282 Dr. 0. R. A. Wright on the Determination of 



after twenty-four hours than at first ; and in some instances, 

 when the pink electro-metal was sensibly browned or disco- 

 loured by oxidation, either all over or here and there in spots, 

 the diminution was even greater, sometimes as much as '010. 

 Amalgamated copper plates, if the surface were still white and 

 brilliant after twenty-four hours, gave the same value as at 

 first : but if the mercury had sunk into the copper, and brown 

 spots of oxidized metal were here and there visible, the E.M.F. 

 was a few thousandths of a volt lower than at first. With the 

 zinc plates greater diminutions were, as a rule, observed. In 

 some cases amalgamated plates showed little or no diminution 

 after twenty-four hours; but generally a diminution of *002 to 

 •005 was observed ; whilst with bright and electro-zinc plates 

 diminutions of from '001 to *015 were noticed. On the whole, 

 after twenty-four hours the E.M.F. was sometimes unchanged, 

 and sometimes less by '020. After forty-eight hours the fall 

 was more perceptible still, the few combinations that had not 

 appreciably altered during twenty-four hours always showing 

 a decided fall after a longer period. It is noticeable in this con- 

 nexion, that cells after Daniell's construction, but containing 

 other metals than copper, did not always give the same results 

 as normal Daniell cells. Thus, for instance, whilst cells con- 

 taining cadmium sulphate and cadmium plates behaved like 

 copper Daniells, in that the E.M.F. was sensibly steady for 

 some hours after first setting up, and only exhibited a mea- 

 surable fall after several hours had elapsed, and not always 

 then, analogous cells containing silver sulphate and silver 

 plates invariably showed a perceptible fall in less than an hour 

 after first setting up, the diminution becoming progressively 

 greater as a longer time elapsed. That this diminution was 

 due to a change (presumably oxidation by dissolved air) 

 induced on the surface of the zinc plate was rendered evident 

 by the fact that, on taking out from such a zinc-silver cell the 

 zinc plate after the lapse of an hour or more, and opposing it 

 to electro-copper in an ordinary zinc-copper Daniell, an E.M.F. 

 was indicated considerably less than the value given by a fresh 

 zinc plate, and usually just about as much less as represented 

 the fall in E.M.F. observed with the zinc-silver cell at the end 

 of the period during which it was observed, as compared with 

 the E.M.F. at the beginning of that period, when it was newly 

 set up. 



109. It is further to be noticed, that all the above-mentioned 

 figures were obtained with cells the nature of the construction 

 of which was such that diffusion of copper-sulphate into the 

 zinc-sulphate solution, and consequent deposition of copper on 

 the surface of the zinc, did not take x>lace at all during the 

 whole time that the observations lasted. With ordinary 



