Chemical Affinity in terms of Electromotive Force. 



29; 



Zinc surrounded by dilate Sulphuric Acid. 



E.M.F. observed 

 Calculated -with electro-copper 

 Strength of acid. electromotive opposed to arnalga- 



force. mated zinc. 



H 2 S0 4 , 358H 2 = sp.gr. 1-010 1 l'llS + x+y 1-129 



H 2 S0 4 , 67H 2 0= „ 1-050; 1-124 + A- + ?/ 1-139 



H 2 S0 4 , 37H 2 0= „ 1-090 1-128 + x+y 1-148 



H 2 S0 4 , 17H 2 0= „ 1-175 ! 1-141 + x+y 1-169 



where y is a small quantity corresponding to 

 the heat absorbed on dilution of the zinc sul- 

 phate to ZnSO 4 ,800H 2 O. 



It is hence evident that in all cases the agreement between 

 the E.M.F. actually developed and that due to the net che- 

 mical and physical actions taking place is so close, that what 

 differences exist lie within the limits of experimental error ; so 

 that, finally, the conclusion may be drawn that, under favourable 

 conditions, the E.M.F. of a Daniell cell is that due to the net 

 resultant of the various physical and chemical actions taking- 

 place, the whole of the energy being adjuvant, viz. that 

 gained by the displacement from copper-sulphate solution of 

 copper by zinc, together with that gained by the transfor- 

 mation into ordinary electro-metal of the " nascent " copper 

 first thrown down by the action; whilst under other conditions 

 the E.M.F. falls below this amount, even after making 

 allowance for the effect of the migration of the ions in 

 causing solutions of different specific gravities to accumulate 

 round the plates, indicating non-adjuvancy of one or other 

 or both of these component portions of the total energy gained. 



Experiments made with a view to find whether the Fall in 

 E.M.F. on increasing the Current-density is mainly depen- 

 dent on changes taking place in connexion ivith the actions at 

 the surface of the Zinc or at that of the Copper plate. 

 115. In order to trace out somewhat more completely, if 

 possible, how far that amount of fall in the E.M.F. of a 

 Daniell cell taking place as the current generated increases, 

 which is not due to the accumulation of solutions of zinc and 

 copper of different densities round the plates, can be attributed 

 to actions taking place at one or the other plate respectively, 

 the experiments described above (§103, 104) were repeated, 

 with the difference that, instead of two sets of readings only 

 being made (viz. when the two larger and the two smaller 

 plates respectively were opposed), four sets were made — (1) 

 when the two larger plates were opposed, (2) with the larger 

 zinc and smaller copper, (3) with the two smaller plates, and 

 Phil Mag. S. 5. Vol. 13. No. 81. April 1882. 2 A 



