum 

 side of the line FF' , F' then lies within the triangle i^XG^ and so the 

 reaction F' "^ F -}- L -j- G takes place. 



