( 203 ) 



or right of this line, dependent on the mutual influence of the 

 vapour tension of the components. 



In order to elucidate this important point I have indicated in the 

 figs. 4, 5 and 6 what shapes the three phase regions hatched in the 

 figs. 1, 2 and 3 can assume in the neighbourhood of the line for 

 the combination, in the case that a slight dissociation takes place in 

 the liquid and gas phase. It is not improbable that there will also 

 be dissociation in the solid phase in this case, but this is not taken 

 into account here, in the first place because it is most likely exceed- 

 ingly slight, and in the second place, because the diagram becomes 

 much more intricate, when this dissociation is taken into account. 



Fig. 4 corresponds to fig. 1, fig. 5 to fig. 2 and fig. 6 to fig. 3. 

 For fig. 4 it may be remarked, that for the case that the vapour 

 tension of the combination lies between the vapour tensions of tlie 

 components, the liquid and the vapour line draw very near to each 

 other somewhat past the line of the combination, on the side of the 

 component with the smaller vapour tension, but that they do not 

 reach each other, so that there remains a gap between them, from which 

 follows, that in the series of liquids and vapours which can coexist 

 with solid AB, not a single point can be pointed out where vapour, 

 liquid, and solid phase have the same concentration, nor is there a 

 point where a vapour and a liquid phase have the same concentration. 



This latter is only the case when the vapour tension of the combination 

 lies between those of the components, for when the vapour tension of 

 the combination is smaller or greater than those of the components, 

 we get according to a rule of Gibbs a three phase region with a 

 minimum, fig. 5, or with a maximum, fig. 6, and at the place of 

 this minimum or maximum the concentration of the vapour and 

 the liquid phase must be identical. 



If we now discuss figs. 4, 5 and 6 at tlie same time, we may 

 remark, that there for a special temperature the situation is indicated 

 of the two three phase pressures ecs and c^e^s^ and the continuous 

 line for solid-fluid or solubility-isotherm. This line must have an 

 horizontal tangent at the point where it cuts the line for the com- 

 bination. For this case is viz. .vf = Xs, or the concentration of the 

 fluid phase is the same as the concentration of the solid phase and 

 then it appears from the equation drawn up by van der Waals for 

 the equilibrium solid fluid: 



dp _ Xs—iCf^ d'g \ 



clvf Vsf X^x^fJPT 

 that 



^ = 0. 



dxf 



14 



Proceedings Royal Acad. Amsterdam. Vol. VIII. 



