POLARIZATION OF THE ELECTRODES. 243 



the voltameter in unit time, and P be the weight of water decom- 



p 

 posed. These two quantities being proportional, the quotient =/ 



expresses the weight of water decomposed by unit electricity. If a 

 is the heat of combination of unit weight of water at constant 

 pressure, JaP represents the energy necessary to decompose a weight 

 of water equal to P. This energy being furnished by the fall of the 

 current, we must have 



W 



from which is deduced 



Hence, between the two electrodes of a voltameter traversed by 

 a current there is, besides the difference of potential due to the 

 resistance of the intermediate conducting liquid, a sudden fall, the 

 exact seat of which is indeterminate, and which may be produced 

 either wholly upon one electrode, or partially on both, and which is 

 numerically expressed by the mechanical work corresponding to the 

 energy absorbed by that quantity of water which a unit of electricity 

 decomposes. 



252. POLARIZATION OF THE ELECTRODES. By what mechanism 

 is this difference of potential produced ? It is clear that before the 

 current passes, the two electrodes, if they are of the same metal, 

 (both of platinum, for instance,) are, by Volta's law, at the same 

 potential, which probably differs from that of water ; but the sudden 

 and opposite changes which then take place at each of the electrodes 

 would produce in the voltameter an amount of work which is ob- 

 viously zero. When the current is started, the two falls are unequal 

 and their difference is equal to H. Following Volta's ideas, we are 

 led to the conclusion that the surfaces in contact are modified. A 

 deposition of the elements of the electrolyte on the electrodes gives 

 a sufficient explanation of this modification. 



For if a plate, which has been used as an electrode, or which has 

 been immersed in a gas, is placed in water in presence of a plate of 

 the same kind, but clean, or recently heated to redness, a difference 

 of potential is set up between the two plates. 



Let us consider, as a particular case, the decomposition of water. 

 The first portions of gas which come in contact with the platinum, 

 if they do not form with it a true combination, seem at any rate to 

 be deposited there in a state of condensation in which the gas has 

 far less potential energy than it has in the free state. This effect 

 of condensation of the gas takes place particularly at the outset, 



R 2 



