as a Standard of Electromotive Force. 135 



gravity by the aid of the specific-gravity bottle or hydrometer. 

 For general use two standard solutions of each salt are speci- 

 ally useful. First, a solution of sulphate of copper very nearly 

 saturated at 15° C. and having a specific gravity of 1'200, and 

 a solution of sulphate of zinc of equal density. Secondly, a 

 solution of sulphate of copper of density 1*100 at 15°, and a 

 solution of sulphate of zinc of 1*4. These last values are 

 chosen because they were employed by Sir W. Thomson in 

 his standard gravity-cell ; and they can be used in either his 

 form of gravity-cell, or in theU-tube form above, or in Raoult's 

 form of separate vessels and siphon *. 



A very large number of comparisons have been made of the 

 E.M.F. of cells set up with these solutions and the E.M.F. 

 of Clark's cells, whose value has been compared directly with 

 cells standardized in absolute measure. 



If a Daniell cell is carefully made up, either in the Raoult 

 form or U-tube form, of solutions of pure zinc and copper 

 sulphate not sensibly interdiffused at the level of contact, and 

 with pure amalgamated cast zinc and freshly electrotyped 

 copper plate, which is evenly plated with anew uniform pink- 

 ish deposit of electro-copper free from all brown spots of oxide, 

 the E.M.F. of this cell, taken at once, is very close to 1*102 

 true volt, and the ratio of the E.M.F. of this cell to a cor- 

 rected Clark cell at 15° C. is very nearly '768 to unity f; 

 the Clark cell being taken as 1*435 at 15° C. 



If, instead of taking the equidense solutions, we take zinc 

 sulphate of specific gravity 1*400 and copper sulphate of 1*100 

 at 15° C, and the same plates, the E.M.F. of the cell lies close 

 to 1*072 volt, and the ratio of the E.M.F. to that of the cor- 

 rected Clark is *747. 



If, however, instead of taking the electromotive force at 

 once after the freshly electroplated copper pole is introduced 

 into the cell, the cell is allowed to stand an hour or so, both 

 the above values will be increased by about *003 volt, pra- 



* If 28*25 parts by weight of pure crystallized sulphate of copper 

 (CuS0 4 , 50H 2 ) are dissolved in 71*75 parts by weight of distilled water, 

 the resulting solution will have very nearly a specific gravity of 1*200 at 

 18° C. ; and if 16*5 parts of the crystals are dissolved in 83*5 parts of 

 water, the solution will have a density of 1*100 at 15° C. 



For the zinc-sulphate solutions take 55*5 parts by weight of crystallized 

 zinc sulphate (ZnS0 4 , 70H 2 ) and dissolve in 44*5 parts of distilled water. 

 The resulting solution will have at 20° *5 C. a specific gravity of 1-400. 



If 32 parts by weight of the crystals are dissolved in 68 of water at the 

 same temperature, the solution will have a density of 1*200. These are 

 useful densities. 



t Lord Rayleigh assigns a value very close to *770 for this ratio, and 

 Dr. Alder Wright -765. The figure in the text is derived from about 50 

 experiments of my own. 



