Chemical Affinity in terms of Electromotive Force. 287 



deduced from the zinc-sulphate cell experiments, representing 

 the difference in E.M.F. produced by a variation in the specific 

 gravity of the copper-sulphate solutions equal to that between 

 the acid and copper sulphate in the cell examined: i. e, if, for 

 example, the E.M.F. of a cell containing both liquids of sp. 

 gr. 1*100 be E, that of a copper-cell containing acid of sp. gr. 

 1*100 and copper-sulphate solution of sp. gr. 1*010 will be 

 E — (1*100 — 1*010) x«, where a is the factor expressing the 

 diminution per unit difference of specific gravity in the E.M.F. 

 between the limits of sp. gr. 1*010 and 1*100 for copper- 

 sulphate solution. Similarly, that of a cell containing acid of 

 sp. gr. 1*100 and copper-sulphate solution of sp. gr. 1*175 

 would be E + (1*175 — l*100)/3, where ft is the corresponding 

 factor for a difference in specific gravity between the limits 

 1*100 and 1*175. 



Thus, for instance, the following values were obtained with 

 a cell containing fluids both of sp. gr. 1*175, the temperature 

 being close to 18° throughout. 



Period since setting up. 



During 

 1st hour. 



2nd hour. 



3rd hour. 



4th hour. 



Average. 



Electro-copper plate 



Bright „ „ 



1-161 

 1-167 



1-162 

 1-168 



1-163 

 1-168 



1-160 

 1-166 



1-1615 

 1-1672 



In most cases, however, a distinct fall of upwards of *005 volt 

 occurred in less than four hours. 



A number of similar series of observations (upwards of 

 thirty) with various other cells, in which the fluids were 

 always of equal specific gravity, gave the following results, 

 the observations being only continued as long as the E.M.F. 

 remained sensibly constant — i. e. for a period of tiiae varying 

 from thirty minutes to four hours, and averaging about two 

 hours. In all these experiments electro-copper and amalga- 

 mated pure zinc plates were employed. 



Specific gravity 

 of fluids. 



Electromotive force set up, in volts. 



Maximum. 



Minimum. 



Average. 



1-010 

 1-050 

 1-090 

 1-175 



1-143 

 1-150 

 1-155 

 1-179 



1-121 



1-128 

 1-137 

 1-161 



1-129 

 1-139 

 1-148 

 1-169 



Evidently, even with the weakest acid, the E.M.F. is sen- 

 sibly above that developed with zinc-sulphate cells — viz. 1*114; 



