Page:Text-book of Electrochemistry.djvu/78

 The experiments of Boberts- Austen (14) and of G. Meyer (lo) on the diffusion of raetala in mercury confirmed the above results.

Solid Solutions. — Van't Hpfif's formula for the calcula- tion of the molecular weight can only be applied provided that the solvent separates in the pure form when the solution freezes, or that when the boiling point method is used the dissolved substance does not volatilise. These conditions are not always fulfilled, and this is particularly the case when the dissolved substance chemically resembles the solvent. Thus when the dissolved substance is /3-naphthol and the solvent is naphthalene, or the dissolved substance is antimony and the solvent tin, it is found that the two separate out in union, and abnormally low depressions of the freezing point are obtained (^6'). A similar behaviour is sometimes noticed, e.g. iodine in benzene, when the substances are in no way chemically related. As a general rule the concentrations of the dissolved substance in the liquid and in the solid (separated) solvent bear a constant ratio to each other (distribution ratio).

In these cases, and in many others, a solid substance acts as a solvent. Van't HoflF (17) has shown that substances dissolved by solids have an osmotic pressure, and so we may speak of " solid solutions." The chief result with respect to the determination of the molecular weight in solids is, as the recent investigations of Bruni (18) and others have proved, that apparently the solid state is in no way connected with a high degree of polymerisation.

Hydrogen, which possesses many metallic properties, is monatomic when dissolved in palladium ; as Hoitsema found, the dissolved molecule has the formula H and not H2.

Experimental Results on the Rise of Boiling Point.

— The correctness of the theoretical formula dT = 002-^

may be gathered from the results obtained by Beckmann •{19) contained in the following table in the column headed dT (observed) : —

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