Chemical Affinity in terms of Electromotive Force. 211 



are of the same specific gravity; but with the other cells 

 containing cadmium this is not so, cadmium-sulphate solution 

 being uniformly more dense than either zinc or copper solution 

 of the same molecular strength. 



(4) The effect on the E.M.F. of a cell of a given alteration 

 in the nature of the surface of either a zinc, copper, cadmium, 

 or silver plate is sensibly the same numerically whichever 

 other one of these four metals be opposed to it ; but the direc- 

 tion of the alteration is opposite according as the plate is the 

 anode or the kathode of the combination. 



(5) Volta's " Law of Summation " universally holds within 

 the limits of experimental error in all the cases examined ; 

 that is, the electromotive forces of zinc-cadmium, cadmium- 

 copper, and copper-silver combinations are such that, for any 

 given kinds of plate-surfaces, the sums of the two first, of the 

 two last, and of the three together are respectively equal to 

 the electromotive forces of zinc-copper, cadmium-silver, and 

 zinc-silver combinations. 



(6) Zinc, copper, and cadmium plates alter superficially 

 (probably in consequence of oxidation by dissolved air) more 

 rapidly when opposed to silver than when opposed to any 

 other one of these four metals, on being immersed in solutions 

 of their respective sulphates forming one half of a cell on 

 Daniell's principle — no current being generated by the cell, 

 the measurements being made by means of a quadrant elec- 

 trometer. 



(7) With all the cells examined the behaviour wdien gene- 

 rating a current is analogous to that of a normal Daniell cell : 

 when the current-density exceeds a few microamperes per 

 square centimetre of plate-surface, a more or less marked 

 diminution in the E.M.F. ensues, the falling-off being the 

 greater the greater the current-density. With moderately 

 strong currents the diminution far exceeds the maximum 

 possible amount due to accumulation of dissolved salt round 

 the plate dissolved, and exhaustion of solution round the other 

 plate. Cceteris jxiribus, the rate of fall in E.M.F. as the 

 current-density increases is the more rapid the lower the heat 

 of formation of the metallic salt decomposed in the cell so as 

 to deposit the metal, and is comparatively but little affected 

 by the nature of the dissolved metal. 



(8) The effect of halving the area of the plate on which 

 metal is deposited is usually to cause a greater diminution in 

 the E.M.F. than is produced by halving the area of the dis- 

 solved plate ; amalgamated cadmium plates in zinc-cadmium 

 cells, however, form an exception to this rule. 



P2 



