II PREFACE. 



The conclusion from all of this work is that, as far at least as aqueous 

 solutions are concerned, a part of the water is combined with the dissolved 

 substance, the amount being a function primarily of the nature of the 

 substance, but for any given compound is a function of the concentration 

 and temperature. 



If a portion of the solvent is combined with the dissolved substance, it 

 is a matter of interest and importance to know what portion. This is 

 not a simple problem, but an approximate solution of it is possible. By 

 determining the freezing-point, the conductivity, and the specific gravity of 

 the solution, we are enabled to calculate approximately the total amount 

 of water held in combination by the dissolved substance, and, therefore, 

 the approximate amount combined with one molecule of the compound 

 or the ions resulting from it. These results are necessarily only approxi- 

 mate, since in this calculation certain assumptions have to be made that 

 are not rigidly true; notably, the validity of Raoult's law for concentrated 

 solutions. 



Notwithstanding these assumptions it seems probable that the composi- 

 tion of the hydrates as calculated is of the right order of magnitude, and 

 this is all that can be claimed at present. These results are of some impor- 

 tance as showing at least the relative hydrating power of the different types 

 of compounds. 



The fact that while the total amount of water held in combination increases 

 with the concentration, the amount combined with one molecule, or the 

 resulting ions, increases with the dilution, would argue that the ions have 

 the greater hydrating power. In the most dilute solutions there are very 

 few molecules present, nearly all of them having been broken down into 

 ions. In such solutions we have the greatest hydration, consequently, the 

 ions have greater hydrating power than the molecules. That molecules, 

 however, can combine with water is shown by the fact that some non- 

 electrolytes, such as glycerol and cane-sugar, show marked hydration. 



The present theory of hydrates differs fundamentally from the older 

 theory proposed by Mendeleeff, in that according to the latter certain com- 

 pounds, such as calcium chloride, sulphuric acid, and the like, form a few 

 definite compounds with the water in which they are dissolved. According 

 to the present theory, combination between the dissolved substance and 

 water is a general phenomenon ; and a given compound, say calcium chloride, 

 forms a complete series of hydrates ranging in composition from a few 

 molecules of water to at least thirty every intermediate stage being repre- 

 sented. It is thus obvious that the two theories are vitally different. 



The question still remains as to whether the property of combination 

 between the dissolved substance and solvent is perfectly general, or is limited 



