22 



to the reaction, S+?; H20 = 8(HjO)„. AGCording to the mass 

 law, ^p^^|/.," = K{p^, where ip^ and <//., represent what is sometimes 

 called the "active mass" of the unhydrated and hydrated solute, 

 respectively, i/'.^ that of the water, xj/^ and i/', are nearly i)ropor- 

 tional respectively to the concentration of S and of S(H20),„ ij/^ to 

 the vapor pressure of water. Since the latter decreases with 

 increasing concentration of the salt, {f/^ must increase faster than 

 tp.j — that is, the degree of hydration must decrease with increas- 

 ing concentration. If instead of one a number of hydrates are 

 formed, this same argument would apply to each reaction. The 

 only case in which hydration could increase with the concen- 

 tration would be the case in which polymerization accompanies 

 hydration. But in the experiments of Jones and Getman there is 

 no evidence of such polymerization, the presence of which would 

 in any case invalidate their calculations. 



Although we can not accept all the conclusions of these authors, 

 nevertheless their experimental data, regarded as evidence in favor 

 of the general idea of hydration, are of great value, and, while 

 probably incapal)le of any such exact quantitative apjilication as 

 the authors have attempted, still serve to give us a very good idea 

 of the prolialjle relative hydration of different salts. 



The method of Jones and Getman is not the only one which 

 has been employed in attacking this problem, and the various 

 methods all give evidence of the existence of hydration, and 

 usually agree also as to which salts are to the highest degree 

 hydrated. All these attempts have been brought together in an 

 excellent monograph by Baur.' 



I wish to approach the subject of hydration from a different 

 point of view, by a study of the colors of certain salts in solution, 

 namely, chloride of coljalt and the chloride and bromide of copper. 

 These sul>stances have different colors when in concentrated 

 and in dilute solution. Cobaltous chloride changes from purple 

 to a clear red, cupric chloride from an olive green to blue, and 

 cupric bromide from a dark l^rown through varying shades of l)rown 

 and green to blue. These changes were in 1887 attril)uted l)y 

 Wiedemann'^ to gradual hydration of the salts. However, this 

 explanation has been succeeded by another, according to which 

 the change of color is due to the electrolytic dissociation of the 



1 Ahrens Samml. Chem. Vortnige. 8, 456 (1903). 

 '^Report Brit. As^oc. for 1887, p. 346. 



