33$ HISTORY OF CHEMISTRY. [LECTURE XVI. 



principally upon the degree of dilution. A number of methods 

 for determining the extent of this dissociation very soon pre- 

 sented themselves, as was pointed out by Arrhenius himself, 12 

 and also by Planck, 13 Ostwald, 14 and others ; and (what is very 

 important) these methods give results that agree with one 

 another. 



The hypothesis of Arrhenius found a great many opponents 

 indeed it could hardly have been expected that it would be 

 otherwise. The assumption that an aqueous solution of 

 common salt contains free sodium and chlorine ions (which, 

 however, are nothing but electrically charged atoms that be- 

 have like free molecules) was certain to meet with opposition 

 from chemists, since it stood in contradiction to observation 

 and thus included something of a metaphysical nature. Be- 

 sides, the explanation of many reactions that had formerly 

 appeared simple Wias rendered much more difficult; as, for 

 example, the decomposition of water by the alkali metals, 15 

 since in this reaction no combination with oxygen and, on the 

 other hand, no displacement of hydrogen ions by sodium 

 ions could be assumed. But of what consequence are con- 

 siderations of this kind in face of the great advantages which 

 the theory of electrolytic dissociation affords ? A large number 

 of otherwise inexplicable facts are satisfactorily explained by 

 means of it. The so-called law of thermo-neutrality, of Hess, 16 

 which has been confirmed, in part at least, by the well-known 

 investigations of Thomsen ir and of Berthelot, 18 is in complete 

 accord with the theory of ionisation, and so are the exceptions 

 to this law which must necessarily exist in cases of incomplete 

 dissociation ; whereas, without this theory the facts concerned 

 constitute an incomprehensible puzzle. 19 



It is similar with the identity of the heat of neutralisation 

 of one and the same acid by means of different bases, and 



12 Z. physik. Chem. 2, 491. ia Wiedem. Ann. 34, 139. 14 Z. physik. 

 Chem. 2, 36 and 270. 15 Compare however Ostwald, Lehrbuch. Second 

 Edition, 2, 989. 16 Pogg. Ann. 52, 97. . 17 Thermochemische Unter- 

 suchnngen. I," 63. 18 Ann. Chim. [5] 6, 325. 19 Compare L. Meyer, Z. 

 physik. Chem. I, 134. 



