Refrigeration 153 



cleansing. But we need to consider here its properties and 

 behavior in relation to refrigeration. 



Ammonia in liquid form evaporates very rapidly ; and the 

 rate of evaporation is greatly accelerated by additional heat 

 or by currents of warm or cold air. Heat is necessary in chang- 

 ing a solid into a liquid, and a liquid into a vapor or gas. But 

 in the case of many substances evaporation may be promoted 

 by mechanical means, as release from high into low pressure, 

 but with no supply of artificial heat. The heat, which is in- 

 dispensable, must be derived from somewhere else. This fact 

 may be strikingly illustrated in the following experiment. 



Exercise : Heat absorbed in evaporation. Pour enough water 

 upon a board or the table top, if level, to make a small pool. Upon 

 this pool place a thin glass dish containing a quantity of sulfuric ether. 

 Apply a forced draft of air, as with a pair of bellows or a fan, to hasten 

 evaporation of the ether. Evaporation requires heat. With forced 

 or hastened evaporation the necessary heat cannot all be taken from 

 the air. It must come from adjacent substances. In a very short 

 time the glass is frozen fast to the board. Heat has evidently been 

 taken from the liquid ether, from the glass, from the water in which 

 the glass is placed, and from the board. 



This illustrates exactly what occurs in an ice-making ma- 

 chine. The chief difference is in the mechanical appliances by 

 which vaporizing is accelerated, and in the use of ammonia 

 instead of ether. 



Heat absorbed in expansion of gases. Besides evaporation, 

 which, as we have seen, may be promoted by mechanical means, 

 there are two other facts in the behavior of a gas which must be 

 emphasized in this connection. The first is that a gas under 

 the influence of heat tends to expand and occupy more space. 

 When it expands with no aid from artificial heat, it must get 

 the necessary heat for expansion from itself and from adjacent 

 substances. The heat it takes from itself and uses in expanding, 

 causes directly a sensible reduction in its own temperature. 

 This you know from common experience. Air expelled from a 



