14G Notices respecting New Boohs. 



chemically equivalent quantities of different substances will pro- 

 duce the same effect in solution. As we proceed to stronger 

 solutions the molecules get closer together, and the theory launches 

 out with the bold assertion that Van der Waals's modification of the 

 law of gases is applicable to the molecules of a dissolved substance 

 also; and the deviations of the properties of a solution from abso- 

 lute proportionality to the amount of dissolved substance present 

 are calculated on this assumption, the agreement being in some 

 cases remarkably good. 



The theory supposes that the molecules of the dissolved substance 

 remain intact when they enter into solution. In the case of solu- 

 tions of salts the laws indicated above are not obeyed, and the 

 explanation given is that the molecules of the salt split up into 

 their constituent atoms, a view which is supported by many phe- 

 nomena of electrolysis. Hence the solution contains molecules of 

 salt and free atoms or " ions " which have been produced by the 

 dissociation of the salt. No quantitative measurements are given, 

 because we have no certain knowledge as to the relative amounts 

 of dissociated and undissociated salt. Recently Arrhenius has 

 worked in this direction with some success. 



In this treatise the case of salt-solutions is only treated in a 

 somewhat superficial manner ; indeed it is at this point that the 

 subject becomes a most difficult one. One hardly justifiable 

 assumption is that the solvent remains inert, although the disso- 

 ciation of the salt is effected by it. Again, there are chemical 

 objections to having free atoms in a liquid, though Prof. Ostwald 

 suggests very reasonably that a free atom and a molecule of an 

 element are not alike, and do not necessarily have similar pro- 

 perties. An attempt is made to account for many physical 

 properties of solutions by supposing them due to the presence of 

 the ions ; for example, colour and refractive indices. It is argued, 

 for instance, that all salts containing a cobalt ion are red, and that 

 therefore the cobalt ion has a red colour ; similarly, the copper ion 

 has a blue colour. This law will, however, not admit of uni- 

 versal application ; an exception may be seen in the case of ferric 

 sulphocyanide, which is of an extremely deep red colour in aqueous 

 solution, although neither the ferric ion nor the sulphocyanogen 

 one has this colour. The only explanation possible is that the salt 

 is deep red when in the molecular state. In that case a very dilute 

 solution should be orange-coloured, owing to the mixture of the 

 red molecular salt and the yellow ferric ion, of which the latter 

 would be present in the greater quantity. 



As a concise account of the new theory of solution Prof. 

 Ostwald's work is most valuable ; but it is somewhat to be regretted 

 that he did not give some indication of the older theories, of which 

 not a word is said. Indeed, a student taking up the book and 

 having no previous knowledge of the subject would be led to sup- 

 pose that the theory here put forward is the universally accepted 

 one, whereas it is really regarded by the majority of chemists as 

 quite untenable. 



