Freezing Mixtures are mixtures of various substances employed for the production of low temperatures. Most depend upon the fact that when a solid liquefies, energy is in general absorbed, and hence the temperature falls. Thus, when potassium sulphocyanide is dissolved in cold water to form a concentrated solution, the temperature falls to - 20° C. If ice, or snow, and salt be mixed together, a strong solution of brine is obtained, and the temperature sinks to - 23° C. To obtain the greatest lowering, the proportions of ice and salt, by weight, should be about 3 to 1. If calcium chloride crystals be mixed with an equal weight of snow, the temperature falls to - 45°, a point at which mercury-solidifies. Also for the production of low temperatures, advantage is frequently taken of the fact that, if a liquid evaporates, energy is absorbed and heat disappears. If, therefore, the evaporation be rapid, the temperature will fall. In this way liquid C02 may be converted into a snow-like solid, which, when mixed with ether, forms a very powerful freezing mixture. By the rapid evaporation, also, of methyl chloride (a liquid boiling .at- - 22° O), extreme cold may be produced, and if a current of air cooled by this means be passed through liquid ethylene, the latter evaporates, and such intense cold is obtained as to cause the liquefaction of oxygen and ordinary air.