24 ELECTROLYSIS. 



DETERMINATION OF THE ELECTEOMOTIVE FORCE. Observ- 

 ing that z H represents the heat given off by one gramme of the 

 substance under consideration, multiplied by the electro-chemical 

 equivalent of the said substance, the electromotive force can be 

 estimated in respect of the chemical equivalents, and instead of 

 E = 4 15944 zH volts, we can write E = 4*16 x 0' 0105 Re 

 = 0434 H e volts, e being the chemical equivalent of the 

 metal acted upon, H e is the number of calories (kilogramme- 

 degree) evolved by the combination of one chemical equivalent 

 of the substance under consideration : 



. 1 volt therefore corresponds to . = 23 calories. 



In order to determine the electromotive force, in volts, 

 necessary for chemical decompositions, it will therefore be suffi- 

 cient to know the quantity of calories evolved by a chemical 

 equivalent of the metal decomposed in the electrolytic bath and 

 to divide that quantity by 23. 



ELECTROMOTIVE FORCE REQUIRED FOR THE DECOMPOSITION 

 OF WATER. Let us take as an example the decomposition of 

 water, in which hydrogen acts as the attacked metal. One 

 gramme of hydrogen in being oxidised, can develop 34*5 

 calories, therefore 



O/f . K 



E = ^^ = 1 -495 ; about 1J volt. 



& 



EEMARK ON THE DECOMPOSITION OF WATER BY MEANS OF 

 A LOW ELECTROMOTIVE FORCE. It will be seen from the fore- 

 going that water cannot be electrically decomposed unless a 

 current of at least 1'495 volts is used. Many physicists, how- 

 ever, maintain that this decomposition can be effected with a 

 lower electromotive force, for example, by means of one Daniell 

 cell, the electromotive force of which is approximately equal to 

 one volt. Doctor Jahn himself, in his remarkable work on the 

 theory of electrolysis,* explicitly says : 



" We have already established that the electromotive force 

 of any circuit must be proportional to the production of heat 



* ' Die Elecktrolyse trad ihre Bedeutung fur die theoretische und angewandte 

 Chemie.' Vienna, 1883. 



