164 



REFRIGERATION AND ITS USES 



cold is produced. This last way of speaking is really not 

 incorrect if we fully realize that cold is simply absence of heat. 

 It is better, however, to speak of the heat's being absorbed. 

 We should also recognize that it is the heat of vaporization 

 (Art. 150) which is absorbed. We also saw that those liquids 

 which evaporate most rapidly absorb heat most rapidly; they 

 feel the coldest on the back of the hand. The substances 



FIG. 124. Diagram of ah ice factory. 



studied were water, alcohol, and gasoline. But each of these 

 substances is a liquid at ordinary temperatures, and each 

 of them boils under the pressure of the atmosphere at a 

 temperature higher than that of the air about us. (Recall 

 the boiling temperature of each of these substances.) Evi- 

 dently, then, each of these three substances will change 

 from the liquid form to vapor form rapidly, that is, it boils 

 only when raised to a rather high temperature. Further, it 

 is only when a liquid is boiling that it absorbs the heat of 

 vaporization rapidly. 



We see now that if we can obtain some substance in the 

 liquid form which evaporates very rapidly, or even boils, at a 



