16 Mr. E. F. Herrouu on the Electromotive 



of *525 volt may be taken as a tolerably accurate average 

 E.M.F. for this kind of cell. 



It would, of course, be un philosophical to assume that an 

 equality in the amount of free acid present in each solution 

 has the same effect as operating with neutral solutions ; but 

 being unable to employ a neutral solution of stannous sul- 

 phate, the addition of an amount of sulphuric acid to the zinc 

 solution equal to that present in the SnS0 4 solution was con- 

 sidered to be the fairest way of meeting the difficulty. 



Increasing the amount of free sulphuric acid in the tin solu- 

 tion only has naturally the effect of lowering the E.M.F., so 

 that with 5 per cent. H 2 S0 4 the E.M.F. is not higher than 

 •49 volt. 



Substituting a 10 per cent, solution of H 2 S0 4 for the zinc 

 sulphate, leaving the stannous sulphate unchanged, raises the 

 E.M.F. to -596 volt. 



The effect of varying the strength of both solutions (keep- 

 ing them of equal molecular strength) is the same in kind as 

 that which occurs with the zinc-tin chloride cell, but is less 

 marked. (See Zinc-Tin Chloride Cell.) 



The heat of formation of stannous sulphate being unknown, 

 it is impossible to compare the value *52o volt with deduc- 

 tions from thermal data. 



II. Tin- Copper Sulphate Cell. 



A cell in which tin in '5SnS0 4 , 100 H 2 is opposed to 

 copper in *5CuS0 4 , 100 H 2 0, containing from 1 to 2 per 

 cent. H 2 S0 4 in both solutions, was found to have an E.M.F. 

 varying from *56 to *572 volt, according to the condition of 

 the surfaces of the plates and the acidity of the solutions. 

 The effect of increasing the proportion of free H 2 S0 4 in the 

 tin solution is to raise the E.M.F. of the cell, and it is, of 

 course, opposite to the effect observed in a cell in which zinc 

 displaces tin. The increase of free sulphuric acid equally in 

 both tin and copper solutions also tends (at least through a 

 certain range up to 3 or 4 per cent.) to slightly raise the 

 E.M.F. ; and from this I conclude that with neutral solutions 

 of the above strength, the value would not exceed, and would 

 probably fall slightly below, '56 volt. Preference would 

 therefore be given to the minimum value '56 instead of the 

 mean value '566. 



Alteration of the strength of the solutions is complicated 

 by the question whether such alteration involves a change in 

 the percentage of contained acid or only of the salts employed. 

 Increase of strength of the solutions as regards the salts only, 

 is attended with a slight depreciation of the E.M.F. ; but if 



