350 TRANSACTIONS OP SECTION B. 



solution is practically nil, the vapour pressure of each constituent is reduced 

 merely as if by dilution with the other constituent, and so on. That there is 

 some action between the two components even in this extreme case must be 

 admitted, but it may be referred entirely to action of a physical kind, such as 

 one finds on mixing one gas with another at considerable pressures. Action of 

 a chemical nature is absent. If it be said that even saturated hydrocarbons 

 have some chemical affinity for each other, recourse may still be had for examples 

 to mixtures of two inactive elements, say liquid argon and liquid krypton, where 

 chemical affinity is non-existent. 



At the other extreme we have such solutions as those of sulphuric acid and 

 water. Here there is every physical evidence of chemical union. The volume 

 of the mixture is by no means the sum of the volumes of the components ; the 

 amount of heat evolved on mixing is very great ; the separate liquids, which are 

 practically non-conductors, yield on mixing a solution which is a good conductor; 

 and so on. There is obviously here a great influence of the solvent water on the 

 solute sulphuric acid, and this influence we can only account for by assuming 

 that it is essentially chemical in character. 



As the influence in such a case is necessarily reciprocal, then if even one of 

 the constituents of the solution is inactive chemically there can plainly be no 

 action of a chemical nature on mixing. Thus, no matter what solvent we take, 

 it can exercise no action other than that of a physical kind on argon, say, which 

 das been dissolved in it; and, again, if liquid argon is chosen as solvent no 

 substance dissolved in it can be affected by it chemically, and we thus obtain only 

 the properties of a physical mixture. It is convenient therefore to classify liquid 

 solvents according to their chemical activity. The saturated hydrocarbons, 

 which are chemically very inert, and, as their name paraffin implies, little dis- 

 posed to chemical action of any kind, may be taken as typically inactive solvents, 

 analogous to liquid argon. Water, on the other hand, as its numerous com- 

 pounds (hydrates) with all kinds of substances testify, may be taken as a 

 typically active solvent. The ordinary organic solvents exhibit intermediate 

 degrees of activity. 



For the purpose of illustrating the effect of solvents on a dissolved substance 

 one may conveniently take a coloured substance in a series of colourless solvents. 

 If the substance is unaffected by the solvent, we might reasonably expect the 

 colour of the solution to be the same as the colour of the vapour of the substance 

 at equal concentration. Iodine, for instance, gives rise to the familiar violet 

 vapour. Its solution in carbon disulphide has a colour practically similar, but 

 its solution in alcohol or water is of a brown tint quite different from the 

 other. In the indifferent hydrocarbons and in chloroform the colour is like that 

 in carbon disulphide, in methyl or ethyl alcohol it is brown. We conclude 

 therefore roughly that iodine dissolved in saturated hydrocarbons, in chloroform, 

 carbon tetrachloride and carbon disulphide is little affected by the solvent, 

 whereas in water and the alcohols it is greatly affected, probably by way of 

 combination, since in all the solvents two atoms of iodine seem to be associated 

 in the molecule. That combination between the iodine and the active solvents 

 has really occurred receives confirmation from the behaviour of iodine in dilute 

 solution in glacial acetic acid. If the colour of this solution is observed in the 

 cold it is seen to be brown, resembling in colour the aqueous solution. If the 

 solution be now heated to the boiling-point, the colour changes to pink, which 

 may be taken to indicate that the compound of iodine and acetic acid which 

 is stable at the ordinary temperature becomes to a large extent dissociated at 100°. 



Now, as I have said, a general theory of solution must be applicable to all 

 classes of solution, and herein lies the importance of van't Hoff's osmotic 

 pressure theory. It applies equally to mixtures of gases, to mixtures of inert 

 liquids, and to mixtures such as those of sulphuric acid and water ; and it has 

 the further advantage that so long as the solutions considered are dilute there are 

 simple relations connecting the osmotic pressure with other easily measurable 

 properties of the solutions. It has been unfortunately the custom to oppose the 

 osmotic pressure theory of solution to the hydrate, or more generally the solvate, 

 theory, in which combination between solute and solvent is assumed. The solvate 

 theory is, in the first place, not a general theory, and in the second place it is 

 perfectly compatible with the osmotic pressure theory. It is in fact with regard 



