=188. Artificial Cooling.=--The fact has been brought out that when a solid is melted, a certain amount of heat, called the heat of fusion, is absorbed or disappears. This absorption of heat is also noticed when a solid is liquefied by dissolving it in a liquid as well as when it is liquefied by simply applying heat. Thus if some table salt is placed in a tumbler of water the temperature of the solution is lowered several degrees below that of the salt and water used. The liquefaction or solution of the salt has been accompanied by an absorption or disappearance of heat. This heat has been taken from the salt and from the water, resulting in a lowered temperature. Sal ammoniac or ammonium nitrate when dissolved in water produce a much more marked cooling effect than does table salt. The dissolving of a crystal in a liquid is something like evaporation, except that the molecules of the liquid attract the molecules of the solid and thus assist the change of state. =189. Freezing Mixtures.=--If one attempts to freeze a solution of salt and water, ice will not form at 0°C. but several degrees lower. The ice formed however is pure. Evidently the attraction of the molecules of salt for the water molecules prevented the formation of ice until the motions of the water molecules had been reduced more than is necessary in pure water. As the temperature of freezing water is that of melting ice, ice in a salt solution melts at lower temperature than in pure water. In a saturated salt solution this temperature is -22°C. It is for this reason that the mixture of ice and salt used in freezing cream is so effective, the salt water in melting the ice, being cooled to a temperature many degrees below the freezing point of the cream. The best proportion for a freezing mixture of salt and ice is one part salt to three parts of finely powdered or shaved ice. =190. Refrigeration by Evaporation.=--Intense cold is also produced by permitting the rapid evaporation of liquids under pressure. Carbon dioxide under high pressure is a liquid, but when allowed to escape into the air evaporates so rapidly that a portion of the liquid is frozen into solid carbon dioxide which has a temperature of -80° C. The evaporation of liquid ammonia by permitting it to escape into a pipe, under reduced pressure, is used on a large scale as a means of producing cold in cold storage and refrigeration plants. (See Fig. 157.) [Illustration: FIG. 157.--Diagram of a refrigerating system.]