UNPOLARIZABLE CELLS. 251 



combination, or even if we allow for the heat of solution, the 

 electromotive force is equal to the algebraical sum of the energies 

 produced at the two electrodes, or at the two poles of the cells. 



Such, for instance, is the couple employed by Joule, in which 

 the copper plate is covered by a layer of oxide, which the hydrogen 

 gradually reduces. The electromotive force is equal to the difference 

 between the heats of oxidation of the copper and of the zinc for the 

 same weight of oxygen. 



In other cases, a salt of the metal which forms the positive 

 electrode is dissolved in the liquid ; for instance, a solution of 

 cadmium sulphate, in which is placed a plate of zinc and a plate 

 of cadmium. The dissolved sulphate undergoes electrolysis when 

 the circuit is closed, and a weight of cadmium is deposited on the 

 cadmium plate which is equivalent to the zinc dissolved. The 

 electromotive force corresponds to the heat of substitution of zinc 

 for cadmium in the sulphate that isj about 8-3 thermal units. 



This condition lasts as long as the weight of zinc dissolved is not 

 so great that the salt itself takes part in the electrolysis. From this 

 time the polarization of the cell is again produced. 



The electromotive force of this cell may be expressed, in terms 

 of the electromotive forces of contact, by the following symbols : 



E = Zn|Cd + Cd|CdO.SO 3 + CdO.SO 3 |Zn. 



263. CELLS WITH Two LIQUIDS. In DanielPs cell two liquids 

 are used : a concentrated solution of copper sulphate surrounding 

 the copper plate, and a solution of zinc sulphate in which is the 

 plate of zinc. The two liquids are separated by a membrane, such 

 as bladder, or a vessel of porous porcelain, so as to hinder the 

 liquids from mixing, without destroying the conductivity. 



The electromotive force is 



,SO 3 + CuO,SO 3 |ZnO,SO 3 + ZnO,S0 3 |Zn. 



While the zinc plate dissolves, the copper arising from the elec- 

 trolysis of copper sulphate is deposited on the copper plate. The 

 electromotive force corresponds to the difference between the heats 

 of formation of the zinc sulphate and of the copper sulphate that 

 is to say, to the heat of substitution of the zinc for the copper in the 

 sulphate, or 24*2 thermal units. 



This cell is remarkably constant, and is one of those which 

 undergo least change from variations of temperature. 



In Grove's cell the copper is replaced by platinum : the hydrogen 



