( 231 ) 



(made for instance of plaster) of the vapour surface (which if we 



choose — and if we nealect the deviations from tiie law of Boyle 



is always the same). 



Instead of executing this process, which directly expresses the 

 theory of van der Waals, on the plastercast, it may also be done 

 entirely by a drawing on the plane as indicated in Comm. 59a ^ 6. 

 The given construction now becomes niuch simpler if only we 

 neglect the thickness of the ridge. In fig. 2 is drawn the rim 

 curve of the liquid plate and the different sections v = const. 

 Then we combine the points where the sections have the same 



— (or s comp. Comm. 59^; § 8) as certain points of the rim curve 



to the substitution curves (comp. Comm. 59a § 5) which therefore 



always belong to the point with the given — - (or s) value on the 



rim curve (with mixtures taken as perfectly gaseous the substitution 



curves on the x\p projection are straight lines at right angles to the 



.r-axis), connect the point on the rimcurve with different points ot 



the substitution curve in the vapour zone, rotate the section with a 



plane at right angles with the ^TV-plane passing through the connecting 



line with the latter on the if??;-plane. The point where the section 



touches the connecting line gives the phase wanted, which is in 



equilibrium with the first. 



The construction becomes still simpler if w^e neglect the distance 



from the rim curve to the plane ?; = 0. Then w^e need only 



B 

 transfer in fig. 2 the sections v = const, by A -\ -f- .... lower, 



V 



and draw a common tangent to the rim curves and each of these 

 transferred lines in order to find x^ap ^nd x^^. 



If finally we neglect the deviations from the law of Boyle, w^e 

 have only to move down unvaried the entire sections v = const, of 

 the vapour phase in fig. 2 in order to obtain the just mentioned 

 system of curves, to which with the rim curves common tangents 

 must be drawn in order to find x^ap and xu^. 



§ 8. Application of the empirical law of reduced vapour tension 

 of pure substances to the phenomena of coexistence in mixtures. In 

 ^ 7 we drew the attention to the circumstance that the liquid branch 

 of the binodal line is obtained sufficiently accurately by substitui ing the 



rim curve, the points with I - 1 == 0, for the points with 



