HETEROGENEOUS EQUILIBRIUM 287 



equation (6) [129], which, as will be shown later, is equivalent to 



dp _ AH 

 dt ~ AF ' 



in which AH is the entropy change, AF the volume change of the 

 reaction in question. Whenever we have a reaction in which 

 the vapor does not take part, e.g., the reaction 



S' + S'" = S" + L, 



the slope of the p-f curve is always very great, because of the 

 small value of AF. In other words, the p-t curves of all con- 

 densed systems are almost vertical, and go from the invariant 

 point to regions of higher pressure, and in almost all cases, 

 higher temperature. W^ien the reaction is one between solid 

 phases and vapor, e.g., the dissociation-pressure curve of S', 



S' = S" + V, 



the 'p-t curve always goes from the invariant point to regions of 

 lower temperature and pressure; since AF is large (except under 

 high pressure), the slope of the curve is comparatively small. 

 In reactions of the type 



S' + L = S" + V, 



in which both liquid and vapor take part, d-p/dt may be large 

 or small, positive or negative. We will consider this case in 

 detail later. 



Consider now the application of the above principles to the 

 determination of the sequence of -p-t curves around an invariant 

 point.* The method used is based on the fact that the order 

 of the slopes dp/dt of the various curves at the invariant point 

 is determined by the masses of the phases which take part in 



* The question of the sequence of p-t curves around an invariant 

 point has been discussed by A. Smits (Proc. Acad. Sci. Amsterdayn, 

 18, 800-804 (1916)), and by F. A. H. Schreinemakers in the series of 

 papers beginning with Proc. Acad. Sci. Amsterdam, 18, 116-26 (1916), 

 and by G. W. Morey and E. D. Williamson, /. Am. Chem. Soc, 40, 59 

 (1918). 



