246 ENERGY OF CURRENTS. 



the oxygen and hydrogen in that state of partial combination in 

 which they exist on the platinum, and the couple thus altered gives 

 rise to the same quantity of water as all the others. 



The law, moreover, is general ; the weight of elements combined 

 or decomposed in any electrolyte is proportional to the quantity of 

 electricity which passes ; and this whether the operation is positive or 

 negative ; whether it takes place with polarization of the electrodes, 

 as in the decomposition of water, or of cupric sulphate with 

 platinum electrodes ; or whether the polarization can be neglected, 

 as in the electrolysis of cupric sulphate by two copper electrodes. 

 This statement includes as a necessary consequence that the electro- 

 lyte never acts as a mere conductor, and never allows any fraction of 

 the current to pass without correlative decomposition. 



In the electrolysis of cupric sulphate by two copper plates, if the 

 two plates are really in the same condition, the electromotive force 

 of contact of the metal with the liquid is the same on both sides, 

 and since just as much copper is dissolved at the positive electrode 

 as is deposited at the negative electrode, the heat produced must be 

 equal to the heat expended. On the other hand, any difference in 

 the state of the two plates would be shown by thermal work. 



256. We may, however, state here an important restriction in 

 the principle of the equivalence between chemical energy and 

 electrical work. It is assumed that, at the place where the chemical 

 action takes place, no external work, and no change of temperature 

 is produced independently of the resistances. If this is not so, we 

 must take into account all the physical or chemical secondary work 

 to which the electrolysis may give rise. 



In the decomposition of water, for instance, the energy of the 

 current first brings about the separation of hydrogen and oxygen, 

 and then does the work required by the gases in occupying a certain 

 volume at the external pressure. 



When the current arises from a Grove's battery, each element 

 performs the same work. So long as Mariotte's law holds, the 

 external work is always the same for the same weight of water 

 decomposed, and therefore for the same expenditure of electricity. 

 Within these limits the condition of equilibrium of the cells and of 

 the electrolytes is independent of the pressure. 



Mariotte's law is far from holding at very high pressures ; the heat 

 of combination of oxygen and hydrogen is thus modified, and it is 

 known that decomposition by the battery requires the employment of 

 far greater electromotive forces. The heat of formation of water is, 

 moreover, a function of the temperature, and the condition of 



