THE REDUCTION OF PURE IRON. 7 



The iron from these reductions was of a gray color and of a porous struc- 

 ture. A piece of it could usually be broken with ease between the fingers 

 and rubbed to powder. This consistency was of course due to its mode of 

 preparation, the powdered oxide having been simply pressed into the boats 

 and the temperature having been only high enough to cause slight cohesion 

 between the particles, not to fuse the mass. 



The cohesion of the particles varied considerably with the temperature at 

 which the reduction was carried on, a product reduced at 6oo having no 

 cohesion at all, while one heated to about i,ioo for some time could be 

 broken only with some difficulty. The color also varied, owing to the differ- 

 ence in degree of subdivision, from almost black to a clear, silvery gray. 



A convenient index of complete reduction was the absence of any trace 

 of black oxide in the cooler portions of the boat ; for water formed in the 

 hotter places is decomposed in the somewhat cooler ones, and if the reduc- 

 tion is not complete water is of course present. Ordinarily, the reduction 

 was carried out by heating the substance in a rapid stream of hydrogen for 

 three or four hours with a Bunsen burner, and then for about half an hour 

 at the highest temperature of the blast lamp. Such iron showed not the 

 slightest trace of black oxide, even when finely powdered in a mortar. After 

 heating the iron remained in a continuous stream of dry hydrogen until 

 quite cold, and was then transferred for preservation to a good desiccator 

 containing finely divided fused caustic potash. No trace of oxidation was 

 ever visible on iron thus preserved; but, as will be explained later, the 

 electromotive force measurements gave clear evidence that an exceedingly 

 thin coating of oxide or adsorbed oxygen had been formed, and therefore, 

 for the more accurate experiments, small boat-loads of pieces about the size 

 of peas were reduced a second time and sealed in hydrogen until just before 

 using, thus avoiding the possibility of oxidation. Whenever this specially 

 sealed iron is used in the experiments below the fact is mentioned. The 

 sealing was done as follows : A hard glass tube was drawn out in several 

 places, having between each contracted place a boat containing porous iron. 

 Three or four such boats in series were laid in a combustion furnace, and 

 after reduction the boats were sealed off by drawing out the contracted places. 



All this iron undoubtedly contained hydrogen in greater or less amounts. 

 Although, as Baxter 8 has shown, the weight of hydrogen retained by pure 

 iron ignited at a high temperature is inconsiderable, some of the other 

 properties of the occluded gas are more important, as will be seen. Most 

 of the hydrogen can be expelled by heating for a long time in vacuum, or 

 by standing for a long time in dry air; but in order to drive out the last 

 traces fusion in vacuum is necessary. 



"Baxter, Am. Chem. Journ., 22, 363 (1899). 



