LECTURE xv.] HISTORY OF CHEMISTRY. 325 



It may be further remarked here, in passing, that the term 

 thermal effect (Warmetonung), and the words exothermic and 

 endothermic as applied to reactions, were introduced by 

 Thomsen l6 ' 2 and by Berthelot 163 respectively. 



Endeavours to discover the relations between electrical and 

 chemical forces have occupied the attention of the most talented 

 investigators since Davy and Berzelius, without, as yet, throwing 

 full light upon this important department. Faraday's electro- 

 lytic law 1(54 is an empirical one. It may to-day be looked upon 

 as one of the most powerful supports of the theory of valency, 165 

 and still a clear theoretical apprehension of it has not yet been 

 obtained. Even the phenomenon of electrolysis itself remains 

 a riddle still unsolved, although various explanations have been 

 furnished by the fine investigations of Daniell and Miller, 106 

 of Hittorf, 107 and of Kohlrausch. 168 One fundamental point, 

 nevertheless, appears to have been settled. I refer to the appli- 

 cation of the law of the conservation of energy to electrolytic 

 processes ; and to the connection between chemical energy and 

 electro-motive force. The elucidation of these relations is due 

 to the investigations of Braun lt39 and of Helmholtz. 170 The 

 former showed that there are galvanic elements whose electro- 

 motive force is less, and also those whose electro-motive force 

 is greater, than that corresponding to the chemical transforma- 

 tion of energy. Helmholtz established the principle that the 

 energy of the current is equal to the chemical energy only when 

 the electro-motive force of the battery is independent of the 

 temperature. When the electro-motive force increases with rise 

 of temperature, heat as well as chemical energy is used up in 

 the production of the current; while in the converse case a 

 part of the chemical energy is liberated in the form of heat. 



The accuracy of this principle, besides having been proved 



162 Pogg. Ann. 88, 351. 163 Mecan. Chim. 2, 18. 164 Phil. Trans. 

 1834, 77; Pogg. Ann. 33, 301. 163 Ladenburg, Berichte. 5, 753. 

 166 Pogg- Ann. Erganzungsband I, 565 ; 64, 18. 167 Ibid. 89, 176; 98, i ; 

 103, i ; 106, 337, 513. 168 Wiedem. Ann. 6, i, 145. 169 Ibid. 5, 182; 

 16, 561 ; 17, 593. 17 Berlin. Akad. Ber. 1882, 22, 825. Gesammelte 

 Abhandlungen. 2, 985. 



