Chemical Affinity in terms of Electromotive Force, 351 



Fig. 5 (PI. III.) represents the four curves thus obtained, — 

 those marked 1 and 3 being the curves obtained with acid of 

 27*0 grammes per 100 cubic centims., and with carbon and gold 

 electrodes respectively ; and those marked 4 and 6 the curves 

 obtained with acid of 1*0 gramme per 100 cubic centims., 

 with carbon and gold electrodes respectively. The other 

 two curves traced, marked 2 and 5, are put in for comparison, 

 being respectively the mean curves obtained as above described 

 with acid of 27*0 and 1*0 grammes per 100 cubic centims. with 

 platinum electrodes. In each case the weaker acid gives the 

 higher counter E.M.F. for a given rate of flow of electricity 

 per square centimetre of electrode surface ; but the effect 

 produced by the variation in the strength of the acid is small 

 compared with that produced by substituting platinum for 

 carbon, or gold for p'atinum *. 



It should be noticed, however, that it is highly probable 

 that the counter E.M.F. observed with the carbon electrodes 

 is somewhat diminished, at any rate for the smaller currents, 

 by the circumstance that electrolytic " nascent " oxygen is 

 capable of attacking carbon, forming CO and C0 2 (Beetz, 

 Wied. Annalen, 1878 [2] v. p. 1 ; also Phil. Mag. 1879 [5] 

 vii. p. 1) ; the effect of this would of course be to diminish e 

 by a quantity equivalent to the heat evolved in the oxidation 

 of the carbon (reckoned per gramme-equivalent of water de- 

 composed). With currents of the m'nuteness of only a few 

 microwebers it was not found practicable to prove directly 

 the formation either of CO or C0 2 ; but the circumstance 

 that the observed values for e fell slightly below 1 # 50 volt with 

 the voltameter containing acid of 27 grammes per 100 cubic 

 centims., and with currents of from 5 to 10 microwebers, or 

 slightly above the current equivalent to the " diffusion dis- 

 charge"" found with the same voltameter substituting platinum 

 electrodes for carbon ones (viz. close to 4*0 microwebers), 

 together with the circumstance that with platinum and gold 

 electrodes the value of e was never below 1*5 volt when the 

 current exceeded that equivalent to the " diffusion discharge," 

 would seem to indicate that the values of e for these currents 

 found for the carbon electrodes were actually slightly below 

 the values that would have been found had no oxidation of 

 the electrode taken place. 



* The two carbon curves lie a little below their true positions, owing- to 

 the effective area of the carbons being a little greater than their superficial 

 measurement, on account of the unavoidable absence of perfect smoothness 

 and slight pitting of the surface. The true area, however, exceeds that 

 measured by only a small amount, the carbon surface having been filed 

 and ground so that the surface was as nearly smooth as it could be made, 



