298 On the Determination of Chemical Affinity. 



ployed are not of the same specific gravity, the E.M.F. of 

 the combination differs from that which would have been set 

 up had both been of the same specific gravity by an amount 

 which increases with the difference in specific gravity of the 

 two solutions: if the copper-sulphate solution is the stronger, the 

 E.M.F. is increased, and vice versa. The amounts of increase 

 and decrease are sensibly the same as the electromotive forces 

 generated when two copper-sulphate or two zinc-sulphate 

 solutions interdiffuse, the specific gravities of which are iden- 

 tical respectively with those of the two fluids actually present in 

 the cell examined. So long as the two solutions are of the 

 same specific gravity, the E.M.F. set up is, cceteris paribus, 

 sensibly independent of the actual value of this specific 

 gravity ; at least the fluctuations observed are not outside the 

 range of experimental errors. 



5. When dilute sulphuric acid is used instead of zinc- 

 sulphate solution, its specific gravity being the same as that 

 of the copper-sulphate solution, an increase in the E.M.F. of 

 the cell is produced which sensibly corresponds with the 

 increase in the "heat of formation " of zinc sulphate when 

 sulphuric acid is employed of the strength used in the cell, as 

 compared with acid diluted to a larger extent (H 2 S0 4 , 800 

 H 2 0). If the copper-sulphate solution differs from the acid in 

 specific gravity, the latter not exceeding 1*18 in specific gra- 

 vity, the E.M.F. is raised above or lowered below what it 

 would have been had the copper sulphate been of the same 

 specific gravity, by an amount which is sensibly the same 

 as the E.M.F. generated by the interdiffusion of two copper- 

 sulphate solutions the specific gravities of which are iden- 

 tical respectively with those of the two fluids actually present 

 in the cell examined. 



6. Owing to the diminution in the E.M.F. of a Daniell 

 cell when generating a current, the fluctuations in the maxi- 

 mum values obtainable with any given cell with the physical 

 condition of the surfaces of the plates and the time that has 

 elapsed since its construction, the tendency to deposition of 

 copper on the zinc by diffusion and the consequent dimi- 

 nution in E.M.F., and the variation in E.M.F. according as 

 dilute acid of different strengths or zinc-sulphate solution is 

 used to surround the zinc plate, it results that " the E.M.F. 

 of a Daniell cell " is a very variable standard of E.M.F. and 

 one singularly devoid of permanence. By taking suitable 

 precautions in the construction of a cell (using pure zinc 

 amalgamated with pure mercury, amalgamated or electro- 

 copper, and pure zinc- and copper-sulphate solutions of the 

 same specific gravity), a cell may be obtained the E.M.F. of 



