Chemical Affinity in terms of Electromotive Force. 247 



ferent compounds (especially oxygen and hydrogen in water), 

 not only when the substances developed by the electrolysis are 

 in their ordinary conditions, but also when they are "nascent " 

 — i. e. with the view of determining, first, the E.M.F. corre- 

 sponding to the "heat of formation" of (for instance) water 

 from gaseous oxygen and hydrogen, and, secondly, that requi- 

 site to produce electrolysis proper apart from secondary physical 

 and chemical changes. It is evident that, if such determina- 

 tions are practicable, the comparison of the two sets of values 

 might lead to interesting conclusions as to the nature of these 

 secondary actions, and might throw light on the internal me- 

 chanism of the chemical changes taking place, and especially 

 on the phenomena connected with the "nascent state." Re- 

 sults in this direction have already been obtained by Favre, 

 with this difference, that the values are expressed in gramme- 

 degrees, being obtained entirely by calorimetric observations. 

 The discussion of the observations of this kind already made 

 by previous experimenters, and of the writer's results (obtained 

 chiefly by electrical measurements), will be resumed in a 

 future paper, the present essay having chiefly reference to the 

 determination of the E.M.F. representing the sum of the 

 physical and chemical actions taking place during electrolysis. 

 It should, however, be noticed in connexion with this subject, 

 that since Sir William Thomson first sketched out the Mecha- 

 nical Theory of Electrolysis (loc. cit. supra), more exact mea- 

 surements have shown that the E.M.F. actually capable of 

 being generated by a given voltaic combination is by no 

 means necessarily equal to the sum of the physical and che- 

 mical changes taking place in the pile, and that it is not ne- 

 cessarily constant for the same ultimate chemical change- 

 varying with the temperature, the state of dilution of the so- 

 lutions used, &c. 



14. For example, when zinc is dissolved in dilute sulphuric 

 acid, the total ultimate change is expressed by the equation 



H 2 S0 4 + Zn = H 2 + ZnS0 4 , 



which corresponds to an evolution of somewhat more than 

 18,000 gramme-degrees per gramme-equivalent of zinc dis- 

 solved or of hydrogen liberated, and representing therefore 

 about 18000 x 0*000105 x 42 x 10 6 = 0*794 x 10 8 C.G.S. units 

 of E.M.F., or about 0*794 volt. Similarly, when zinc or any 

 analogous metal is dissolved in any other acid (e. g. hydro- 

 chloric) with evolution of hydrogen by a parallel reaction, a 

 heat-evolution of h gramme -degrees results per gramme-equi- 

 valent, corresponding to A%J C.G.S. units of E.M.F. But 

 when this change takes place in an electromotor with a second 



