Efforts have been directed to expediting the spontaneous evaporation of water; a reduction of pressure being effected by a vacuum pump, and the vapors removed by a suitable absorbing medium. The utilization of cold obtained by the evaporation of other liquids, more volatile than water, have received attention. There have been discovered a number of freezing mixtures, some of which produce wonderfully low temperatures. The addition of salt to broken ice is the best known of these; it is commonly employed in making ice creams and ices, and is of great commercial importance as applied to refrigerator cars and cold storage houses. Of all these methods the compression, absorption and air machines, and the freezing mixture of ice and salt, have entered into commercial uses in this country. The details of the latter method are referred to in Chapter VI. Regarding the others, some account of the principal operations involved, and of the natural phenomena upon which they are based, will be given. PRINCIPLES OF ICE MACHINES.--It has been observed of gases, that by compressing them to a fraction of their original volume, heat was produced. If the compression was great, most gases were liquified by it. A few gases were found which would not liquify, and were designated as permanent. By relieving the pressure, gases will resume their original bulk, and the heat of compression is gathered up or reabsorbed by them. There is, naturally, a constant tendency toward the preservation of an equilibrium of temperature among the atoms of any body, and also between different bodies. This exchange is carried on upon a grand scale, in the economy of nature. Where the difference in degree is small the exchange is effected slowly, but where it is great the initial transfers proceed rapidly and with vigor. By means of efficient insulation we can cut off, in a great measure, a room, or a part of a room, from the influence of the outer, or general exchanges of temperature. By this means we are able to bring this natural law into service, by controlling the exchanges of temperature of the objects we place in such rooms. By the union of heat with a fluid, the latter is converted into a vapor, and the abstraction of heat from a gas converts it into a liquid. When a fluid or a gas is at the temperature at which a change in its condition is effected, the continued application or withdrawal of heat does not increase or diminish this temperature. This heat is termed latent, or hidden. The temperature at which ebullition takes place varies greatly with different liquids. Water boils at 212° F., while ammonia boils at 32° F. Some substances act as absorbents. Water will absorb about seven hundred times its bulk of ammonia gas when they are brought in contact.