390 SCIENCE PROGRESS 



Jones l and his co-workers find by this method that, 

 whilst the great majority of organic substances are not 

 hydrated in solution, glycerine, cane sugar, and fructose are 

 exceptions. 



Biltz, 2 and also Jones, 3 have attempted to determine the 

 " hydration " of electrolytes — the number of molecules of the 

 solvent associated with a molecule of the solute — but the results 

 are very uncertain, and must be regarded as of a preliminary 

 nature. The values for potassium chloride, according to Biltz, 

 vary from 25 to 19 with o - i to 0*5 normal solutions, whilst Jones 

 finds for 3'2, C32, and 0*08 normal solutions of lithium chloride, 

 the values 10, 18*5, and 53 respectively. Biltz finds that the 

 number of associated water molecules increases with dilution, 

 whilst Jones was at first of opinion that the hydration decreases 

 on progressive dilution, but in his most recent paper has 

 adopted the contrary view, as the numbers just quoted indicate. 

 Bousfield, 4 from entirely different considerations, also arrives 

 at the conclusion that the hydration increases progressively 

 and continuously with increasing dilution. 



The application of the law of mass action to this problem 

 is a matter of considerable importance ; it was first discussed 

 by Nernst. 5 If one molecule of a salt S forms a hydrate with 

 11 molecules of water, according to the equation S -f «H 2 = 

 S(H 2 0)„, we obtain, on applying the law of mass action, 

 C s C" H ,o = KCs(h.,o)„ , where C s and C H ,o denote the con- 

 centration of salt and water respectively and Cs(H 2 o)„ denotes 

 the hydrate concentration. In dilute solution Chl<o is much 

 greater than the other two ; it is therefore practically constant 

 when the others are varied, and thus the amount of water 

 in combination with the salt is nearly proportional to the salt 

 concentration ; in other words, the hydration is practically 

 independent of the dilution. It can also be shown by thermo- 

 dynamical considerations that the active mass of the water is 

 proportional to its vapour pressure ; and as, in dilute solution, 

 the latter is only slightly affected by further dilution, a rapid 

 increase of hydration on dilution, as postulated by Jones and 



1 Loc. cit. 



2 Loc. cit. 

 Zcit. physikal. Chem. 1906, 55, 385. 



* Proc. Roy. Soc. 1906, 77 A, 377 ; Zcit. physikal. Chem. 1905, 53, 257. 

 5 TJieoretische C/iemie, 4th ed., p. 457. 



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