A high-temperature cloud chamber is described in which a bead of alkali halide is supported on a heater coil mounted in the roof. By passing the current through the coil the temperature of the bead may be momentarily raised by several hundred degrees, producing salt vapour at high supersaturation. Condensation ensues in the presence of the inert supporting gas, and clouds of droplets or solid particles appear depending on the chamber temperature. Light scattered from the clouds under strong illumination is examined with a telescope, and the presence of crystalline particles is detected by their capacity to scintillate, or ‘twinkle’. It is found that twinkling in clouds of alkali halides appears sharply as the temperature is lowered below the melting point, defining a critical temperature of solidification for each salt. Reasons are given for regarding this temperature as the freezing threshold of molten salt droplets, for which supercoolings of about 150 °C are indicated. A reduced temperature, given by the ratio of the freezing threshold to the melting point, has the value of approximately 0.8 for all the alkali halides examined.
Relations between the Thermodynamic Properties of Alkali Halides at the Melting Point
