Electrolytic Dissociation versus Hydration. 31 



Group A. — (1) Osmotic pressure. (2) Lowering of 

 freezing-point. (3) Lowering of vapour-pressure. (4) Raising 

 of boiling-point. (5) Electromotive force of concentration- 

 currents in solutions. 



Group B. — (6) Electric conductivity of electrolytes. 



Group C. — (7) Diffusion of electrolytic solutions. 



Group D. — (8) Change of the degree of dissociation of 

 weak acids with dilution. (9) Conductivity of mixed solu- 

 tions. (10) Change of the strength of weak bases and acids 

 by the addition of neutral salts. (11) Distribution of bases 

 amongst different acids (Thomsen's i( avidity"). 



Group E. — (12) Velocity of reactions of various chemical 

 processes caused by the presence of acids or bases. 



Group F. — Additive properties of electrolytic solutions, 

 such as (13) specific volume and specific gravity. (14) Heat 

 of neutralization. (15) Compressibility. (16) Internal fric- 

 tion. (17) Colour, rotatory power, and index of refraction. 



It is by means of the two hypotheses named above that for 

 the first time it has been made possible to calculate the nume- 

 rical values of several thousand observations in these seventeen 

 widely different fields ; and with such success that no con- 

 siderable contradiction between theory and experiment has 

 arisen. Are we to assume that the view that hydrates exist 

 in solutions can render such service ? So far as I am aware 

 not a single numerical datum has hitherto been deduced from 

 this hypothesis. 



I may be permitted to discuss this last question in a few 

 words. In the first place it may be considered indubitable 

 that it is impossible to determine whether a salt occurs in 

 solution as hydrate or not by any of the methods for deter- 

 mining the properties enumerated in group A (except perhaps 

 No. 5). Here we perfectly agree with Prof. Mendelejeff ; 

 his remarks besides are to be found almost word for word in 

 a memoir of Raoult {Ann. de Chim< et Phys. [6] viii. p. 291). 

 From this it follows that the conclusions, drawn by Kiidorff 

 and Wiillner, from the lowering of the freezing-point and 

 vapour-pressure of solutions, in favour of the existence of 

 hydrates in them, are unfounded, as Tammann and others have 

 already shown. These inadmissible conclusions of Riidorff 

 and Wiillner were formerly, however, considered as the chief 

 proof of the existence of hydrates in salt solutions. 



On the other hand, there are other phenomena belonging 

 to the groups B, C, and F which are opposed to this assump- 

 tion. If, for instance, an electric current is passed through a 

 solution of KC1, of which we assume that it exists as the hy- 

 drate KC1 . mH 2 0, then the ions of this salt are K . nH 2 0, and 



