THE QUENCHING OF IRON. 33 



A slight indication of an excessive value of this kind had already appeared 

 in other previous samples, many of which after a short immersion in ferrous 

 sulphate showed a slight maximum. This was noticeable even in samples 

 which had been ignited in a vacuum. In the cases mentioned at the begin- 

 ning of this paragraph, after standing some time, bubbles of hydrogen 

 appeared around the metal, as the electromotive force decreased. This 

 seemed to show that in some way the excessive electromotive force was con- 

 nected with hydrogen. But hydrogen gas has a lower, not a higher potential 

 than iron. Therefore, it is clear that if the abnormality is produced by 

 hydrogen, this impurity must exist in the metal in a different form no 

 longer as merely adsorbed gas on the surface of the fine powder, but in some 

 new state, inclosed in the less open structure of the sintered iron. 



More light was thrown upon all these matters by interesting series of 

 experiments which showed that the dissolved active hydrogen could be more 

 easily introduced into the iron in other ways. There follows a brief descrip- 

 tion of these experiments. 



The first series furnishing this further light was a set of experiments 

 originally begun as a preliminary attempt to cause a change in the internal 

 structure of iron by quick cooling. In it iron was plunged while hot into 

 water. 



In order to quench iron suddenly from a high temperature without coating 

 the metal with a hardly soluble film of oxide, it was clearly necessary to 

 conduct in an atmosphere free from oxygen both the heating of the metal 

 and the transference to the cooling agent. 



An atmosphere of hydrogen was first used for this purpose. As before, 

 a stout tube of Berlin porcelain was erected in a vertical position, the middle 

 portion being heated by a Fletcher furnace. Pure electrolytic hydrogen, 

 thoroughly washed and dried, was supplied at the top of the tube through a 

 cooled Hempel stopper, which was further protected against the rapid con- 

 vection of hot hydrogen by a pipe-bowl suspended at the upper limit of the 

 heated zone by means of an iron wire. The iron was heated at first in an 

 unglazed basket of the best porcelain, and in the later experiments on a 

 shelf or disk of the same material. Simple mechanical devices as before 

 enabled the iron to be plunged or dropped quickly into the cooling agent at 

 the base of the tube. The temperature of heating was determined by a 

 Le Chatelier platinum-rhodium thermoelectric junction. Cold boiled water 

 was used as the cooling agent at the base of the tube, the operation being, 

 therefore, merely the quenching of pure iron without exposure to oxygen gas. 



Even the first experiments gave interesting results. Porous iron reduced 

 at 8oo was quenched from i,ooo c '. Twenty minutes afterwards the super- 



