A Practical Manual On Sea Water Distillation
With A Description Of The Necessary Machinery For The Process

140 SEA WATER DISTILLATION. the purpose required—viz., that of abstracting the latent heat of the steam. 11. There is only one other medium which suggests itself for cooling purposes, ready to hand, and that is the air. This, however, as a cooling medium, is out of the question, for the following reasons :—First, its weight, as compared with that of water, would necessitate an enor- mous amount of surface. Thus, 1 cubic foot of air, at normal pressure, weighs 0'071 lb., as against 64 lbs., the weight of 1 cubic foot of sea water, so that, to begin with, one would require about 100 times more surface with air than with water. Secondly, apart from the in- feriority of weight, is its inferiority in specific heat. Air has only a specific heat of 0'238 when at a constant pressure, and only 0'169 when at a constant volume. So that when comparing what is required for using air in- stead of water as a cooling medium, both these inferiorities, as compared with water, would have to be allowed for. Note.—Specific heat, it may not be out of place to observe, is the capacity for heat which a substance has as compared with the same weight of water. Thus if the amount of heat required to raise 1 lb. of water 1° (391° to 40' 1° F.) is one unit (1 B.T.U.), it would take only 0'238 B.T.U. to heat 1 lb. of air (if kept at the same pressure), or 0'163 B.T.U. to heat 1 lb. of air (if kept at the same volume), the above being the heat capacity, or power of absorbing heat which air has ; its power of emitting heat would therefore be in the same ratio (reversed) with regard to water. 12. The following short Table of a few substances with their specific heat given (according to Petit, Dulong, and others) may be found useful:—