Chemical Affinity in terms of Electromotive Force. 269 



It is evident that, when the density of the current is the same, 

 and the surfaces of the two copper and of the two zinc plates 

 are respectively in the same conditions, the effect on the 

 E.M.F. caused by the passage of a current must be sensibly 

 the same, inasmuch as the same amount of zinc is dissolved 

 and of sulphate of zinc formed, and the same quantity of cop- 

 per is deposited and of copper sulphate decomposed, per square 

 centimetre of plate surface ; so that e ± must sensibly =e 2 . 

 Hence, since 



E 1 + 2-5OR=E 2 + 5'0OR, 

 it results that 



R: 



Ei-E 



2-5 D 



By contrasting in this way the values of the E's obtained with 

 various current-densities, a set of values for R. are obtained, 

 fairly concordant when the observations are carefully made and 

 the plates of such materials as to remain in the same condition of 

 surface throughout the experiment, or nearly so, so that on in- 

 troducing a given external resistance into the circuit, sensibly 

 the same values for the E's are uniformly obtained. With 

 sulphate-of-zinc solution surrounding the zinc plates, and with 

 amalgamated plates (copper as well as zinc), this permanence 

 is more readily ensured than when dilute sulphuric acid is used 

 (with amalgamated zinc plates), or when the copper plate is 

 not amalgamated, but only freshly coated with electro-deposited 

 metal. Indeed, to obtain a sufficient number of readings when 

 dilute sulphuric acid is employed, it is preferable to discharge 

 the cell after an hour's use and recharge it, amalgamating the 

 plates afresh, and filling up with the same solutions as before 

 to exactly the same levels (ensured by suitably marking the 

 glass), so that the resistance of the cell may vary as little as 

 possible, the temperature being so adjusted as to be sensibly 

 the same on the average throughout. Thus the following 

 series of values was obtained as the average result of four sets 

 of readings alternately with gradually increasing and gradually 

 diminishing external resistances, with a cell containing nearly 

 saturated copper-sulphate solution, freshly electro-coated cop- 

 per plates, amalgamated zinc plates, and dilute sulphuric acid 

 of 1*045 sp. gr., the plates being reamalgamated and reelectro- 

 coated respectively for each successive series — all the observa- 

 tions being reduced to the same standard as that adopted 

 throughout this paper, viz. the average reading at 15*5 of a 

 large number of Clark's cells taken as 1*457 volt*. 



* The exactness of this value depends not only on how far the average of 

 these cells is identical with the average of those which served as the basis 



