134 THEORY OF CONCENTRATED SOLUTIONS 



combination between the two liquids. There seems to be no 

 direct foundation for this assumption, since, as far as I know, 

 no analogous gaseous system has been completely studied. 

 There is, however, a fair amount of evidence that the conclusion 

 is correct. It is found to be almost invariably the case that those 

 liquid mixtures which give a maximum in the property-concen- 

 tration curve are formed with evolution of heat and contraction 

 in volume. Moreover, it is often, but by no means always, the 

 case that the maximum occurs at a definite molecular com- 

 position of the mixture, i.e., at a mixture representing simple 

 molecular proportions of the components. Naturally, then, 

 there has been a tendency to take this composition as the formula 

 of the compound in solution. So often does this method of 

 deducing the formulae of hypothetical compounds in solution 

 appear in chemical and physical literature that it cannot be too 

 clearly pointed out that the method is purely arbitrary, and rests 

 on no scientific foundation. Again, the composition of the 

 mixture corresponding to the maximum in the curve very often 

 varies with temperature. This had to be explained by another 

 assumption, viz., that the compound undergoes dissociation to 

 a greater or less extent depending on the temperature. Owing 

 to the number of assumptions which have to be made in order 

 to arrive at the formula of a compound in solution on these 

 lines, most chemists have abandoned the method as too unsatis- 

 factory for use. The recognition of the weakness of this method 

 I as tended to throw very considerable doubt on the possibility 

 of arriving at the formula? of compounds in solution, or even 

 of deducing their presence by a study of the physical properties 

 of mixtures. Washburn, in a recent review, sums up the 

 question as follows: — 



" We are therefore forced to conclude, in spite of the extensive litera- 

 ture on this phase of the subject, that methods of studying hydration (i.e.. 

 combination in aqueous solution) which depend upon the deviations of 

 any physical property of a solution from the law of mixtures are incapable 

 of yielding any conclusive information regarding" the complexity or even 

 the existence of hydrates in aqueous solution." 



In spite, however, of this seemingly logical conclusion, it is 

 found that on reviewing the cases where a maximum occurs in 

 the property-composition curve, the number of mixtures, which 

 exhibit the maximum at or very near a composition representing 

 a simple molecular proportion of the two components is too great 

 to be fortuitous. There appear to be some possibilities in the 

 method as a means of deducing combination in solution, but 

 before these can be realised it is essential to remove some of the 

 arbitrary character of the method, and place it in a nearer- 

 relationship to the established principles of chemistry. This 

 seems to me to be quite possible. 



If chemical combination takes place when two liquids are 

 mixed, then the mixture consists of at least three kinds of mole- 

 cules, viz., pure A, pure B, and pure compound, which we will 



