ACTIVITY OF IRON OXIDE IN CaF//2-CaO-Fe//tO AND CaCl//2-CaO-Fe//tO MELTS.

The EMF method with a solid electrolyte (ZrO//2 plus 9 mol%MgO) was used for determination of the activity of Fe//tO in CaF//2-CaO-Fe//tO (1673-1733 K) and CaCl//2-CaO-Fe//tO (1623-1673 K) systems. No appreciable effect on Fe//tO activity was found when Fe//tO was replaced by CaF//2 or CaCl//2 at a constant CaO content. The Fe**3** plus /Fe**2** plus ratio increased with increasing CaF//2 content and decreased with increasing CaCl//2 content.