391 



d no longer takes place; 



~\- vapour (/i, one of tlie reactions c 

 instead one of the reactions 



e) F^F':^L^G; ƒ) Ft:F'^L^(r, 

 takes place. 



xls follows from the position of the points g and g^ in fig. 3, the 

 reaction e takes place. Hence it follows that the pi-essure decreases 

 from g along the two curves. 



IV. The temperature is equal to the minimum meltingpoint^ Ts . 

 The two vapour points are situated in conjunction with respect 



to the line FF' ; one of the liquid points of intersection is situated 

 on the line FF' . We shall call this ligure "fig. 3a". When a con- 

 gruent melting takes place, g is situated between the other two 

 points ; when an incongruent melting takes place, F or F' is situated 

 there. 



We have seen before that the minimum melting point Ts of the 

 complex F -\- F' can be lower than that of one of the two components 

 F or F' taken separately. Imagining in fig. 3^ the saturation curve 

 of F to be exphased, then 7^ is lower than the minimum melting 

 point of F; when both the saturation curves in fig. 3a are exphased, 

 then Ts is lower than the minimum melting point of either of the 

 two compounds F and F' . As the exphased saturation curves are 

 generally not very far removed from F and F' , this last mentioned 

 case can only occur when the points F and F' are situated close 

 to each other, that is if they differ only a little in composition. 



V. The temperature is higher than the minimum melting point, 

 Ts , but lower than the point of maximum temperature JV- 



The four points of intersection are all situated on the same side 

 ^--'"""•«^ of the line FF' . In fig. 4 one of the many 



possible diagrams is drawn. As long as 

 the point g is still situated in the vicinity 

 of the line FF' , reaction a cannot take 

 place in the four-phase equilibrium F-\-F'-\- 

 liqnid g -{• vapour g^, but instead one 

 of the reactions /; — rf takes place. As follows 

 from the position of the points g and (/, 

 in fig. 4, the reaction that takes place 

 is d, therefore : L-^ F -^ F' -\- G. 



Vapour is formed as the reaction 

 proceeds from left ^ to right. Although in such a case the volume 

 generally increases, in this particular instance the volume, however, 

 decreases. We have namely seen before that this formation of vapour 

 in points between N and S takes place with decrease of volume. 



