Chemical Affinity in terms of Electromotive Force. 363 



ments and of the valuations of the E.M.F. of a Daniell cell 

 made by previous observers, and of the bearing on the discre- 

 pancies between these valuations of the point just substan- 

 tiated (viz. the dependence of the E.M.F. on the current gene- 

 rated) will be resumed in a future paper. It may be noticed, 

 however, that even with a not very enormous rate of current- 

 flow per unit area, the diminution in E.M.F. amounts to several 

 per cent, of the highest value, a quantity quite sufficient to 

 vitiate observations in which the E.M.F. is assumed to remain 

 constant. 



98. Similarly the E.M.F. of a Grove's cell is not wholly 

 independent of the current generated, although the effect is 

 somewhat less marked than with a Daniell cell. The end 

 result of the chemical changes taking place in this cell are 

 expressed by one or other of the equations 



Zn + H 2 S0 4 + 2HN0 3 = ZnS0 4 + 2H 2 + 2N0 2 , 

 2Zn + 2H 2 S0 4 + 2H]S T 03 = 2ZnS0 4 + 3H 2 0+ N 2 3 , 

 3Zn + 3H 2 S0 4 + 2HN0 3 = 3ZnS0 4 + 4H 2 + 2]SrO;' 



so that, in addition to the production of gases (more or less 

 completely absorbed by the nitric acid), dilution of the nitric 

 acid takes places in the proximity of the platinum plate, thereby 

 causing 1\! (the correction for the effects of differences in the 

 density of the fluids surrounding the electrodes) to have an ap- 

 preciable value. It is therefore to be expected that the E.M.F. 

 of a Grove's cell will, like that of a Daniell's or an ordinary 

 voltaic circle, decrease with the current generated. That this 

 is actually the case is indicated by the following figures ob- 

 tained with a "gravity" Grove's cell, the plates of which 

 exposed an actual surface of 8 square centims. each, the cal- 

 culated resistance of the column of fluid (partly concentrated 

 nitric acid, partly 5-per-cent. sulphuric acid solution) between 

 the plates being about 2 ohms, and the zinc plate being well 

 amalgamated. 



Current, in 

 microwebers. 



Mi crowebers 

 per square 

 centimetre. 



E, in volts. I 



1 



CR. 



-e=-(E-CR). 











-1-872 





1-872 



94 



1-2 



-1-872 





1-872 



18-7 



2-3 



-1-872 





1-872 



36-4 



4-6 



-1-872 





1-872 



936 



11-7 



-1-872 





1-872 



187-2 



23-4 



-1-872 





1-872 



466-2 



58-3 



-1-867 



•66i 



1-768 



930 5 



116-3 



-1-861 



•002 



1-863 



1850-0 



231-3 



-1-850 



•004 



1-854 



3678-0 



459-8 



-1-839 . 



•007 



1-846 



8955-0 



1119-4 



-1-791 



•018 



1-809 



17480-0 



2185-0 



-i-747 



•035 



1-782 



2D2 



