RELATION OF ELECTROMOTIVE FORCE TO CHEMICAL ENERGY. 445 



following law : The absolute value of the electromotive force of a 

 hydroelectric couple is equal to the mechanical equivalent of the chemical 

 action which is developed in it by unit electricity. 



It must, however, be observed that the elements in question 

 present exceptional conditions : they give rise to no disengagement 

 of gas, to no secondary action ; they do not heat during the passage 

 of the current, or at any rate the calorific effects of the two elec- 

 trodes are virtually equal and of opposite signs; they are, lastly, 

 completely reversible. The agreement does not seem to be so 

 complete with other couples. 



1038. The application of the law requires an exact knowledge 

 of the reactions which take place in the element when at work, 

 which in the present state of our knowledge is at times impossible. 

 Voltaic couples in which amalgamated zinc is combined with metals 

 supposed to be unalterable, such as copper, silver, platinum, etc., 

 have not the same electromotive force, although the same quantity 

 of zinc is dissolved for unit electricity. The unattacked metal does 

 therefore exert an influence which it seems difficult to define with 

 precision. 



A distinction must further be made between the direct effect 

 of the current and the secondary effects. The direct effect is the 

 separation of the elements of the electrolysis, the secondary effects 

 depend on the nature of these elements and of that of the elec- 

 trodes. They are due to the properties of the bodies concerned, 

 and are, apparently at least, independent of the current. To this 

 must be added changes of state, disengagement of gas arising from 

 the decomposition of water, the crystallisation of a salt formed by 

 electrolysis in a saturated solution, etc. Among all these actions, 

 which are we to allow for and which must be disregarded in cal- 

 culating the electromotive force ? 



Let us consider with Berthelot* the electrolysis of potassium 

 sulphate. For each unit of electricity, an equivalent of oxygen is 

 liberated at the positive and an equivalent of hydrogen at the 

 negative electrode. Sulphuric acid, further, is liberated about the 

 former and potass about the latter. This result is usually explained 

 by assuming that the decomposition of potassic sulphate takes place 

 in the same way as that of copper sulphate, with this difference, that 

 the potassium set free at the negative electrode decomposes water by 

 a secondary action, and in turn liberates hydrogen. The sulphuric 



* BERTHELOT. Comptes rendus, Vol. xcin., p. 661. 1881. Ann. de Chim. 

 ct de Phys. [5], Vol. xxvn., p. 89. 1882. 



