The theory which underlies the process of drying is that dry air is capable of absorbing moisture; hence by circulating currents of dry air in and around wet substances, the absorbing power of the air draws off the moisture. For continuous drying, free circulation is a necessity, as air soon becomes saturated and incapable of taking up more moisture. Warming the air increases its capacity to absorb moisture; thus air at a high temperature is capable of drying material much more quickly than the same volume of air would at a low temperature. A free circulation of air at 85° to 100° F., evenly distributed, and with ample provision for the escape of the saturated air, is essential for good drying work.

Experience shows that when a liquid passes into a gaseous state it absorbs heat from the surrounding bodies. To illustrate: If a few drops of ether were placed on your hand you would notice the ether disappear in the form of a vapor by reason of the process termed evaporation, and your hand would feel cold. Evaporation produces coldness. Experience also shows that in condensing a gas by pressing the particles together, heat is given off. Thus the pressure on a gas, that is, its compression, generates heat, while the liberation of particles produces cold.

All gases may be liquefied by increasing the pressure sufficiently. If this pressure is suddenly removed the gas will evaporate quickly and expand, thereby absorbing heat and reducing the temperature of the surrounding bodies.

These scientific facts are taken advantage of in refrigerating plants, described in Chapter VIII (Properties Of Gases. 84. Gas Pressure And Industry), where ice is manufactured by means of the expansion of ammonia which is the most economical gas to liquefy.