PART II. THE ELECTROMOTIVE FORCE OF IRON CON- 

 TAINING OCCLUDED HYDROGEN. 



The fact that hydrogen freely penetrates iron at red heat and that iron 

 retains some of this hydrogen on cooling was discovered as long ago as 1866 

 by Graham ; but the more detailed study of this and related phenomena has 

 been undertaken only in recent years, after its importance in the metallurgy 

 of the metal had been established. Many papers have been published on this 

 topic, but the field has been by no means thoroughly treated. 



Our attention during this research was first directed to the well-known 

 affinity of iron for hydrogen by observing the copious evolution of this gas 

 from metal reduced at a low temperature. 82 Iron obtained from the oxide at 

 570 is, as well known, a fine powder, which oxidizes easily, and hence after 

 even a short exposure to the air gives a low electromotive force. In order 

 to test properly the free energy of such iron, it must be cooled in pure hydro- 

 gen and tested instantly after opening the reduction-tube. In this way it 

 was found that the potential is not essentially different from iron reduced 

 at 700 , being about 0.795 v0 ^ in conjunction with the decinormal electrode. 



Nevertheless, such powder, upon immersion in ferrous sulphate evolved 

 copious bubbles of gas, which were proved to be hydrogen. That this hydro- 

 gen was not in any extraordinary condition is shown by the fact that it 

 neither altered the potential of the iron nor caused the iron containing it to 

 produce any unusual reducing effect on mildly oxidizing solutions. It is 

 probable that the hydrogen thus held by the fine powder was merely adsorbed, 

 as the gas is adsorbed by charcoal. 



That the gas could exist in the metal in a radically different condition we 

 did not at first suspect ; but this conclusion was forced upon us by the sur- 

 prising and quite unexpected results of several other experiments. 



In the first place, several pieces of porous iron which had been reduced at 

 high temperature, slightly oxidized by long standing in the air, and then 

 reheated in hydrogen at 575 gave at first an extraordinarily high value for 

 the usual cell couple, namely 0.825. In seven hours this had decreased to 

 0.810, and in five days became constant at 0.798, essentially the normal value. 



82 Baxter has shown that pure iron reduced at a high temperature contains but little 

 occluded hydrogen. Am. Chem. Journ., 22, 363 (1899). 



32 



