J • 



Chemical Affinity in terms of Electromotive Force. 251 



The difference of potential set up between two points connected 

 by a medium of resistance r traversed by a current C 2 is 



R + r n ± 



and bence tbe expression for tbe E.M.F. equivalent to the 

 affinity to be found, E — H%J, becomes 



n x n 2 



where a is the equivalent of water =8*98. 



19. Experiments on the electrolysis of water have been 

 made by the writer in accordance with this method, and have 

 furnished moderately concordant results approximating to the 

 value of the E.M.F. representing the affinity to be measured, 

 calculated from the heat of combustion of hydrogen (§ 32) ; 

 but as the experimental errors attendant on the method are 

 considerably greater than those involved when the value of 



r 

 P ^E' is directly determined by means of a quadrant elec- 

 trometer, this latter mode of operating has been adopted in 

 preference : the details of observations thus made in the case 

 of water are given in Part II. Meanwhile it is of interest to 

 notice that if a tangent galvanometer be substituted for a vol- 

 tameter (the quantity of electricity passing being determined 

 by observing the quantity of electrolyte w decomposed whilst 

 a quantity of heat h is evolved), the value of the affinity to be 

 determined is given by the analogous formula 



tan a — tan ft ^^ hay J 



6= t J±j — ; 



tana w 



where a and ft are the deflections observed with the cur- 

 rents Ci and C 2 respectively. The experiments of Joule 

 above referred to (Phil. Mag. vol. iii. p. 481), although made 

 for the purpose of calculating the heat of formation of water, 

 copper sulphate, and zinc sulphate in a wholly different way, 

 contain the data requisite for the application of this mode of 

 observation, based on the use of the tangent galvanometer ; it 

 being taken for granted that W, the value of the E.M.F. of 

 the Daniell battery used, was m x 1*10 volt, where m is the 

 number of cells used in the experiment recalculated. Thus, 

 Joule found the following numbers in a series of four observa- 

 tions on the electrolysis of water in which six Daniell cells 

 were used : — 



