2924.     RUFF, O., AND GERSTEN, E.  [Triferrocarbide (Cementite) Fe3C.]  Ber. deut. chem. Gesell., vol. 45, 1912, pp. 63-72; Chem. Abs., vol. 6, 1912, p. 1409.

        Campbell (Jour. Iron Steel Inst. (London), vol. 59, 1901, p. 217) and others hold that Fe3C is an exothermic compound, 8,940 cal. being given off in its formation.  Ruff (abs. 2922) believes that it is endothermic and reports the following experimental work in confirmation of his theory.  Pure Fe3C, prepared by Werkmeister’s method (Dissertation, Karlsruhe, 1910), is dark gray; its hardness is 3.2-33, sp. gr. 7,396 at 21°, mol. vol. =1/7=24.34.  The molecular volume for 3 Fe and C is 27.84, giving a molecular contraction of 3.5.  Burning Fe3C with O2 in a Berthelot bomb, under a pressure of 25 atm., gave 375.1 cal. for 1 mol. Fe3C.  In burning Fe to Fe3O4 and C to CO2, 360 cal. are evolved, a difference of 15.1 cal.  Therefore, the heat of formation of Fe3C is 15.1 cal., and Fe3C is endothermic.  3Fe+C→Fe3C+15.1 cal.