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 + Fe_{3}O_{4} + 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 + Fe_{3}O_{4} + CO + CO_{2} at a temperature of about 670°, and then add carbon to the system, the reaction C + CO_{2} = 2CO will occur, because the concentration of CO_{2} is greater than what corresponds with the system FeO + Fe_{3}O_{4} + C in equilibrium with carbon monoxide and dioxide. In consequence of this reaction, the equilibrium between FeO + Fe_{3}O_{4} and the gas phase is disturbed, and the change in the composition of the gas phase is opposed by the reaction Fe_{3}O_{4} + CO = 3FeO + CO_{2}, which continues until either all the carbon {309} or all the ferric oxide is used up. If the ferric oxide first disappears, the equilibrium corresponds with a point on the dotted line in the middle area of Fig. 121, which represents equilibria between FeO + C as solid phases, and a mixture of carbon monoxide and dioxide as gas phase. If the temperature is higher than 685°, at which temperature the curve for C--CO--CO_{2} cuts that for Fe--FeO--CO--CO_{2}; then, when all the ferric oxide has disappeared, the concentration of CO_{2} is still too great for the coexistence of FeO and C. Consequently, there occurs the reaction C + CO_{2} = 2CO, and the composition of the gas phase alters until a point on the upper curve is reached. A further increase in the concentration of CO is opposed by the reaction FeO + CO = Fe + CO_{2}, and the pressure remains constant until all the ferrous oxide is reduced and only iron and carbon remain in equilibrium with gas. If the quantities of the substances have been rightly chosen, we ultimately reach a point on the dotted curve in the upper part of Fig. 121.