28 ELECTROMOTIVE FORCE OF IRON AND OCCLUDED HYDROGEN. 



The permanence in the value in the potential is in itself the best evidence 

 that the hydrogen, even if present, is so shielded from the electrolyte as not 

 to affect the measurement of electromotive force. It is improbable that a 

 trace of occluded gas could exercise a lasting- effect in maintaining the 

 potential above that of the reversible electrode of iron in its sulphate. This 

 matter will be discussed in detail in the second section of the paper. 



There are two other hypotheses capable of explaining the abnormal poten- 

 tial of the spongy iron. It is certain that the " sintering ' is increased and 

 the surface of the iron diminished in this way ; and it is possible that change 

 in the degree of polymerization of the iron, resulting in the formation of a 

 more stable modification, is likewise effected. 80 Either of these changes 

 might affect the electromotive force. A method of deciding between these 

 two would demand either the formation of compact iron at low temperature, 

 or else of loose spongy iron at high temperature and neither is capable of 

 certain preparation. The former was nearly obtained in the experiment first 

 recounted, in which the spongy iron was rendered compact by great pressure ; 

 the latter can hardly be obtained by mechanical subdivision. 



It is to be noticed that of the first three specimens used in the experiments 

 recorded in table 3, the untouched porous pieces gave an average potential 

 of 0.783, while the same pieces after cold-welding at very high pressure gave 

 the essentially identical value 0.782. The cold-welded pieces were very hard 

 and tenacious, and appeared qualitatively in every way similar to iron which 

 had been fused (which gave only 0.765 volt) ; but it is not certain that some 

 particles on the surface were not as loosely coherent as in the sponge, and 

 hence capable of giving as high a potential. It is true that one fact mili- 

 tates against this possibility the fact, namely, that the compressed pieces 

 steadily rose to the maximum potential and staid there a long time, which 

 could have hardly been the case if the high potential had depended upon a 

 few particles. 



Because of the experimental difficulties it is impossible now to decide defi- 

 nitely between these possible causes of difference between the spongy and 

 compact metal. It is possible that all three are superposed ; but of the three, 



80 The allotropic theory of iron need not be discussed here. It has satisfactorily 

 explained so many phenomena and has had such wide usefulness that it is universally 

 accepted by scientific metallurgists. Its theoretical side from the standpoint of the 

 phase rule is thoroughly treated by Roozeboom, Z. f. Phys., ch., 34, 436 (1900). The 

 metallurgist's use of it is shown by Baron von Jiiptner in his work " Grundziige der 

 Siderologie," vol. 2, 1901. See also Berichte der d. Chem. Gesell., 39, 2381 (1006). 



A short explanation of the terminology may not be out of place. Pure iron has at 

 least two well-defined transition points. Iron stable below 766 is known as alpha 

 iron, between 766 and 895 as beta iron; and the phase formed above 895 is called 

 gamma iron. 



