The activities of manganese in Mn-C, Mn-Si, Mn-Si-C_sat. and Mn-Fe-Si-C_sat. solutions at 1400°C have been determined. The transportation method was applied to the vapor pressure measurement. The results obtained are as follows:
(1) The values of the vapor pressure of pure manganese at 1350 and 1400°C in this work agreed approximately with those obtained by previous measurements in the literature.
(2) The activities of manganese in the Mn-C solutions, as referred to pure manganese were determined from the vapor pressure data. It was found that the activities of manganese in these solutions show a positive deviation from ideality at less than 0.17 of N_C and deviate negatively above this value.
(3) The activities of manganese in the Mn-Si solutions were determined in the same way. The activities of silicon as referred to pure silicon in the liquid state in the Mn-Si solutions were calculated from the activities of manganese obtained by means of the Gibbs-D\v{u}hem integration and conversion of the reference state. These activities showed a remarkable negative deviation from ideality. Some thermodynamic quantities at 1500°C were calculated on the assumption that the Mn-Si solutions are considered to be a regular solution.
(4) The activities of manganese obtained in Mn-Si-C_sat. and Mn-Fe-Si-C_sat. solutions were expressed in the form of an activity curve and isoactivity curves, respectively.