Chemical Affinity in terms of Electromotive Force. 245 

 since, by definition of electrochemical equivalents (§ 8), 





(7= — 



a X 



Finally, therefore, 







e =E-H % J, 



where 







XT ha 





H=— , 

 n 



i. e. where H is the heat evolved per gramme-equivalent of 

 substance decomposed. 



Experiments made in this way (detailed in Part II., vide 

 also § 32) have given numbers for the value of e in the case 

 of water closely corresponding with the value of e deducible 

 from direct combustion-experiments by the formula arrived at 

 above (§ 10), viz. 



j, e 



af = — j 



J X 

 which becomes 



* = a fx = ali ' J % = H 'x J > 



where 7/ is the heat liberated per gramme, and H / that per 

 gramme-equivalent of water formed by combustion of hydrogen 

 and oxygen. 



12. On attentively considering the phenomena attendant on 

 electrolysis, it is evident that it by no means follows that the 

 direct action of the current causes the entire change taking 

 place ; physical changes of state and chemical actions second- 

 ary to those due to the action of this current alone may occur ; 

 so that the value of e thus deduced, although representing the 

 E.M.F. corresponding to the nett work done in the sum of the 

 total changes taking place, and consequently representing the 

 " affinity " between the ultimate products in the original com- 

 pound as above defined, is not necessarily a measure of the ten- 

 dency to combine together of the substances primarily formed by 

 the electrolytic decomposition. For instance, suppose that in 

 the electrolysis of water the primary products are ordinary 

 gaseous hydrogen and ozone, the latter becoming subsequently 

 transformed (by a secondary chemical action) into ordinary 

 gaseous oxygen : then, since heat is evolved in the conversion 

 of ozone into oxygen to the extent of 14,800 gramme-degrees 

 per 24 grammes of oxygen (Berthelot, Bull. Soc. Chim. Paris, 

 1876, vol. xxvi. p. 56), or 4933 gramme-degrees per 8 grammes 

 of oxygen (corresponding to 9 grammes of water), the value 

 of e deduced as just indicated will be below the true value by 



