Force of certain Tin Cells* 17 



the addition of more salt causes an increase in the acidity of 

 the tin solution, the effect is to heighten the E.M.F., so that 

 these opposite effects may equal each other, or one may ex- 

 ceed the other, according to their relative magnitude. 



Cells were set up in which tin in a solution of 20 per cent. 

 H 2 S0 4 (sp. gr. 1*141) was opposed respectively to tin in 

 •5SnS0 4 , 100 H 2 0, containing 1^ per cent. H 2 S0 4 , and to 

 •5CuS0 4 , 100 H 2 0, and the following values obtained : — 



Tin-tin sulphate cell *058-*062 volt. 



Tin sulphuric-acid-copper sulphate cell. '59 volt. 



(It will be seen that the last-mentioned cell does not exceed 

 the tin-copper sulphate cell (*566) by an amount equal to the 

 tin-tin cell, the lower E.M.F. being probably due to an ad- 

 verse E.M.F. resulting from diffusion into the comparatively 

 strong sulphuric-acid solution.) 



If the mean values assigned to the zinc-tin cell and the 

 tin- copper cell are added together, they are found to exceed 

 the zinc-copper cell, which they should theoretically equal 



(•525 + '566 = 1*091 -1*08= *011 excess).. 



This result confirms the opinion that for neutral solutions 

 the value *566 is probably too high for the tin-copper cell, 

 and justifies the minimum value *56 being taken as correct. 

 This still gives a slight excess 1*085 — 1*08 = 0*005, and in 

 practice it is found that a zinc- tin cell and a tin-copper cell 

 united in series actually give an E.M.F. slightly in excess of 

 the zinc-copper cell. This difference may be due either to 

 the different conditions of the surfaces of the two tin plates 

 or to some less obvious cause * but as it has been found to 

 exist on several occasions in different cells, it can hardly be 

 a mere coincidence. 



III. Cadmium- Tin Sulphate Cell. 



A cell set up with cadmium in *5CdS0 4 , 100 H 2 0, opposed 

 to tin in *5SnS0 4 , 100 H 2 0, both containing about 1*5 per 

 cent. H 2 S0 4 , was found to have an E.M.F. of *189 volt. 



In order to compare the summation with the zinc-tin cell, 

 a determination of the zinc-cadmium sulphate cell was made 

 (using ^"SO^ 100H 2 O), andit was found to have an E.M.F. 

 of *335 volt. (The value given by Messrs. Wright and 

 Thompson for the zinc-cadmium cell where m=*5 and the 

 solutions are neutral is *361, which, correcting for difference of 



1 *08 

 standard, gives ttttt x '361 = "349 or somewhat higher.) 



Phil. Mag. S. 5. Vol. 21. No. 128. Jan. 1886. C 



