364 The New Theories of Solution. 



and could be separated by diffusion or otherwise, which is 

 contrary to all experience. The assumption of a merely 

 potential dissociation thus fails to account for the known 

 facts, whereas an actual dissociation at once affords the neces- 

 sary explanation. Many interesting examples of the appli- 

 cation of the dissociation theory to explain the electrical 

 behaviour of salt-solutions are to be found in a paper by 

 Ostwald, in the Zeitschr. physikal. Chem. ii. p. 270. 



With reference to the existence of hydrates in solution, 

 Mr. Pickering (Phil. Mag. xxxii. p. 91) brings against Mr. 

 S. Lupton the charge of entirely ignoring the cumulative 

 character of the evidence brought forward in favour of the 

 hydrate hypothesis. Now this cumulative character of the 

 evidence is precisely on what the theories of osmotic pressure 

 and of electrolytic dissociation chiefly rely, and what Mr. 

 Pickering altogether neglects. In the short paper by Arrhenius 

 already referred to (Phil. Mag. xxviii. p. 36) attention is drawn 

 to the great number of subjects dealt with by these theories 

 jointly : in almost all cases a quantitative comparison of the 

 results of experiment with deductions from the theory has been 

 possible, with the most satisfactory results. 



The only rival hypothesis which has been actively pursued 

 is the hydrate theory, according to which the dissolved sub- 

 stance in an aqueous solution is combined with all the water 

 (except in very dilute solutions) to form one or more hydrates. 

 This is of course a perfectly legitimate hypothesis so far as it 

 goes ; but besides suffering at present from a slight arbi- 

 trariness in its employment where the interpretation of expe- 

 rimental data is in question, it also labours under the dis- 

 advantage of being unable to afford any secure basis for 

 calculation. Mr. Pickering, it is true, has calculated the 

 freezing-points of sulphuric-acid solutions by its aid, and on 

 account of the apparent great elasticity of his method has 

 obtained results in perfect accord with his experiments (Brit. 

 Assoc. Report, 1890, p. 321 ; < Nature/ xlii. p. 631). When 

 one looks, however, a little closely into the details of the cal- 

 culation, the concordance appears of questionable value. He 

 attributes the lowering produced by the dissolved substance 

 to three distinct causes, which he classifies as (1) mechanical, 

 (2) physical, and (3) chemical. The actual depression is 

 made equal to the sum of three components depending on the 

 above causes. First of all, we have the mechanical lowering 

 based on Person's so-called absolute zero for liquids, i. e. the 

 temperature below which a liquid will not solidify (Proc. 

 Chem. Soc. 1889-90, p. 150). Mr. Pickering himself has 

 subsequently proved Person's views to be utterly untenable 



