APPLICATIONS OF CRYSTALLOGRAPHY, ETC. 579 



liquid solution; these mixed crystals are termed "solid 

 solutions " by van't HofT, partly because their properties 

 are in the main means of the corresponding properties of 

 the two crystalline salts composing them, in the same way 

 that the properties of a solution of one liquid in another is 

 in the main a certain mean of the corresponding properties 

 of the constituent liquids. If two non-miscible liquids lie 

 in contact with each other, and a third substance A soluble 

 in both these liquids be added to the mixture, equilibrium 

 is after a time set up in the following manner ; the substance 

 A distributes itself between the two non-miscible solvents 

 in such a way that the ratio of the amounts of A dissolved 

 in equal parts of the two solvents (the so-called partition- 

 coefficient) is independent of the actual concentration, pro- 

 vided only that the concentration is not great, that the 

 temperature remains constant, and that the molecular 

 weight of A is the same in each of the two solvents. 

 Should, however, the molecular weight be twice as great 

 in one solvent as in the other, the partition-coefficient is 

 not constant, but becomes constant if a particular term of 

 the ratio is squared before calculating. This method of 

 determining molecular weights is applicable not only to 

 two liquid solvents in contact but also to a liquid solution 

 in contact with a solid solution of the same substance ; thus 

 if a series of aqueous solutions containing the isomorphous 

 salts, potassium chlorate and thallium chlorate in various 

 proportions be caused to crystallise at a fixed temperature, 

 mixed crystals are deposited of such composition that the 

 proportion of potassium chlorate in the mixed crystals 

 divided by the proportion of the same salt in the liquid 

 solution in contact with, and therefore in equilibrium, with 

 them is a constant, and thus independent of the actual pro- 

 portions of the two salts present in either liquid or solid 

 solution, provided that the potassium chlorate is not present 

 in large proportion. It is hence evident that in the crystal- 

 line state potassium chlorate has the same molecular weight 

 as in solution, and that, therefore, the physical molecule is 

 identical with the chemical molecule in the crystalline sub- 

 stance. Several disadvantages embarass the easy application 



