------+-------------+-------------+--------------+--------------- | | Duration of | | Percentage of No. | Tube filled | experiment | Temperature. | | with | in hours. | | CO_{2} | CO ------+-------------+-------------+--------------+--------+------ I. | CO | 15 | 800° | 35.2 | 64.8 II. | CO | 18 | 530° | 29.1 | 70.9 III. | CO | 13 | 880° | 30.2 | 69.6 IV. | CO_{2} | 24 | 870° | 32.3 | 67.7 V. | CO | 18 | 760° | 36.9 | 63.1 VI. | CO_{2} | 16 | 820° | 34.7 | 65.3 VII. | CO_{2} | 18 | 730° | 41.1 | 58.9 VIII. | CO | 18 | 630° | 34.9 | 65.1 IX. | CO_{2} | 17 | 630° | 61.6 | 58.4 X. | CO | 18 | 540° | 25.0 | 75.0 XI. | CO_{2} | 25 | 540° | 36.5 | 63.5 ------+-------------+-------------+--------------+--------+------ As is evident from the above tables and from the curves in Fig. 121, the curve of equilibrium in the case of the reaction Fe_{3}O_{4} + CO = 3FeO + CO_{2} exhibits a maximum for the ratio CO : CO_{2}, at 490°, while, for the reaction FeO + CO = Fe + CO_{2} 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 Fe_{3}O_{4} can coexist with a mixture of 65.5 per cent. of CO_{2} and 34.5 per cent. of CO. If the partial pressure of CO_{2} is increased, there occurs the reaction 3FeO + CO_{2} = Fe_{3}O_{4} + 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 Fe_{3}O_{4} + CO = 3FeO + CO_{2} and all the ferric oxide can be made to disappear. We see, therefore, that Fe_{3}O_{4} can only exist at temperatures and in {308} 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.