601 



Because we have in this system a temperature between T^ and Ty, 

 between the three piiases the reaction : F"^ liquid n -\- vapour n^ 

 takes place from left to riLiht with increase of vohune. We compare 

 now the ternary equilibrium F -\- liquid h -\- vapour h^, wherein 

 b and J>^ differ but little from n and n^, with Ihe previous system. 

 We then see that as well the foi-mation of vapour from F -\- 

 vapour b as also the formation of liquid from F -\- vapour /^j takes 

 place with increase of volume. According to the rule deduced in 

 communication VI, the sides solid-liquid and solid-vapour of the 

 threephasetriangle must then approach one another on increase of 

 P and separate from one another on decrease of P. We see that 

 the movement of Fbb^ in fig. 8 is in accordance with this rule. 



The occurrence of a point of maximum- or minimumpressure can 

 be easily examined by the reader. 



Tf<C 1- W® "t^^v take a temperature T a little higher than the 

 minimummeltingpoint Tf of the substance F. We then must distinguish 

 two cases, according as the substance expands or contracts on 

 melting. We only take the first case. In a similar way as is deduced 

 for the general case fig. 12 (I), we now find a diagram as fig. 4, 

 wherein the two curves are exphased. Further, it is assumed again 

 that on these curves neither a point of maximum nor a point of 



minimumpressure occurs. From the deduction 

 of the diagram, it follows that Fa^^ of the 

 threephasetriangle Faa^ is situated always 

 between Fa and Fli^. When the points a 

 and a^ are situated in the vicinity of?zand 

 n^, Faa^ turns, as is clear from the figure, 

 in such a way on decrease of pressure, that 

 the line Fa goes ahead. When a and a^ 

 are situated however, in the vicinity of h 

 and Aj, Faa^ turns on decrease of pressure 

 in such a way that Fa^ goes ahead. This 

 turning is in accordance with the rules, 

 deduced in communication VI. Let us firstly 

 contemplate the equilibrium F -{- L„ -j- Gn^ 

 of the binary system BC. Herein the reaction 

 L„'^F-\- G„^ takes place from left to right 



with decrease of volume. Let us now take the system /^^-f- ^^ + ^^'ai 

 of which the points a and a^ are situated in the immediate vicinity 

 of n and ^i. At the formation of vapour from F -{- La the volume 

 will decrease, at the formation of liquid from F -\- (xax the volume 

 will increase. 



Fie. 4. 



