AS is well known, the progress in the physical chemistry of solutions which has been made during the last thirty years, though extensive and detailed in a certain sense,. has nevertheless suffered not a little from the fact that fully 90 per cent, of the investigations have been restricted to the study of the behaviour of substances dissolved in water. At the present time, therefore, whilst a very large amount of data has been accumulated upon the subject of aqueous solutions, our knowledge of the behaviour of non-aqueous solutions and solutions formed in mixed solvents is deplorably scanty. Of course, here and there the subject has been attacked, especially within the last decade, and a few general conclusions have been laboriously attained. Many of the rules, however, which serve as a trustworthy guide in the case of aqueous solutions have to be considerably modified or even discarded altogether when we come to non-aqueous solutions. At the same time, it is clear that the problem of solution in general cannot be regarded as in a satisfactory state, so long as generalisations applicable to a large number of solvents at least are wanting.