6 ELECTROMOTIVE FORCE OF IRON AND OCCLUDED HYDROGEN. 



Most of the iron used in this research was prepared by a method which 

 had been found in an exhaustive investigation of the atomic weight of the 

 metal to yield good results. 1 It is true that not quite all the steps were used 

 in the present work, but the more essential ones were retained, and a very- 

 pure sample of the metal was prepared. " Piano " wire of good quality 

 served as the original substance ; it was carefully sandpapered and wiped, 

 and dissolved in dilute nitric acid. Concentrated acid was first used, but this 

 oxidized the silicon to silicic acid, and filtration from the silicon itself (or 

 silicide of iron, SiFe 2 ), 2 which the dilute acid leaves unchanged, was con- 

 sidered more safe, besides being more convenient. All iron carbide or 

 graphite is either completely oxidized or else is unchanged and filtered out. 

 The dilute acid also lessens the chances of any sulphur being oxidized to 

 sulphate. 



The nitrate formed was then concentrated and recrystallized three or four 

 times. Recrystallization is here a peculiarly good method of purification, 

 because the ferric nitrate has an unusual crystal form ; therefore most im- 

 purities can only be held mechanically. 3 The recrystallized salt was then 

 heated in a platinum dish at a moderate temperature in an electric oven. 

 The nitric acid was partly driven off and the dry, dark-red basic nitrate was 

 then powdered in a Hempel steel mortar 4 and heated to full redness to 

 complete the denitration. It is important that this operation be carried out 

 in steps as described, because in this way a fine, spongy powder is obtained 

 which allows of perfect reduction. If denitrated too quickly by too high a 

 temperature the oxide obtained is exceedingly hard and the powder con- 

 sists of very compact particles, in which the complete reduction of the core 

 is more or less uncertain and decidedly less rapid. 



The oxide so produced was reduced in a stream of electrolytic hydrogen ' 

 in an unglazed porcelain boat e set in a hard glass combustion-tube when a 

 low-temperature reduction was desired, or in a large glazed porcelain tube T 

 in a gas-furnace when a higher temperature was used. 



'Richards and Baxter, Z. f. anorg. ch., 23, 245 (1900) ; Proc. Amer. Acad., 35, 253 

 (1900). 



2 Lebeau, Bull. Soc. Chim., Paris, (3) 27, 39-42 (1902); Ann. Chim. Phys. (7), 26, 

 5-31 (1902). 



3 T. W. Richards, Zeitschr. f. phys. Chem., 46, 189 (1903). 



4 W. Hempel, Z. f. ang. Chem. 14, 843 (1901). 



The generators used were zinc amalgam, dilute hydrochloric acid, platinum cells, 

 and were of the convenient form used by Cooke and Richards, Proc. Amer. Acad. 23, 

 168 (1887). The hydrogen was twice washed in strong caustic solution and dried 

 by passing through two caustic potash towers. 



6 Unglazed, because at the high temperature used the glaze frequently fused with 

 the iron. 



' Hempel water-cooled stoppers were used in the porcelain tube. 



