GENERAL RELATIONS. 143 



of water held in combination by the dissolved substance is undoubtedly 

 subject to the law of mass action. 



NATURE OF THE COMPOUNDS FORMED. 



The question arises whether these hydrates are true chemical compounds, 

 as that term is ordinarily used, or whether they represent some less stable 

 form of combination. That they are unstable is shown by the ease with 

 which they are broken down by heat. Most of the water can be driven off, 

 from the above solutions, at a temperature only a little above 100. The 

 more complex hydrates are, then, decomposed in solution at a comparatively 

 low temperature and the water given off in the form of vapor. In the light 

 of these facts the hydrates can scarcely be regarded as true chemical com- 

 pounds. If, however, we insist on calling them chemical compounds, we 

 must admit that they represent a very low order of stability. 



DO IONS OR MOLECULES FORM HYDRATES? 



We are of the opinion that both the molecules and ions combine with 

 water, forming hydrates. It seems that the molecules are certainly capable 

 of forming hydrates, because, in very concentrated solutions where the mole- 

 cules are present in large quantities, we often have considerable hydration; 

 and, further^ Jones and Getman* have shown that certain non-electrolytes, 

 such as glycerol, cane-sugar, fructose, etc., which are undissociated, still have 

 the power of combining with water in solution, forming hydrates of even 

 a high order of complexity. 



That ions are capable of combining with water in solution is shown by the 

 magnitude of the hydration in many of the dilute solutions, where chiefly 

 ions and only a few molecules are present. 



Attention should be called to the fact, that in the preceding tables the 

 number of molecules of water in combination with one molecule of the dis- 

 solved substance is calculated for a molecule of the dissolved substance. 

 If this molecule is dissociated into 2, 3, or 4 ions, the number given in column 

 H in the tables would have to be divided by 2, 3, or 4, in order to ascertain 

 how many molecules of water are in combination with an ion resulting from 

 the dissociation of the molecule in question. 



THE OLD AND THE NEW HYDRATE THEORY. 



The theory of hydrates in aqueous solution, which we believe is estab- 

 lished by this work, is to be sharply distinguished from the old hydrate 

 theory of Mendeleeff, which, having long since been shown to be untenable, 

 has been abandoned. According to the older theory, when a substance like 

 calcium chloride is dissolved in water, there are formed certain definite 

 chemical compounds, having perfectly definite compositions and containing 



*Amer. Chem. Journ., 32, 308 (1904). 



