Chemical Affinity in terms of Electromotive Force. 205 



Average difference in case A = +'0042 



n B = + '°027 



,. .. .. \^ i . . . . 



Mean. 



= --0032 

 = +-0012 



On carrying out twin-cell experiments like those described 

 in § 122, it was found that when a silver plate was placed in 

 the central beaker, and either copper and zinc, copper and 

 cadmium, or zinc and cadmium plates were used in the other 

 beakers, together with solutions of the respective metallic 

 sulphates of the same molecular strengths, the difference 

 between the electromotive forces determined in the twin cell 

 was always sensibly equal to the E.M.F. developed by the pair of 

 plates other than silver employed when taken out and opposed 

 to each other in an ordinary cell containing the same metallic 

 solutions ; and this was found to be the case, not only with 

 freshly-prepared plates, but also with plates that had been 

 immersed for hours and had become oxidized on the surface. 

 For instance, in a pair of experiments with amalgamated zinc, 

 bright silver, and electro-copper plates: — 



Cells newly set up. 

 Zinc-copper in single cell . 

 Copper-silver in twin cell . 



Sum . . . 

 Zinc-silver in twin cell . . 



Difference . 



1-115 



•417 



1-532 

 1-534 



-•002 



After 24 hours. 

 1-098 

 •425 



1-523 

 1-522 



+ •001 



Similar results were obtained in several other experiments 

 of the same kind, both with zinc-copper plates opposed to 

 silver and with the other two pairs (zinc-cadmium and 

 cadmium- copper). The average difference in each case was 

 considerably less than + *001 volt. 



Relations between the Electromotive Forces of Zinc-Silver, 

 Copper-Silver, and Cadmium-Silver Cells and those corre- 

 sponding to the net Chemical Actions taking place therein. 



129. In all the cells hitherto examined in this series of 

 researches there has been shown to be a sensible equality 

 between the electromotive forces generated with clean plate- 

 surfaces of pure metals and those corresponding to the net 

 chemical and physical actions taking place therein. A priori, 



