Accurate lattice constants for ferrous oxide (wustite) have been determined at 1oOO" over the composition range in which this non-stoichiometric phase is stable ; the limits of the range, at 10o0", have been established by extending the measurements into the two-phase regions on either side. The effects of varying conditions of heat-treatment have given information on the mode of decomposition of wustite into magnetite (Fe304) and iron.In preparation for later work, the curve of lattice constant against composition for solid solutions of wustite and manganous oxide has been established with increased accuracy. The effect of dissolved manganous oxide on the composition range for wustite has been determined at one Fe : Mn ratio.The measurements of lattice constant were carried out in conjunction with a thermodynamic study of the three-phase system : 0 2 (g) I " FeO+ MnO " solid solution I Fe+ Mn solid alloy.

Equilibrium oxygen pressures (at $SO", 1OOO" and 1150" C) have been determined for reduction of solid solutions of ferrous oxide (wustite) and manganous oxide to metal ; the equilibrium was established in controlled atmospheres containing hydrogen and water vapour, covering a wide composition range. Comparison of experimental oxygen pressures with calculated values shows that the oxide solid-solution is thermodynamically ideal, within the limits of error, over the whole range of composition. An "oxygenpressure meter '' based on reduction of a solid solution is envisaged.Thermodynamic activity relationships in solid solutions have received little study in the past, and few data exist to indicate whether such solutions may be considered, even approximately, as thermodynamically ideal. The results presented in this paper show, for the first time, that the activities of ferrous oxide (wustite) and manganous oxide in their solid solutions correspond closely with ideal behaviour over the whole composition range.Standard free-energies of formation of numerous pure metal oxides have been determined experimentally by measuring the equilibrium oxygen pressures relative to their dissociation into metal; these pressures are usually measured indirectly by observing the H20/H2 or CO21CO ratio in a gas atmosphere in equilibrium with oxide and metal at the desired temperature. Extensive compilations of freeenergy data (see, for example, Richardson and Jeffes 1) have resulted from such measurements, and accurate values are now available for the more important oxides, including wustite and manganous oxide. When an oxide solid solution

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