36 ELECTROMOTIVE FORCE OF IRON AND OCCLUDED HYDROGEN. 



immersed in the ferrous sulphate solution five minutes after quenching, 

 while the other was thoroughly dried in alcohol and ether and put into the 

 cell to be measured on the following day. 



The results agreed in a general way with those already obtained by 

 quenching from an atmosphere of hydrogen. They differed in two respects. 

 The maximum electromotive force was 0.03 volt higher than any other thus 

 far obtained, rising to 0.94 volt, and this high electromotive force was 

 retained for a much longer period of time. These are differences in degree 

 only, and since the other results differed among themselves, although to a 

 less extent, the higher value is to be ascribed merely to more favorable 

 experimental conditions. 



Could the formation of iron nitride in any way have produced the high 

 potential in this case? Reflection promptly decided this question in the 

 negative. Iron nitride is formed only at low temperatures and is decom- 

 posed even in a stream of ammonia at higher temperatures ; " free nitrogen 

 is not able to unite with iron, even when the latter is reduced in nitrogen, 

 and the formation of nitride is dependent on the coming in contact of nascent 

 nitrogen with iron." M Moreover, the nitride begins to decompose in a 

 stream of nitrogen at 6oo, and the nitride Fe 2 N is an exothermic compound. 

 Hence probably it has a lower electromotive force than iron, 34 not a higher 

 one, and the nitride is evidently out of the question. 



Upon standing in ferrous solution the quenched iron evolved an appre- 

 ciable volume of gas, which was shown to be hydrogen by explosion with 

 oxygen in a micro-eudiometer. 



In view of this fact, and because iron quenched from nitrogen gave the 

 same high potential as iron quenched from hydrogen, there can be no doubt 

 that the bulk of the occluded hydrogen must have come from the water at 

 the moment of quenching. Some of the iron must have been oxidized, and 

 the resulting nascent hydrogen must have dissolved in the iron, in its active 

 state. No other explanation seemed to be compatible with the extraordinary 

 potential observed. 



Other workers on this subject (for example Heyn, 35 who found that 

 hydrogen exists in iron quenched from above 730 in an atmosphere of 

 hydrogen) have usually supposed that the occluded impurity was taken 

 from the gas and not from the water, but our experiments show con- 

 clusively that the water is the source of the greater part of the hydrogen. 



83 Stahlschmidt, Pogg. Ann., 125, 37 (1865). 



"Assuming total energy change to be an approximate guide to free energy change 

 in reactions of this kind. 



58 Stahl und Eisen, 20, 837 (1900). See also Roberts-Austen, Fifth report of the 

 Alloys Research Committee, Proc. Inst. Mech. Eng., 1899, p. 35. 



