LECTURE XVI.] HISTORY OF CHEMISTRY. 337 



particular), now attain their theoretical significance for the first 

 time. As the outcome of this, and also in consequence of 

 improvements and simplifications that Raoult's methods of 

 molecular weight determination underwent, 10 these methods 

 very soon obtained a footing; and their results, especially those 

 from the depression of the freezing point, are considered to be 

 just as accurate as those from the vapour density. 



Raoult had already pointed out, however, that aqueous 

 solutions of salts, of bases, and of acids, in particular, did not 

 agree with his rules ; but always yielded results that were too 

 low, and only attained to a value from one half to one-third of 

 that which had to be regarded as the normal number. All 

 explanation of this anomaly was at first wanting, so that the 

 general applicability of van 't Hoff's theory appeared to be 

 placed in doubt. The difficulty was got rid of in the same 

 way as in the case of the abnormal vapour densities (compare 

 p. 304). 



Arrhenius dealt with this matter by exactly the same method 

 that Cannizzaro, Kekule, and Kopp had adopted in solving the 

 other difficulty. His theory, advanced in iSSy, 11 adopts as 

 actually existent that condition which must be assumed to 

 exist in order to arrive at an agreement between the theory of 

 van 't Hoff and the numbers furnished by Raoult's rules. He 

 draws attention to the fact that it is in the cases of solutions of 

 those substances which are electrolytes and break up, under the 

 influence of the electrical current, into their ions, that numbers 

 are obtained which do not agree with theory. He now assumes 

 that the ionisation does not merely take place as a result of the 

 passage of the current, but that it occurs during the dissolution; 

 and that the latter is thus accompanied by a more or less com- 

 plete (electrolytic) dissociation, the extent of which depends 



10 Compare especially, Beckmann, Z. physik. Chem. 2, 638 ; 4, 532 ; 

 8,223; 18,473; etc. u Z. physik. Chem. I, 631. Clausius (Pogg. 

 Ann. 101, 338) and Helmholtz (Weidem. Ann. II, 737) must be mentioned 

 as predecessors of Arrhenius. Planck (Z. physik. Chem. I, 577) also 

 clearly stated the idea of the dissociation of salts in aqueous solution 

 simultaneously with Arrhenius. 



Y 



