Chemical Affinity in terms of Electromotive Force. 207 



effected, more or less past y amalgam being formed, and a smaller 

 heat-development being then noticeable. The most trustworthy 

 determinations made indicated that during the solution of 

 108 grams of silver, 2070 gramme-degrees are evolved. 

 Hence the heat of formation of silver sulphate, when the 

 silver is dissolved in mercury, is, per gramme-molecule, 

 20390-2 x 2070 = 16250] so that the heat of displacement of 

 silver from silver sulphate, when the metal is ultimately obtained 

 in mercurial solution, is greater than the values above calcu- 

 lated for zinc, cadmium, and copper respectively as precipi- 

 tating metals by 4140 gramme-degrees per gramme -molecule, 

 or 2070 per gramme-equivalent, corresponding with # 0915 volt. 

 The average difference in E.M.F. between cells otherwise 

 alike, in which crystalline electro-silver and amalgamated 

 silver (fluid) are respectively used, is almost identical with 

 this, being -101 -'008 = -093 volt. As already noticed (§ 120), 

 silver and cadmium differ in their behaviour in this connexion 

 when amalgamated, the inequality between the effect on the 

 E.M.F. actually produced by amalgamation of cadmium and 

 that corresponding with the heat of solution in mercury 

 being probably due to the oxidation of cadmium by dissolved 



Rate of Fall in Electromotive Force through so-called " Polari- 

 zation" for definite Amount of Increase in Rate of Current- 

 flow. 



130. In order to see whether the electromotive forces of the 

 cells above described are rendered less (when no current 

 passes) than they otherwise would be, through the interfering 

 action of dissolved air or some other similar obscure cause, a 

 number of experiments were made like those described in 

 Part V. (§§ 103-105), with the general result of showing that 

 the deficiency observed in the E.M.F. actually generated when 

 no current passes (as compared with that calculated from the 

 thermo-chemical data) is at any rate not due to any such 

 cause, inasmuch as the E.M.F. generated when a current does 

 pass is always more or less beloiv that set up when no current 

 passes, just as with normal Daniell cells and with the zinc- 

 cadmium and cadmium-copper cells above described. Thus, 

 for example, the following numbers were obtained in three 

 experiments, in each of which bright zinc plates were em- 

 ployed, and one in which electro-copper plates were used, the 

 silver plates being in each instance uppermost and surrounded 

 by saturated silver sulphate solution. 



