n.l-tTHOLYTIC LAWS. 23 



j>o-iiii,' produced more work than was required for their decora- 

 tion, the result would be tho en at ion of energy, which is 

 ijuitc as impracticable as the realisation of perpetual motion. 



ELECTROMOTIVE FORCES. Faraday's law respecting the 

 quantities of substances liberated by one unit of intensity, and 

 the preceding one as regards the work required for effecting a 

 chemical decomposition, establish that, to electrolyse a given 

 compound it is necessary to use a given electromotive force, and 

 whatever may be the intensity of the current, no decomposition 

 of the electrolyte occurs if that electromotive force is not reached. 

 Calling E the electromotive force necessary for the decom- 

 position of the bath, and Q the number of coulombs flowing 

 througli the bath, the work of decomposition will be expressed 

 by the formula : 



W = ^~- kilogrammetres, 



whence 



_, Wx9-81 



Ft z= '. 



If z is the electro-chemical equivalent of the liberated sub- 

 stance, the total weight liberated by Q coulombs will be equal 

 to Q z. Calling H the number of calories emitted by one gramme 

 of the substance liberated by the electrolysis for returning to the 

 state of combination it was in at the beginning of the operation, 

 the heat produced by QB will be QzH, and, as the mechanical 

 equivalent of heat is 0*424 kilogrammetres for one calory 

 (gramme-degree), the corresponding work will therefore be 

 W = ' 424 Q z H kilogrammetres. 



As we have already shown that 



W = jS-Sr , 



after simplifying we will have 



E = 4 -15944 z H volts. 



THOMSON'S LAW. Thomson's law is but a paraphrase of the 

 above formula ; it can be thus formulated : The electromotive 

 force of an electrolyte is, in absolute measure, equal to the 

 mechanical equivalent of the chemical action to which an 

 electro-chemical equivalent of the decomposed metal is subject. 



