FARADAY'S CONCEPTION OF ELECTRICITY. 249 



are separated at the same tircie from the liquid ; some of them are 

 evolved as gases, others are deposited as rigid strata on the surface of 

 the electrodes, like galvanoplastic copper. But the union of two con- 

 stituents having powerful affinity to form a chemical compound, as 

 you know very well, produces always a great amount of heat, and heat 

 is equivalent to work. On the contrary, decomposition of the com- 

 pound substances requires work, because it restores the energy of the 

 chemical forces which has been spent by the act of combination. 



Metals uniting with oxygen or halogens produce heat in the same 

 way, some of them, like potassium, sodium, zinc, even more heat than 

 an equivalent quantity of hydrogen ; less oxidizable metals, like copper, 

 silver, platinum, less. We find, therefore, that heat is generated when 

 zinc drives copper out of its combination with the compound halogen 

 of sulphuric acid, as is the case in a Daniell's cell. 



If a galvanic current passes through any conductor, a metallic wire, 

 or an electrolytic fluid, it evolves heat. Mr. Prescott Joule was the 

 first who proved experimentally that, if no other work is done by the 

 current, the total amount of heat evolved in a galvanic circuit during 

 a certain time is exactly equal to that which ought to have been gen- 

 erated by the chemical actions which have been performed during that 

 time. But this heat is not evolved at the surface of the electrodes, 

 where these chemical actions take place, but is evolved in all the parts 

 of the circuit, proportionally to the galvanic resistance of every part. 

 From this it is evident that the heat evolved is an immediate effect, 

 not of the chemical action, but of the galvanic current, and that the 

 chemical work of the battery has been spent in producing only the 

 electric action. 



If we apply Faraday's law, a definite amount of electricity passing 

 through the circuit corresponds to a definite amount of chemical de- 

 composition going on in every electrolytic cell of the same circuit. 

 According to the theory of electricity, the work done by such a defi- 

 nite quantity of electricity which passes, producing a current, is pro- 

 portionate to the electro-motive force acting between both ends of the 

 conductor. You see, therefore, that the electro-motive force of a gal- 

 vanic circuit must be, and is, indeed, proportionate to the heat gener- 

 ated by the sum of all the chemical actions going on in all the electro- 

 lytic cells during the passage of the same quantity of electricity. In 

 cells of the galvanic battery chemical forces are brought into action 

 able to produce work ; in cells in which decomposition is occurring 

 work must be done against opposing chemical forces ; the rest of the 

 work done appears as heat evolved by the current, as far as it is 

 not used up to produce motions of magnets or other equivalents of 

 work. 



Hitherto we have supposed that the ion with its electric charge is 

 separated from the fluid. But the ponderable atoms can give off their 

 electricity to the electrode and remain in the liquid, being now elec- 



