THE POSSIBLE EXPLANATION. 41 



These considerations show further that the final potential which the iron 

 attains after long immersion in ferrous sulphate is probably the true poten- 

 tial, because, since both iron and ferrous sulphate are greatly in excess, the 

 equilibrium finally attained must correspond to these two alone. Even if 

 some dissolved hydrogen is retained by the iron it can not possess a perma- 

 nent potential above that of the iron. That this is true is shown by the 

 fact that quenched iron finally attains a potential equal to that finally 

 attained by a similar sample which has been long exposed to the air one 

 approaching this value from above, the other from below. 



The attempt to explain the nature and condition of the active hydrogen in 

 iron is assisted by a suggestive paragraph which occurs in a paper by Thos. 

 Graham 40 " On the relation of hydrogen to palladium and on hydro- 

 genium " : 



The chemical properties of hydrogenium u also distinguish it from ordinary hydro- 

 gen. The palladium alloy precipitates mercury and calomel from a solution of the 

 chloride of mercury without any disengagement of hydrogen ; that is, hydrogenium 

 decomposes chloride of mercury, while hydrogen does not. . . . Hydrogenium 

 (associated with palladium) unites with chlorine and iodine in the dark, converts red 

 prussiate of potash into yellow prussiate, and has considerable deoxidizing powers. It 

 appears to be the active form of hydrogen as ozone is of oxygen. 



Of these chemical tests one was easily extended to hydrogen contained 

 in iron. It was found that carefully cleaned pure iron wire, and also porous 

 iron which had been kept for a year in the pure air of a desiccator over 

 potash, had no appreciable tendency to reduce neutral potassic ferricyanide. 

 Even after the iron had stood in a solution of this salt for five days the 

 solution gave no precipitate with ferric chloride. Such iron was evidently 

 practically free from active hydrogen. On the other hand, iron which had 

 been freshly made showed a marked reducing tendency, and this tendency 

 was even greater in quenched iron and iron which had been used as a 

 cathode (or had been simply immersed in acid) and thoroughly washed. 

 These results are quite in accord with the potential measurements already 

 recorded, and furnish additional evidences of active hydrogen in freshly 

 reduced or quenched iron, but point to its absence on the surface of iron 

 long exposed to the air. 



In what form may this hydrogen be supposed to exist? Ramsay was per- 

 haps the first to suggest that hydrogen undergoes dissociation when passing 

 through a hot metal, or through one made a cathode in acid solution. 42 

 Our experience supports this hypothesis, and seems to be explicable in no 



40 Proc. Roy. Soc. (London), 17, 219 (1869); Collected Papers, p. 290-299; Pogg. 

 Ann., 138, 49 (1869). 



41 Hydrogenium = hydrogen in a metal. 



"Sir William Ramsay, Phil. Mag., 5, 38, 206 (1894). 



