FREEZING MIXTURES, AND OTITER :MEANS OF COOLING. When matter passes from the solid into the liquid state, heat in large quantity disappears, and ceases to affect the thermometer. See HEAT. The chemist avails himself of the fact that heat disappears during liquefaction, for the purpose of procuring artificial cold. When a piece of ice having a temperature of 32' F. is placed in its own weight of water at 174', we find, on testing time water with the thermometer after the ice has melted, that its temperature is 32'; the heat which the water contained having disappeared during the melting of the ice. As water in passing from the solid to the fluidstate possesses the property of ren dering latent a greater amount of heat than any other substance, it is, when in a solid form, as ice or snow, or when combined with salts, as water of crystallization, a power ful agent in producing artificial cold.
The substance employed in freezing mixtures should be finely powdered, rapidly mixed, and placed in vessels with little conducting power. The following are a few of the important fornnilm for these mixtures: 1. A mixture of 2 parts of pounded ice or of fresh snow and 1 part of common salt, causes the thehnotneter to fall to — 4*. 2. A mixture of 5 parts of commercial hydrochloric acid and 8 parts of powdered crystallized sulphate of soda, causes a reduction.of temperature from 50' to 0'. 3. Equal parts of water, of powdered crystallized nitrate of ammonia, and of powdered crystallized car bonate of soda, produce a cold of = 7'. 4. A mixture of 3 parts of crystallized chloride of calcium, previously cooled to 32', and 2 parts of snow, produces a cold of which is sufficient to freeze' mercury.- Z. By solid carbonic acid, or solid nitrous oxide gas, in sulphurie ether, tediperatures of from — 120' to — 146° may be obtained, at which alcohol passes to the consistency of oil, and finally to that of melted wax. This is the most powerful freezing Mixture that is known.
The freezing mixtures used by confectioners and those that are most ocLivenient for ordinary experimental purposes. are the first and second of the above list.
When matter passes from the liquid to the ac‘riform stale, heat also disappears, and the knowledge of this fact has been applied to the cooling of liquids, and ,to the actual production of ice. If a glass bottle containing water be covered with a cloth, which is kept constantly wet by the application of water, the evaporation from the wet cloth will soon diminish the temperature of the contents of the bottle, and if the cloffiwere moist ened with alcohol or with ether, the cold would be proportionally greater, the degree of cold varying with the rapidity and extent of the evaporation. Wine-coolers, or water coolers, made of porous earthenware, act in the same manner as the cloth. They are soakeddn, and saturated by water, which by its evaporation occasions cold. Coolers of this kind are common in most hot countries. On the ancient monuments of Egypt, a man is sometimes represented as fanning these vessels with a palm-leaf, to promote evaporation, and the Arabs in that country still practice this custom. See REFRIGER ATING MAciHNEs.
In some parts of India, where the dryness of the air allows a considerable evapora tion to take place, ice is obtained in the following manner: "FlaL•shallow excavations, from 1 to 2 ft. deep, are loosely lined with rice-straw, or some similar bad conductor of heat, and upon the surface of this layer are placed shallow pans of porous earthenware, filled with water to the depth of 1 or 2 inches. Radiation (see HEAT) rapidly reduces the temperature below the freezing-point, and ice is formed in thin crusts, which are removed as fast as they are produced, and stowed away in suitable ice-houses."—Mil ler's Elements of ,Chernistrg, 2d ed., vol. i. p. 220.