Chemical Affinity in terms of Electromotive Force. 353 



the above experimenters show that the effect of the different 

 densities of the solutions round the two electrodes is to cause 

 a current to flow through the fluid from the weaker to the 

 stronger solution, i. e. from the — to the + electrode * 3 or in 

 the opposite direction to the current producing electrolysis. 



If, then, dilute sulphuric acid, for instance, be decomposed 

 by means of oxidizable electrodes, say of copper, the value of 

 e will, under any given condition, be given by the formula 



where A // X J = E 1 -E 3 . > 



The same formula will also apply when a metallic salt is 

 decomposed, A X/ %J still representing E x — E 3 , whatever values 

 these two quantities may happen to possess. If the metallic 

 salt decomposed contain the same metal as that dissolved 

 from the + electrode, E x will approximately = E 3 , and W 

 will nearly =0. Since, however, the physical condition of the 

 deposited and dissolved metal will rarely, if ever, be identical, 

 Ej will not exactly =E 3 ; h ,r will represent the heat that 

 would be evolved in the transformation of a gramme-equiva- 

 lent of deposited metal into the physical condition of the 

 dissolved electrode. 



Since 1^ = when a solid is deposited, the formula simplifies 

 to this, e={h' + h" + (l-n,)H. l } X J. 



So-called non-polarizable electrodes are, strictly speaking, 

 misnomers ; they are only cases where the value of e is very 

 small, owing to the particular conditions of the experiment 

 rendering (1— n 1 )H 1 + A / + h ff but little greater than 0. 



If the value of n x decreases as the current increases, when 

 it refers to a metal deposited, as appears to be the case when 

 it refers to hydrogen, from the above described experiments 

 it results that the value of e will continually increase with the 

 current, if the conditions of the experiment are such that N 

 and h" remain constant (or if they are negligible). 



92. Decomposition of Acidulated Water with Copper Elec- 

 trodes. — In order to examine these points experimentally, known 

 currents were passed through a U-voltameter filled with well- 

 boiled, just cooled, dilute sulphuric acid containing 11*5 

 grammes of H 2 S0 4 per 100 cubic centims., and fitted with 

 copper electrodes each exposing a surface of 8*5 square 

 centimetres. After allowing a current of some 50 to 100 



* Presumably this is only tlie case when heat is developed by the 

 mixture of the strong and weak solutions ; were heat absorbed during 

 the intermixture, prooably the current would be reversed in direction ; 

 for in that case the expenditure of work in order to effect electrolysis 

 would be less instead of greater when the strong and weak solutions ac- 

 cumulated round the electrodes than it would be did diffusion and inter- 

 mixture take place instantaneously. 



