(457 ) 



from the point of view of the liidden equilibria which continuously 

 connect with each other the lines of the liquids and vapours coexis- 

 ting with the solid phase. 



In the systems which formerly came most to the front, the diffe- 

 rence in volatility between the two components was so large — 

 such as with water and salts — that on the whole three-phase line 

 no vapour occurred which had the same composition as the compound. 



If however, the difference in volatility is less pronounced, a case 

 may occur where the equality in composition between vapour and 

 compound is attained somewhere. Van der Waals foresaw that pos- 

 sibility in 1885, bat not until 1897 did he point out how such a 

 point, occurring on the three-phase line below the point F, indi- 

 cates the maximum temperature at which the compound may still 

 evaporate in its entirety, and how in that point the subliming line 

 of the compound meets the three-phase line. Such a point is indi- 

 cated in fig. 1 by G, the subliming line by GL. 



It was, however, thought very desirable to elucidate the manner 

 in which, in such a case, the equilibria solid-vapour, solid-liquid and 

 liquid-vapour join each other on the three-phase line by a repre- 

 sentation in which is also shown the change of the concentrations 

 'of liquid and vapour along the three-phase line of the compound 

 with increasing temperature. 



Dr. Smits ^) recently gave a representation of this by working out 

 a connected series p , .I'-sections of a spacial figure, which in the case 

 of a binary compound takes the place of my spacial figure, where 

 only the components occur as solid phases. 



A good example may be fouad in Stortenbeker's ^) research on the 

 system chlorine -\- iodine. There it is found that both the compounds 

 JCl and JCI3 yield at their melting point a vapour containing more 

 CI, but at a lower temperature ihey have a point on their three- 

 phase line where the vapour becomes the same as the compound. 

 Stortenbeker had noticed this fact during his research, but h^d not 

 followed the matter up. After I had completed in 1896 my p, t, x- 

 figure for binary mixtures, I also projected the spacial representation 

 for this case, and I had then already come to the view, by graphical 

 methods, that the point G is the highest temperature at which a 

 compound can exist near vapour of equal composition. 



Bancroft ^), in consequence of van der Waals' publication, tried 

 to elucidate the case of JCl by a representation of partial pressures, 



1) These Proc. June 1905. 



2) Rec. Trav. Ghim. 7. 183. 1888. 



3) Journ. Phys. Chemistry 3. 72. 1899. 



