DISCUSSION OF EVIDENCE. 191 



results were predicted from this theory before a single experiment 

 was carried out. Solvation, then, being accepted, as now seems pretty 

 generally the case, the question arises, how does the present solvate 

 theory of solution differ from the older hydrate theory of Mendeleeff, 

 which has long since been abandoned as untenable? 



Mendeleeff's theory was that certain hydroscopic substances, such 

 as calcium chloride, sulphuric acid, and the like, formed a few definite 

 hydrates when in the presence of water. Thus, sulphuric acid formed 

 the hydrates H 2 S0 4 .2H 2 0, H 2 S0 4 .6H 2 0, H 2 S0 4 .100H 2 0. 



This view of Mendeleeff was proposed as the result especially of 

 measuring the specific gravities of aqueous solutions of such com- 

 pounds at different dilutions. When the specific gravities were plotted 

 against the concentrations, the curve was not a continuous one, but 

 showed a number of breaks. These breaks Mendeleeff could account 

 for by assuming that certain definite hydrates or compounds between 

 water and the dissolved substances existed at these concentrations. 

 This was among the most important evidence breught to light bearing 

 on the so-called hydrate theory of Mendeleeff. 



This suggestion of Mendeleeff, based upon such inadequate evidence, 

 should not be called a theory. It is scarcely worthy of the name 

 hypothesis. Before a suggestion becomes a theory there should be a 

 fair amount of evidence supporting it, and showing not only that the sug- 

 gestion accounts for the facts, but that it is the only suggestion which 

 will account for them. This was lacking in the so-called Mendeleeff 

 hydrate theory. 



The present solvate theory of solution may claim to have a fairly 

 good experimental support, as the above review of the evidence ob- 

 tained in this laboratory will show. In aqueous solutions hydration 

 is a general phenomenon. Some substances combine with very little 

 water, but most salts combine with very large amounts of water, the 

 amount of combined water for any given substance being a function 

 of the concentration of the solution and of the temperature. The 

 more dilute the solution the larger the amount of the solvent com- 

 bined with the dissolved substance the more complex the hydrate. 

 The lower the temperature the more complex the solvate. These 

 solvates are very unstable; indeed, so unstable that it seems better to 

 call them systems than definite chemical compounds. Anything so 

 easily broken down by rise in temperature could hardly be called a 

 chemical compound. Here, again, the present solvate theory differs 

 from the older hydrate theory. 



While there is some spectroscopic evidence pointing to the existence 

 in solution of a certain definite hydrate, or certain definite hydrates, 

 we have obtained a large amount of evidence which seems to indicate 

 the existence in aqueous solutions of a large number of hydrates, or 

 indeed of a whole series of hydrates, the composition depending pri- 

 marily on the concentration of the solution. While this is not essential 



