Chemical Affinity in terms of Electromotive Force. 297 



up varying within certain small limits according to the 

 precise condition of the surfaces of the metals as regards 

 polish, oxidation, &c, but always lying fairly close to 1415 

 Volt, and practically identical -with the E.M.F. corresponding 

 to the energy gained in the net chemical change ensuing, 

 viz. the displacement of copper from copper-sulphate solution 

 bv zinc ; so that under these conditions practically all the 

 energy gained is adjuvant, whether due to displacement of 

 copper by zinc, or to transformation into ordinary metal of 

 the " nascent " copper thus set free. 



2. "When impure zinc, or pure zinc amalgamated with 

 impure mercury, is used, a greater or less amount of the 

 energy gained is non-adjuvant, even under conditions such as 

 would with pure zinc cause all the energy to be adjuvant. 

 The source of this non-adjuvancy evidently lies in the nature 

 of the actions taking place at the surface of the zinc plate : 

 the maximum E.M.F. that such a cell can generate is more 

 or less considerably below 1*115 volt, in some instances by 

 several per cent. 



3. When the density of the current exceeds 8 micro- 

 amperes per square centimetre, the E.M.F. of the cell falls 

 more or less below 1*115 volt, owing to three causes, each of 

 which produces an effect in the direction of diminishing the 

 E.M.F. First, according as the current density is greater 

 or smaller, a greater or lesser degree of non-adjuvancy of the 

 energy gained in the conversion of metallic zinc into zinc- 

 sulphate solution is brought about. Secondly, a greater or 

 lesser degree of non-adjuvancy is similarly brought about in 

 the energy gained by the transformation into ordinary copper 

 of the "nascent " metal liberated at the surface of the copper 

 plate ; other things being equal, this source of non-adjuvancy 

 distinctly predominates over the other source just mentioned. 

 Thirdly, the passage of the current causes a weaker solution 

 of copper sulphate to be formed round the copper plate, and a 

 stronger one of zinc sulphate to be produced round the zinc 

 plate, than were originally used ; this sets up an inverse 

 E.M.F., and diminishes the effective E.M.F. of the cell. 

 The maximum possible diminution due to this cause does not 

 exceed *04 volt ; whilst with a current the density of which 

 amounts to '003 ampere per square centimetre and upwards, 

 the total diminution due to this cause together with the non- 

 adjuvancy amounts to several times this maximum possible 

 value. The diminution due to these various causes jointly 

 constitutes what is sometimes spoken of as the " polarization " 

 of the cell. 



4. When the solutions of zinc and copper sulphate em- 



