286 Dr. C. R A. Wright on the Determination of 



Moreover the effect of a given alteration in the strength of 

 the zinc-sulphate solution (every thing else remaining the 

 same) is sensibly equal in amount, but opposite in sign, to 

 that of a similar alteration in the strength of the copper- 

 sulphate solution; it is evident that only when this is the case 

 can the E.M.F. of the cell be the same whether the solutions 

 be strong or weak. It will be shown in a future paper that 

 this property, though not absolutely peculiar to the normal 

 Daniell cell, is still rather the exception than the rule with 

 analogous voltaic combinations. 



Experiments with Daniell Cells containing dilute Sulphuric 

 Acid. 



111. In all the above described experiments, the solution 

 surrounding the zinc plate was one of pure zinc sulphate. 

 Various previous experimenters, notably H. F. Weber, have 

 found higher values for the electromotive forces of Daniell 

 cells containing dilute sulphuric acid than for those containing 

 zinc-sulphate solution (vide § 113): a priori a higher value 

 might be anticipated, because a notable amount of heat is 

 evolved on further diluting even weak sulphuric acid; so that 

 the total energy gained in the cell is not merely that due to 

 the displacement of copper from copper sulphate by zinc, but 

 also that gained in the dilution of the sulphuric acid through 

 the diffusion which necessarily goes on. On trying experi- 

 ments of the same kind as those just described with cells con- 

 taining dilute sulphuric acid of various strengths (the zinc 

 being amalgamated), it was found that not only was there a 

 considerable want of permanence in the E.M.F. set up, the 

 values perceptibly decreasing after a period of time (varying 

 in different cases from half an hour to several hours) had 

 elapsed since setting up the cells, but, further, that two cells, 

 apparently set up in identically the same way, exhibited much 

 greater differences in their readings during the period before 

 the E.M.F. began to diminish, than were observed in the zinc- 

 sulphate cells examined as above described. On the whole, 

 however, the average values obtained distinctly pointed to the 

 conclusion that, when the acid and copper-sulphate solutions 

 are of the same specific gravity, the E.M.F. rises with 

 strength of the solution ; and that when they are not of the 

 same specific gravity (the acid not being stronger than spe- 

 cific gravity 1'18), the E.M.F. is sensibly that due to a cell 

 containing liquids both of specific gravity equal to that of 

 the acid in the cell examined, corrected by the addition (or 

 subtraction) of a quantity representing the difference in spe- 

 cific gravity of the solutions multiplied by the numerical value 



