126 SCIENCE PROGRESS. 



with the calculated depression, and van't Hoff from theo- 

 retical considerations showed how the divergence could be 

 estimated from a knowledge of the composition of the solid 

 which actually separated out from the solution on cooling. 

 That the abnormal values for the points of solidification 

 depend on the separation of the dissolved substance along 

 with the solvent has now been experimentally verified in a 

 considerable number of cases. Heycock and Neville found 

 that for the case of solutions of antimony in molten tin, the 

 freezing-point of the tin was raised instead of lowered by 

 the presence of the second metal. Kiister has shown that 

 this and similar instances are susceptible of a very simple 

 explanation. The two metals separate out together in very 

 nearly the same proportion as that in which they remain 

 behind in the liquid, so that the solution solidifies as a 

 whole. In such circumstances the point of solidification of 

 the liquid can be calculated by the simple mixing formula. 

 If the melting-point of each pure substance is multiplied by 

 the proportion in which it exists in the mixture, the sum of 

 the two numbers thus obtained will give the point of solidi- 

 fication of the solution. As antimony melts 200 degrees 

 higher than tin, the admixture of the former in however 

 small proportion will, since the mixture freezes as a whole, 

 raise the point of solidification instead of lowering it, as 

 would be the case if pure solid tin separated from the liquid 

 on cooling. 



Not only do solutions exhibit a lower freezing-point than 

 that of the pure solvent, but they also exhibit a lower 

 vapour tension. The pressure of aqueous vapour over 

 salts containing water of crystallisation may in many cases 

 be measured with accuracy, and there it is found that the 

 isomorphous admixture of another salt lowers the vapour 

 pressure of water which is in equilibrium with the solid. 

 Thus the vapour tension of a mixed crystal of ordinary alum 

 with iron alum is less than the vapour tension of either of 

 its components. In this respect then the mixed crystal 

 behaves as a solid solution. Again, the solubility of a sub- 

 stance is diminished when it itself acts as a solvent for 

 another substance insoluble in the original solvent. Of the 



