LIQUEFACTION PROCESS. Liquefaction is one of the most ancient methods employed for arti ficial cooling. The reduction of the temperature of water by the melting of saltpetre is said to have been known in India at a very remote period. The Romans are said to have cooled wine by immersing the bottle containing it in a second vessel filled with cold water, into which salt petre was gradually thrown. while at the same time the bottle was rapidly rotated. Freezing water by the use of a mixture of snow or pow dered ice and saltpetre was mentioned by Litinus Tancredus in 1607, and wine by means of snow and common salt by Santorio in 1926. The best among the many forms of apparatus for making ice on this principle are probably those of To.elli and Siemens. In Toselli's machine the frigeritie agent employed is a mixture of ammonium ni trate and water, which produces a reduction temperature of about F. The apparatus con sists of it vessel in which the solution of the ammonium nitrate i; effected, and of a can wherein are placed a number of circular molds of different sizes. These molds previously tilled with water are inserted in the freezing Mixture and a thin film of ice is formed round their edges; these tapered tubes of ice are then with drawn from the molds and placed one inside the other. thus forming a snmll stick of ice. In
Siemens's apparatus the frigerifie agent is eal eium chloride, whose dissolution in water pro duces a reduction of temperature of only 30° F., and to admit. of this reduction being sullieient to produce ice with water at an initial temperature of 65° F., a heat interchanger is provided in which the spent liquor. which is at a temperature of about 30° F., is employed to cool the water before it is mixed with the salt. The Siemens apparatus has been used in making artificial ice with much success, but, being less economical than more modern icc-machines, has never conic into general use. When these devices or others of the same type are used for cooling purposes brine is cooled in them and then circulated in the usual manner through a system of circulating pipes. The general law governing the production of cold by frigerific mixtures is that during the liquefaction of a solid a certain amount of heat not indicated by or sensible to the thermometer is absorbed, which heat is abstracted from any surrounding bodies. The absorption of heat from the surrounding bodies is the greater the more rapidly the solid is liquefied.