134 Dr. J. A. Fleming on the Use of DanielVs Cell 



and also zinc-sulphate solutions deprived of free acid, the ele- 

 ment zinc, zinc sulphate, and amalgamated zinc will not manifest 

 any appreciable E.M.F. The superiority of amalgamated zinc 

 over ordinary zinc is easily explained ; but when pure zinc is 

 used, deprived of any oxide on the surface by slight rinsing in 

 dilute sulphuric acid before placing it in the sulphate of zinc, 

 there is little or no certain difference, to the extent of 1 part 

 in 1000 in the case of the normal Daniell, between the pure 

 zinc amalgamated or unamalgamated. 



3. The Influence of Density of the Solutions on the 

 Electromotive Force. 



The chief reason for the differences in the values assigned 

 by various observers to the E.M.F. of the normal Daniell is 

 due to the great influence that the specific gravity of the two 

 exciting solutions exerts on the resultant E.M.F. of the com- 

 bination ; and the general law of the effect is this — If a 

 Daniell cell be formed of amalgamated pure zinc, freshly- 

 electrotyped copper, and solutions of pure zinc sulphate and 

 copper sulphate of equal specific gravity, then, taking this 

 cell as a standard of reference, increasing the density of the 

 zinc sulphate lowers the electromotive force, and increasing 

 the density of the copper sulphate raises the electromotive 

 force, within, at any rate, the limits of density from 1*01 as an 

 inferior, up to saturation as a superior limit. Zinc sulphate 

 saturated at 15° C. has a density of a little above 1*4, and 

 copper sulphate similarly saturated 1*2 ; and Dr. Alder Wright 

 shows by experiments that the increment and decrement of 

 E.M.F. for identical increment or decrement of density of 

 both solutions are so nearly equal that for equally dense 

 solutions, within limits, the E.M.F. is independent of the 

 absolute density of both. This fact has been confirmed by 

 many other observers. Amongst other careful experiments 

 may be noticed those of Carhart*, whose experiments were 

 specially directed towards ascertaining the influence which 

 variation in the density of the zinc sulphate exerts on the 

 resultant E.M.F. 



In using a Daniell cell as a laboratory standard it is neces- 

 sary, therefore, to prepare and standardize the solutions of 

 zinc and copper sulphates with the same care as if they were 

 to be used for volumetric analysis. It is a good plan to pre- 

 pare two large stock bottles of solutions, and having carefully 

 determined the density to adjust them to known specific 



* A paper read before the American Association for the Advancement 

 of Science, Sept. 5, 1884. See ' Telegraphic Journal and Electrical 

 Review,' vol. xv. p. 250. 



