this ratio has a minimum value at 680°. From these curves can be derived the conditions under which the different solid phases can exist in contact with gas. Thus, for example, at a temperature of 690°, FeO and Fe3O4 can coexist with a mixture of 65.5 per cent. of CO2 and 34.5 per cent. of CO. If the partial pressure of CO2 is increased, there occurs the reaction
3FeO + CO2 = Fe3O4 + CO
and if carbon dioxide is added in sufficient amount, the ferrous oxide finally disappears completely. If, on the other hand, the partial pressure of CO is increased, there occurs the reaction
Fe3O4 + CO = 3FeO + CO2
and all the ferric oxide can be made to disappear. We see, therefore, that Fe3O4 can only exist at temperatures and in
contact with mixtures of carbon monoxide and dioxide, represented by the area which lies below the under curve in Fig. 121. Similarly, the region of existence of FeO is that represented by the area between the two curves; while metallic iron can exist under the conditions of temperature and composition of gas phase represented by the area above the upper curve in Fig. 121. If, therefore, ferric oxide or metallic iron is heated for a sufficiently long time at temperatures above 700° (to the right of the dotted line; vide infra), complete transformation to ferrous oxide finally occurs.
In another series of equilibria which can be obtained, carbon is one of the solid phases. In Fig. 121 the equilibria between carbon, carbon monoxide, and carbon dioxide under pressures of one and of a quarter atmosphere, are represented by dotted lines.[[379]]
If we consider only the dotted line on the right, representing the equilibria under atmospheric pressure, we see that the points in which the dotted line cuts the other two curves must represent systems in which carbon monoxide and carbon dioxide are in equilibrium with FeO + Fe3O4 + C, on the one hand, and with Fe + FeO + C on the other. These systems can only exist at one definite temperature, if we make the restriction that the pressure is maintained constant (atmospheric pressure). Starting, therefore, with the equilibrium FeO + Fe3O4 + CO + CO2 at a temperature of about 670°, and then add carbon to the system, the reaction
C + CO2 = 2CO
will occur, because the concentration of CO2 is greater than what corresponds with the system FeO + Fe3O4 + C in equilibrium with carbon monoxide and dioxide. In consequence of this reaction, the equilibrium between FeO + Fe3O4 and the gas phase is disturbed, and the change in the composition of the gas phase is opposed by the reaction Fe3O4 + CO = 3FeO + CO2, which continues until either all the carbon