<Original Papers>On the Molar Volume of the MgF_2-CaF_2 Binary System in Solid and Liquid States

The density of the molten MgF_2-CaF_2 system was measured by the Archimedean method with two bobs. In addition, the linear thermal expansivity in the solid state was measured by an improved sessile drop method which consists of an arrangment to observe it from three directions. The density in the solid state was obtained from the expansivity and the density which was measured at room temperature. The eutectic temperature of this system is considerably lower than the melting point of the pure components, about 270℃ lower than the melting point of MgF_2 and about 430℃ lower than that of CaF_2. It is rather difficult, therefore, to discuss the change in volume with composition under isothermal conditions in which only the liquid exists. This is due to the fact that the gap between the above temperature and the liquidus temperature also varies greatly with composition. Thus, it is proposed that the true effect of composition can be observed along a temperature contour directly related to the liquidus curve. It is difficult to define the volume change upon fusion in the case of a solid-liquid mixture due to the the presence of the two, coexisting phases. According to the experimenatal results, the volume change upon fusion can be reasonably defined by extrapolating the liquid volume to the eutectic temperature and comparing this with the solid volume