(12) 



— ^ = O and Vsf = O, when connecting the corresponding points 



we obtain loci of these points, indicated by lines. 



As, however, we simplify the discussion, as van der Waals has 



a> ^ . 



shown, when we consider the quantity - — . V^f instead of the 



• dv/ 



quantity Vs/, because this product can never become infinitely great 



and is yet zero when F,/ = 0, the locus of the points where 



d'lp ^^ ^ . . • /. ^ 

 — - . Vsf=0 IS given in fig. 1. 

 dv/ 



We know then too that this quantity on the left of the line ot 



the compound is negative outside this locus, and positive within it. 



ö> 

 Further the locus of - — =: is indicated, and we see that these 



two lines intersect at the point where they pass through the line of 

 the compound. 



In his lectures van der Waals has lately proved in the following 



way that this must necessarily be so : If we write for -— . Vg/ 



we see that when this quantity = 0, and when at the same time 



Xs = ay : 



or 



^ = 0. 



dv/ 



I, too, had already arrived at the conclusion that in the left half of 

 our diagram the two loci mentioned had interchanged places, by 

 assuming that there existed a three-phase equilibrium also on the 

 right, and by drawing the corresponding isobar M^Q^BJiJi^'B^'Q.'N'. 

 It appears then that here the points R^ and R^' lie within the points 

 Z)i and i)/, which points to a reversed situation (compared with 



the left half) of the loci --^ . Vsf = and —-==0. Van dek Waals 



dl'/ • dv/ 



has also given this graphical proof. 



As for the sign of the quantity r — on the right of the line 



Ov/ 



