SOLID SOLUTIONS. 125 



but the results achieved have on the whole been small, owing 

 chiefly to the experimental difficulties encountered. 



An important application of the idea of solid solutions 

 was made by van't Hoff in explaining the abnormalities 

 that are sometimes met with in the determination of mole- 

 cular weights by the lowering of the freezing-point in solu- 

 tions. It had been proved theoretically that the freezing- 

 point of a given solvent should be depressed to a certain 

 value (calculable from the freezing-point and the latent 

 heat of fusion of the solvent) when the solution was of 

 normal concentration, i.e., contained one gram-molecule 

 of dissolved substance per litre. The nature of the dis- 

 solved substance should be without influence on this value. 

 Now, whilst it was ascertained experimentally that this 

 theoretical relation was in the vast majority of cases ac- 

 curately fulfilled, yet there remained certain combinations 

 of dissolved substance and solvent which gave values of the 

 depression constant altogether at variance with the cal- 

 culated value. Thus, metacresol dissolved in phenol gave 

 a depression of 48 instead of 74, and thiophene dissolved in 

 benzene a depression of 34 instead of 53. Van't Hoff's 

 explanation of these and similar abnormally low values of 

 the depression was that the freezing-point observed was not 

 in the strict sense the freezing-point which had been assumed 

 in the theoretical reasoning. The true freezing-point of a 

 solution is the temperature at which the liquid is in equi- 

 librium with the solid solvent. The freezing-point of an 

 aqueous salt solution, for example, is the temperature at 

 which it can exist in contact with pure ice without the ice 

 melting or without fresh ice being deposited from the solu- 

 tion. Now, in the exceptional cases above alluded to it is 

 known that the solid and the solvent have a tendency to 

 crystallise together, i.e., to form mixed crystals, so that the 

 substance that separates out is not the pure solvent but 

 rather a solid solution. The temperature at which such a 

 solid solution would be in equilibrium with the liquid solu- 

 tion might not by any means be the freezing-point of the 

 solution as above defined. The apparent observed freezing- 

 point of the solution, therefore, would not in general coincide 



