THE HYDRATE THEORY OF SOLUTIONS 141 



The half-hydrate S0 3 iHX), in spite of its great stability, does 

 not reveal itself by any abrupt change in the density curve, but 

 produces a remarkable break in the capillarity curve for 22 C, 

 when the hydrate is studied as an over-cooled liquid ; the 

 viscosity curve for the over-cooled liquid also reveals the 

 presence of the half-hydrate by rising to a maximum of 

 altogether exceptional sharpness. 



The less stable dihydrate, on the other hand, produces no 

 marked inflection of the density or capillarity curves, and (apart 

 from the conductivity) is only revealed to the eye by an ill- 

 defined maximum in the viscosity curve at 87 per cent. H 2 S0 4 — 

 theory 84*48 per cent. 



In the case of the more dilute solutions no evidence of the 

 formation of hydrates can be obtained by direct observation of 

 the experimental curves ; the only hope of detecting their 

 presence in the solution consists in recording the slight changes 

 of curvature which may be revealed by Mendeleef s method of 

 differentiation or by the use of a bent lath. The difficulties 

 associated with the application of these methods have already 

 been referred to ; but it will be appropriate at this point to state 

 specifically that, even after differentiation, most of the curves 

 appear to be reasonably smooth, and that an observer who was 

 not on the alert to detect discontinuities would probably be 

 content to represent even the derived experimental points by 

 continuous rather than by broken curves. Mendeleef s anticipa- 

 tion that the examination of a single curve, such as the density 

 curve for sulphuric acid, would reveal all the hydrates that 

 might be formed in the solution has therefore proved to be 

 unfounded in practice as well as in theory. 



Later workers have been driven to base their conclusions in 

 reference to the existence of hydrate in solution on " cumulative 

 evidence derived from independent sources " rather than on the 

 results obtained from the study of any one property of the 

 solution. 1 This involves a vast amount of work, and the only 

 case that has been adequately studied is that of sulphuric acid, 



1 " From the study of any one, or any few, particular breaks I concluded — 

 nothing; from a study of a whole series of density results I only concluded that 

 it was advisable to make other series at other temperatures ; from the study of the 

 series at four different temperatures I concluded only that I had ' strong pre- 

 sumptive evidence ' of the existence of changes, but that confirmatory evidence 



