Force of certain Tin Cells. 15 



The electromotive forces of the following combinations have 

 been measured : — tin and copper, zinc and tin, cadmium and 

 tin, in solutions of their sulphates and chlorides ; and zinc 

 and tin in solutions of their iodides. 



The stannous sulphate employed was obtained by the action 

 of sulphuric acid with the -aid of heat upon finely granulated 

 tin, in presence of platinum or copper to aid the action. The 

 solution of stannous sulphate contained some free sulphuric 

 acid ; and in fact it appears to be impossible to prepare a 

 neutral solution suitable for these experiments, as on dilution 

 to the required strength it deposits a basic salt, the solution 

 becoming acid. Moreover a nearly neutral solution of stan- 

 nous sulphate when kept is easily acted on by atmospheric 

 oxygen, becoming cloudy and finally depositing what is pro- 

 bably either a mixture of basic stannous sulphate and stannic 

 hydrate, or a basic stannic sulphate (?) . With free sulphuric 

 acid present in the proportion of from 1 to 2 per cent., the 

 solution was satisfactory, and the effect of the free acid is 

 considered below. For similar reasons it was found necessary 

 to operate with solutions of stannous chloride containing a 

 small amount of free hydrochloric acid, and with stannous 

 iodide containing free hydriodic acid. 



The metallic tin employed as plates in these cells was 

 chemically pure, it having been obtained by electrolysis of a 

 pure solution of the sulphate. The electro-deposited metal 

 was carefully fused and rolled into strips. 



The copper plates employed had in all cases been well 

 covered with the electro-deposited metal. The zinc used was 

 a good specimen of commercially " pure " metal, and was 

 amalgamated. The cadmium was fairly pure, and was covered 

 with the electro-deposited metal. 



Electromotive Force of Cells consisting of the Metals immersed 

 in Solutions of their Sulphates. 



I. Zinc-Tin Sulphate Cell. 



A cell in which zinc in *5ZnS0 4 , 100 H 2 is opposed to tin 

 in '5SnS0 4 , 100 H 2 0, both solutions containing about 1*5 per 

 cent. H 2 S0 4 , was found to have an E.M.F. of from "519 to 

 •535 volt, the variation being due to two causes : (1) slight 

 differences in the percentage of free H 2 S0 4 , and (2) differences 

 in the condition of the surface of the tin plate. 



It is thus difficult to assign an exact value except between 

 such limits ; but with solutions of the above strength, and 

 with a tin plate covered with freshly deposited tin, the value 



