296 



SCHREINEMAKERS 



ART. H 



of the phase are first degree homogeneous functions of the 

 mass variables. This proviso means that we assume we can 

 neglect the surface effects which enter into the consideration 

 of micro-heterogeneous systems. For given t and p, the f of a 

 phase will depend, therefore, only on its composition. In the 

 case of a binary system this composition is defined by the 

 value of X (the composition parameter). 



Fig. 1 



In Fig. 1, in which WX = 1, the point a represents a phase 

 containing Wa{= x) mols X and aX(= 1 — a;) mols W. If we 

 now draw the ordinate aa' equal to the ^ of this phase, we shall 

 call the point a' the f-point of the phase a. If we give all 

 compositions, from pure W to pure X, to the phase a, then the 

 point a runs along the line WX, whilst the point a' traverses a 

 curve W'a'X', which, at constant t and p is called the f-curve 

 (free energy curve). Clearly W is the f-point of the pure 

 substance W and X' the f-point of the pure substance X. It 

 can be shown that the f-curve touches the lines WW and XX' 

 at the points W' and X' respectively (for proof see note at the 

 end of this article). 



When all points of WX represent liquids, then W'a'X' is the 

 f -curve of these hquids, whilst W' and X' are the ^-points of the 

 pure liquids and a' that of liquid a. When the points of WX 

 represent vapors (gases), then W'a'X' is the f-curve of these 

 vapors, whilst W' and X' are the respective f-points of the pure 



