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Artificial Cold (When a solid body becomes)

When a solid body becomes liquid, a liquid vapor, or, when a gas or vapor expands, heat is abstracted from neighboring bodies, and the phenomena or sensation of cold is produced. Evaporation produces cold, as is seen familiarly in the chilliness caused by a draught of air blowing on the moist skin. Water may be cooled to 60°, in warm climates, by keeping it in jars of porous earthenware; a flower-pot, moistened and kept in a draught of air, will keep butter, placed beneath it, hard in warm weather. In India water is exposed at night in shallow pans, placed on straw in trenches, and freezes even when the thermometer does not fall below 40° Water may be frozen by its own evaporation under the receiver of an air-pump over sulphuric acid; the process is a delicate one, and not adapted for use on the large scale.

Twining's ice-machine freezes water by the evaporation of ether, aided by the vacuum produced by a pump worked by a steam-engine. The same ether may be used over again indefinitely.

The apparatus works well, but, in case of a leak, the ether vapor, mixed with air, would explode; there is always danger of fire.

Carré's Apparatus Freezes by the evaporation of liquid ammonia, the ammoniacal gas produced being absorbed by water which will take up over 500 times its bulk of the ammonia, which it gives out again on heating. As liquid ammonia boils at 42° below zero, an intense cold is produced. This apparatus is efficient, but as the internal pressure rises sometimes to over 100 lbs. to the inch, it is not quite safe, although no accidents have yet been reported.

Compressed Air.

Air, when compressed, gives out heat which is re-absorbed when it is allowed to expand. By forcing the air into a strong receiver and carrying off the heat developed by a stream of water, it may, on expanding, re-absorb enough to reduce the temperature below 32°. It is thus used in the paraffine works in England, and would be an excellent method of at once ventilating and cooling large buildings.

Freezing Mixtures Depend upon the conversion of solid bodies into liquids. There are two classes, those used with out ice and those in which it is employed. Where extreme cold is required, the body to be frozen should be first cooled as much as possible by one portion of the mixture, and then by a succeeding one.