sosman: problems of the oxides of iron 57 



been that the product was a mixture of iron (or iron carbide) 

 with magnetite or with some oxide intermediate in composition 

 between FeO and Fe 3 4 . The best work on FeO has been that 

 done by Hilpert, and his conclusion, likewise, is that the vari- 

 ous methods commonly cited do not yield FeO, although the 

 product sometimes has an average composition nearly equiv- 

 alent to the composition FeO. 



Hilpert and Beyer 5 found that by reduction of Fe 2 3 by hydro- 

 gen saturated with water vapor they could obtain products 

 which were richer in ferrous iron than Fe 3 4 , but which con- 

 tained no metallic iron. The higher the temperature of reduc- 

 tion, the higher the percentage of "FeO." A product containing 

 only 1.5 per cent of Fe 2 3 was obtained by reduction at 1100°; 

 at 700° the upper limit was 85 per cent of FeO. The reaction 

 velocity is so slow, as equilibrium is approached, that it is difficult 

 to say what the final solid phases would be. Hilpert believes 

 that solid solutions exist between Fe 3 4 and FeO, perhaps with 

 limited miscibility, or even with intermediate compounds. 



We can find a possible explanation of the case of FeO by ref- 

 erence to the somewhat analogous oxides of two other metals 

 of the eighth group, namely, platinum and iridium. 6 The evi- 

 dence indicates that the oxide IrO, within a certain range of 

 temperature, has a higher dissociation pressure than the oxide 

 Ir0 2 ; hence IrO will spontaneously change into a mixture of 

 Ir0 2 and metallic iridium, according to the reaction: 



2IrO = Ir0 2 + Ir 



Thus, if FeO happens, over some particular range of tempera- 

 ture, to have a higher dissociation pressure than Fe 3 4 , it is 

 possible for the following reaction to occur: 



4FeO = Fe 3 4 + Fe 



The existence of such a relation would explain many of the ex- 

 perimental results already obtained, such as the fact that, in 



5 Hilpert, S., and Beyer. J. Ber. deu. chem. Ges., 44: 1608-1619. 1911. 



6 Wohler and Witzmann. Zs. Elektrochem., 14: 97-107. 1908. Wohler and 

 Frey. Ibid.,. 15: 129-142. 1909. A solid solution of Ir0 2 with IrO or Ir forms, 

 rather than the pure oxide Ir0 2 . 



