On the Determination of Chemical Affinity. 189 



when tolerably concentrated solutions of strength MS0 4 

 50H 2 O are compared with similar solutions of only one 

 twenty-fifth the strength, MSO 4 1250H 2 O. 



On varying the nature of the surface of the cadmium plate 

 (by employing bright cast metal, electro-deposited cadmium, 

 or amalgamated cadmium), it was found that whatever result 

 was produced in the cadmium-copper cells by a given altera- 

 tion of the cadmium plate, every thing else remaining unal- 

 tered, precisely the same numerical result, but with the opposite 

 sign, icas produced in the zinc-cadmium cells by that alteration. 

 Thus, substituting electro-cadmium for bright cadmium plates 

 in the cadmium-copper cells caused an increase in the E.M.F. 

 varying from '002 to "006 volt in numerous experiments, and 

 averaging '004 volt ; whilst with the zinc-cadmium cells the 

 same substitution caused a decrease in the E.M.F. varying be- 

 tween almost the same limits, '002 and "007 volt, and averaging 

 almost the same value as before, viz. '0045 volt. Similarly, 

 on substituting amalgamated cadmium plates for bright cad- 

 mium in the cadmium-copper cells, the average effect was a 

 decrease of "0415 volt when the mercurial amalgam was fluid, 

 and of '015 volt when it had become solid and crystalline on 

 standing ; whilst with the zinc-cadmium cells, substitution for 

 bright cadmium of amalgamated metal caused on an average 

 an increase in E.M.F. of '043 volt when the amalgam was 

 fluid, and of '016 volt when it had become solid and crys- 

 talline. 



Cells containing Cadmium opposed to Copper. 



119. On comparing together a number of similar pairs of 

 cells containing in the one case electro-copper and in the other 

 amalgamated copper, it was found that the average difference 

 was sensibly the same as that observed when the same two 

 kinds of copper plates were opposed to zinc (§ 107), viz. that, 

 cceteris paribus, the cell containing amalgamated copper read 

 on an average '001 volt lower than the one containing freshly 

 deposited electro-copper : the actually observed differences 

 ranged from +'003 to —'003 volt, but were more usually 

 negative. 



As just stated, when the cadmium plate was amalgamated a 

 decrease in E.M.F. was brought about, averaging '0415 volt 

 when the amalgam on the surface of the plate was fresh and 

 perfectly fluid, and '015 when perfectly solid and crystalline. 

 Intermediate numbers were given by plates on the surface of 

 which crystallization of the amalgam had begun but was not 

 complete, the gradation being regular as the crystallization 

 progressed . 



