( 576 ) 



supposed case may be realized without excessive experimental 

 difficulties. This may succeed with NHy — HCl. A system for which 

 fig. 3 holds, presents also this particularity, that we have here a 

 P, T, X-surface of two sheets with a minimum curve bounded on the 

 upper side by a continuous plaitpoint cur\e, which, in consequence 

 of the great difference between the critical temperatures of the 

 compound and the components might possibly have the shape 

 described here. 



Prof. Van uek W.\als was so kind as to draw my attention to 

 the particularities of the P, T, X-surface of two sheets, which may 

 be derived directly from tliose of a surface witii a maximum curve, 

 by simply reversing everything. The minimum curve, i. e. the locus 

 of all points for which the concentration of liquid and vapour are 

 the same, forms here the lower boundary of the projection of the 

 P, T, X-surface of two sheets on the P, T-plane. This curve is repre- 

 sented in tig. 3 by the dotted line LL', which touches the plaitpoint 

 curve at L, and the continuous three phase line at N. This point 

 -iV, lying between tlie minimum M in the three phase line and the 

 maximum sublimation point F\ as I have shown in a paper forwarded 

 to the Zeitschr. f. phys. Chem. towards the end of September, is 

 a point wliere tiie concentration of tlie vapour is equal to that of 

 the liquid, and is tlierefore at the same line a point of the minimum 

 curve, which becomes metastable on the left of N. The peculiar 

 feature in the P, T, X-surface of. two sheets drawn here manifests 

 itself, wlien the bounding curves are traced for different concentrations. 



It appears then, that if we come from the side of B, the con- 

 centration of the point L is the first, at which the bounding curve 

 presents some particularity. At this concentration we get, viz., two 

 bounding curxes, whicli starting from (^ and Ö, terminate at L in 

 a so-called cusp, as is here once more separately represented. 



With a concentration somewhat richer iu A we get now two 

 bounding curves which pass continuously info each other. The con- 

 tinuous transition lakes place where the bounding curve touches the 

 plaitpoint curve. Further this continuous bounding curve shows this 

 particularity llial llie two luauches loucli each other near the critical 



