CHAPTER XXXII.

ICE MACHINES. GENERAL PRINCIPLES--FREEZING MIXTURES--PERKINS’ ICE MACHINE, 1834-- PICTET’S APPARATUS--CARRÉ’S AMMONIA ABSORPTION PROCESS--DIRECT COMPRESSION AND CAN SYSTEM--THE HOLDEN ICE MACHINE--SKATING RINKS-- WINDHAUSEN’S APPARATUS FOR COOLING AND VENTILATING SHIPS. Very few people have any correct conception of the principles of ice-making. Most persons have heard in a vague sort of way that chemicals are employed in its manufacture, and many a fastidious individual has been known to object to artificial ice on the ground that he could taste the chemicals, and that it could not therefore be wholesome. Such is the power of imagination, and such the misconception in the public mind. Nothing could be more erroneous, nor more amusing to the physicist, since no chemicals ever come in contact with either the water or the ice. An intelligent understanding of the operations of an ice machine involves only a correct appreciation of one of the physical laws governing the relation of heat to matter, and the forms which matter assumes under different degrees of heat. We see water passing from solid ice to liquid water and gaseous steam, by a mere rise in temperature, and conversely, by abstraction of heat, steam passes back to water, and then to ice. When one’s hands get wet they get cold. A commonplace, but convenient proof of this is to wet the finger in the mouth and hold it in the air. A sensible reduction of temperature is instantly noticeable. A more pronounced illustration is to wet the hands in a basin of water, and then plunge them in the blast of hot, dry air coming from a furnace register. Instead of warming the hands, as many would suppose, this will, as long as the hands are wet, produce a distinct sensation of increased cold. It is due to rapid evaporation, which in changing the water from a liquid to a gaseous form, abstracts heat from the hands. Evaporation may be effected in two ways. The common one is by applying extraneous heat, as under a tea kettle, in which case the evaporated vapor is hot by virtue of the heat absorbed from the fire. The other way is to reduce pressure or produce a partial vacuum over the liquid without any application of heat, in which case the vapor is made cold. As early as 1755 Dr. Cullen observed this, and discovered that the cold thus produced was sufficient to make ice. An incident of evaporation is the passing from the limited volume of a liquid to the greatly increased volume of a gas. Water, for instance, when it changes to a vapor, increases in volume about 1,700 times; that is, a cubic inch of water makes about a cubic foot of steam, and when evaporation takes place from a reduction of pressure, this involves a dissipation of heat throughout the increased volume, and the corresponding production of cold. When, however, matter changes from a liquid to a gas, or from a solid to a liquid, a peculiar phenomenon manifests itself, in that a great amount of heat is absorbed and, so far as the evidence of the senses goes, disappears in the mere change of state. It is called _latent heat_. In such case the heat becomes hidden from the senses by being converted into some other form of intermolecular force not appreciable as sensible heat, and producing no elevation of temperature. In illustration, if a pound of water at 212° F. be mixed with a pound of water at 34° (both being matter in the same state), there results two pounds of water at the mean temperature of 123°. If, however, a pound of water at 212° be mixed with a pound of _ice_ at 32° (matter in another state), there will not be two pounds of water at the mean temperature of 122°, as might be expected, but two pounds at 51° only, an amount of heat sufficient to raise two pounds of water 71° being absorbed in the mere change of ice to water without any sensible raise in temperature. This absorbed heat is called latent heat, and it plays an important part in artificial freezing. A familiar illustration of the absorption of heat in changing from a solid to a liquid is found in the admixture of salt and ice around an ice-cream freezer. These two solids, when brought together, liquefy rapidly with an absorption of heat that produces in a limited time a far greater degree of cold than that which could be obtained from the ice by mere conduction, since the reduction of temperature proceeds faster by liquefaction than can be compensated for by the absorption of heat from the air. Were this not true, ice cream could not be frozen by a mixture of salt and ice. Many such freezing mixtures are known, and a few have been made commercially available, but they cannot be economically employed in ice-making, and it is therefore only necessary to consider the development of the more common principle of evaporation and expansion, in which the change from a liquid to a gas occurs. The volatile liquid which was first employed was water, but as it did not vaporize as readily as some other liquids, more volatile substitutes were soon found, among which may be named ether, ammonia, liquid carbonic acid, liquid sulphurous acid, bisulphide of carbon and chymogene, which latter is a petroleum product lighter and more volatile than benzine or gasoline. As these liquids were expensive, it is obvious that their vaporization could not be allowed to take place in the open air, since the reagent would thus be quickly dissipated and lost, and hence means were devised to condense and save this valuable volatile liquid to be used over again. The vaporization of the volatile liquid to produce cold, and its re-condensation to liquid form to be used over again in an endless cycle of circulation, seems to have been first effected by Mr. Perkins, of England, whose British patent No. 6,662, of 1834, affords a simple and clear illustration of the fundamental principles of most modern ice machines. His apparatus is shown in Fig.