72 MOLECULAR WEIGHT AND POLYMERISM 



naphthol and naphthalene 1 , chlorobromo- and iodoform 2 ,&c., 

 excludes at once the assumption that only one body, that 

 present in great excess, the so-called solvent, is volatile. 

 The condition of equilibrium at the freezing point is then 

 no longer merely equality in the partial pressures of the 

 solvent in the solid and liquid conditions ; only if the same 

 equality of pressure holds for the dissolved substance do 

 the conditions suffice for equilibrium. The question arises 

 whether the fixed composition thus arrived at for the 

 isomorphous mixture separating from a solution of given 

 composition agrees with the facts, or, in other words, 

 whether the cause that produces mixed crystallization does 

 not involve a factor that influences the composition of the 

 mixture. This is probable, on the one hand, because iso- 

 morphous mixtures of all compositions cannot always be 

 obtained (Part I, p. 55); on the other hand this may perhaps 

 correspond to a simultaneous formation of two layers in 

 the liquid. The whole thing awaits further investigation. 

 Another point that may be added in considering the 

 qualitative relations is that the diminution in vapour 

 pressure which a solid suffers when a non-volatile body is 

 taken up by it, involves a diminution of solubility, so that 

 e. g. an isomorphous mixture should separate out when 

 saturated solutions of alum and iron-alum are mixed. 



B. Quantitative Results. 



As regards the quantitative side of the problem the 

 experimental data are not yet very complete or conclusive. 

 The important common result that workers in this field 

 have so far arrived at is that the solid state is not distin- 

 guished by a more complex molecular structure, but that in 

 solids, too, the dissolved molecule has as a rule the smallest 

 size compatible with chemical facts, and at most double 

 that size, as was found to be the case in liquid solutions. 

 This result would be particularly important for isomorphous 



1 Kiister, Zeitschr.f. Pkys. Chem. 13. 452, 17. 355. 



2 Brunni, Atti R. Accad. del Linceij Roma, 7. 166. 



