154 Introduction to the Study of Science 



tightly inflated football or a tire tube feels cold. In the foot- 

 ball it was probably of the same temperature as the surrounding 

 air. When it is liberated from a high pressure into a space of 

 lower pressure it must expand. In expanding it uses enough 

 of its own heat to lower its temperature sensibly. 



Heat liberated in condensation. The second fact is that a 

 gas, when compressed and cooled below its boiling temperature, 

 condenses in giving up heat, and becomes liquid. The con- 

 densation of water vapor in the formation of clouds and rain, 

 with the release of the heat absorbed in evaporation, is an 

 example of the same fact on a large scale. When compression 

 and condensation are effected by mechanical means, the heat 

 that a gas contains is largely forced out of it. In the ice ma- 

 chine the expelled heat is taken up and carried away by running 

 water. 



In compensation for this loss, a liquid in expanding and 

 vaporizing again must take up precisely as much heat as it lost 

 in condensing and liquefying. If this heat is not provided 

 artificially, it must be taken from the vaporizing liquid and the 

 surrounding substances which are thereby cooled and perhaps 

 frozen. 



Application to refrigeration. With these facts concerning 

 the behavior of gases, the process involved in the manufacture 

 of ice and in refrigeration is easily understood. Ammonia 

 and certain other substances have their boiling temperatures 

 changed in accordance with the pressure to which they are 

 subjected. Pressure is readily controlled by mechanical means, 

 such as the compressor pump ; and the higher the pressure to 

 which such a substance as ammonia is subjected, the higher 

 becomes its boiling temperature (page 300 ff .) . The boiling tem- 

 perature of liquid ammonia at ordinary atmospheric pressure 

 is about 29 F. Under an absolute pressure of 156 pounds 

 (absolute pressure means " gage " pressure plus atmospheric 

 pressure or 14.7 pounds per square inch), ammonia boils at a 

 temperature of 80.8 F. Water at a much lower temperature 



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