(3){ 



Chemical Affinity in terms of Electromotive Force. 181 



films of attracted or occluded gas, or when they are chemically 

 acfed upon by the electrodes or other substances in contact 

 with them, e. g. during the so-called " electrolytic convection " 

 of Hemholtz) and the limiting maximum reached with an 

 indefinitely large current. 



65. Before passing from this point it is worth noticing that 

 the circumstance that the limiting value of E^ for water must lie 

 above 3 volts shows that the first action of the current can 

 be neither of the three following changes : — 



(1) 2H 2 0=H 2 2 +H 2 ; 



(2) 3H 2 = 3H 2 + 3 ; 



H 2 0+H 2 S0 4 =H 2 +H 2 S0 5 



or H 2 + 2H 2 S0 4 =H 2 + H 4 S 2 9 , 



forming respectively gaseous hydrogen and hydrogen dioxide, 

 gaseous hydrogen and ozone, and gaseous hydrogen and the 

 " persulphuric acid" of Berthelot ; for that observer has 

 shown (Bulletin Soc. Chim. Paris, 1876, xxvi. p. 56, and 

 1880, xxxiii. p. 246), that the amounts of heat absorbed by 

 the addition of 8 grammes of oxygen to 9 of water to form 

 hydrogen dioxide, to 16 of oxygen forming ozone, and to 

 sulphuric acid forming persulphuric acid, are respec- 

 tively 10,800, 14,800, and 13,800 gramme-degrees ; so 

 that the transformation of 8 grammes of ordinary oxygen 



into 8 of ozone would absorb — ^ — = 4933 gramme-degrees, 



o 



and the above three decompositions would absorb per gramme- 

 equivalent (1 gramme of hydrogen' evolved) respectively 

 34,100 + 10,800 =44,900, 34,100 + 4933 = 39,033, and 34,100 

 + 13,800 = 47,900 gramme-degrees, corresponding to the 

 setting-up of counter electromotive forces amounting re- 

 spectively to only 1*98, 1'72, and 2'11 volts, or far below 

 the actual maximum. Even, therefore, if it be admitted that 

 persulphuric acid is the body of which the oxygen that 

 finally escapes is first produced as a constituent, it must be 

 supposed that the hydrogen is evolved as an allotropic modi- 

 fication absorbing heat in its formation from ordinary gaseous 

 hydrogen ; whilst if this be admitted for hydrogen, it seems 

 at least probable that the same is true for oxygen, and that 

 the hydrogen dioxide, persulphuric acid, and ozone, found to 

 be formed under certain conditions, are secondary products due 

 to the reaction of the "nascent" oxygen on water or 

 sulphuric acid, or to its " rearrangement " into ozone, just 

 as the oxygen ultimately developed is due to a further re- 

 arrangement. 



