RELATION OF ELECTROMOTIVE FORCE TO CHEMICAL ENERGY. 447 



measured directly, differences are observed which it is difficult to 

 explain, owing to insufficient knowledge either of the reaction or 

 of the data necessary for calculating them. Chemical energy is 

 generally greater than electrical energy, but it is sometimes less. 

 In the former case the couple becomes heated during the course ; 

 it becomes cooled in the second, and borrows heat from the ex- 

 ternal medium. 



Braun* has investigated from this point of view a great num- 

 ber of elements on the Daniell type, where the sulphates are 

 replaced by chlorides, bromides, and iodides, and the copper by 

 mercury and silver. He finds that Thomson's law often fails, and 

 particularly in the case of couples in which the salt which sur- 

 rounds the positive pole is insoluble. 



Braun assumes that only a portion of the chemical heat can 

 be converted into electrical work, and that for each compound 

 there is a constant ratio between these two quantities, which he 

 calls the "useful effect." The heat which is not converted into 

 electrical work heats the couple and produces the rise of tem- 

 perature which is ordinarily attributed to secondary actions. From 

 this point of view a couple is analogous to a gas machine in 

 which, from Carnot's principle, only a fraction of the disposable 

 chemical energy is converted into mechanical work. Pushing the 

 analogy still further, Chaperon f assumes that Braun's coefficient 

 is that which would be defined by Carnot's theorem for a thermal 

 machine working between the actual temperature and the tem- 

 perature of dissociation of the compound ; but the experimental 

 data which are necessary for the verification of this hypothesis are 

 at present wanting. 



1040. Von HelmholtzJ has endeavoured to bring these phe- 

 nomena within a strict theory only taking into consideration 

 reversible couples ; and he arrived at this conclusion, that chemical 

 energy is not in general transformable into electrical energy. We 

 shall give this theory in the form in which it has been stated by 

 M. Lippmann. 



* BRAUN. VViedemann's Annakn, Vol. v., p. 182, 1878; Vol. xvi., p. 561, 

 1882; Vol. xvii., p. 593, 1882. 



t CHAPERON. Comptes rendus, Vol. xcii., p. 786. 1881. 



t HELMHOLTZ. Wiedemann's Annalen, Vol. in., p. 201. 1877. Wissen- 

 s chaff. A b 'hand 7. , Vol. I., p. 840. 



LIPPMANN. Comptes rendus, Vol. XLIX., p. 845. 1884. 



