LECTURE XVI.] HISTORY OF CHEMISTRY. 339 



of that of one and the same base by means of different 

 acids ; also with the law of Oudemans 20 and Landolt 21 (in 

 accordance with which the salts of optically active alkaloids 

 and of optically active acids exhibit the same rotation in solu- 

 tions of equivalent concentration), with the magnetic rotatory 

 power, 22 and with the atomic magnetism. 23 Further, the 

 principle in accordance with which the spectra of dilute 

 solutions of different salts with similarly coloured ions are 

 identical, 24 and that according to which the molecular re- 

 fractive power of the salts present in aqueous solution is an 

 additive property, 25 are explained in the same way. But 

 probably the most important fact of this kind is that of the 

 proportionality that exists between electrolytic conductivity 

 and avidity in the case of acids, 20 with which may be coupled 

 the proof, furnished by Arrhenius, 27 that the extent of the 

 dissociation calculated from the electrolytic conductivity leads 

 to very nearly the same results as that calculated from the 

 depression of the freezing point. In these circumstances we 

 cannot be in doubt as to whether the hypothesis of Arrhenius 

 is warranted. 



This ionisation theory, as it is now commonly called, leads 

 us directly to electro-chemistry, which has made advances 

 that were undreamt of twenty years ago, and has now 

 developed into a separate branch of science that constantly 

 leads to new scientific and practical results. The enthusiasm 

 with which the discovery of the galvanic current and of the 

 voltaic pile was welcomed, as sketched in Lecture V., was, as 

 we now know, perfectly justified. And even although disillu- 

 sionment followed the great discoveries of Ritter, Davy, 

 Berzelius, and Faraday, and although this branch remained 

 unproductive for decades, still the opinion has been verified 



20 Wiedem. Beibl. 9, 635. 2l Berichte. 6, 1073. ^ Jahn, Wiedem. 

 Ann. 43, 280. <ja Wiedemann, in Ladenburg's Handworterbuch. 7, 31. 

 - 4 Ostwald, Z. physik. Chem. 9, 579. 25 Gladstone, Proc. Roy. Soc. 16, 

 439 ; Kanonnikof, J. pr. Chem. [2] 31, 339. 26 Arrhenius, Bihang Svensk. 

 Handlingar, 8, No. 13, 1884; Ostwald, J. pr. Chem. [2]' 30, 93. 27 Z. 

 physik. Chem. I, 631. 



