Artificial Freezing, the reduction of the temperature of fluids to such an extent as to render them solid. It is usually applied to the freezing of water and of articles of food. There are two general methods of effecting artificial freezing, viz., by liquefaction and by vaporization and expansion. The method by liquefaction is performed by freezing mixtures, which are formed by mixing together two or more bodies, one or all of which may be solid. They are generally used in vessels having three or four concentric apartments: an inner one, containing the article to be frozen; one eccentric to this, containing the freezing mixture, provided with some contrivance for agitation; one again outside of this, filled with a non-conductor of heat, as powdered charcoal, gypsum, or cotton wool; and sometimes one between them for holding water. The following table contains a list of the more important freezing mixtures, with the reduction of temperature each is capable of effecting:

The method of freezing by vaporization and expansion depends upon principles explained in the articles Boiling Point, Evaporation, and Heat. Among the most efficient apparatus for conducting the process is that of M. Carre of France. A strong galvanized wrought-iron boiler, capable of sustaining a pressure of eight or ten atmospheres, is connected by a tube with a freezer, also made of galvanized iron and of corresponding strength, consisting of two compartments, an outer annular one, connected with the boiler, and an inner one, for receiving the vessel which contains the water or liquid to be frozen. The connection between the boiler and freezer may be controlled either by stopcocks or by self-acting valves. A saturated solution of ammonia is introduced into the boiler, and the freezer is placed in a cold bath. Heat sufficient to produce a pressure of five or six atmospheres is applied to the boiler, which expels the gas from the water in which it is dissolved, and forces it into the annular compartment of the freezer, where it is condensed by its own pressure, aided by the cool bath, along with about one tenth its weight of water.

When sufficient ammonia has been condensed, which is shown by the pressure indicated by a gauge, or approximately by a thermometer, the boiler itself is placed in a cold bath; the cylinder containing the water to be frozen is placed in the inner compartment of the freezer, and to insure contact the interstice is filled with alcohol. As the boiler cools, the pressure which had been produced by heat is gradually removed, and the liquid ammonia in the freezer becomes vaporized, producing an intense degree of cold. In a little more than an hour a block of ice may be frozen. An apparatus in use is said to be capable of producing 800 lbs. of ice in an hour.