UNEXPECTED TRANSFORMATIONS 97 



91. Cooling by Expansion. If a gas expands suddenly, its 

 temperature falls ; for example, if a mass of gas at 87 C. is 

 allowed to expand rapidly to twice its original volume, its 

 temperature falls to o C. If the compressed air of a bicycle 

 tire is allowed to expand and a sensitive thermometer is held 

 in the path of the escaping air, the thermometer will show a 

 decided drop in temperature. 



The low temperature obtained by the expansion of air or 

 other gases is utilized commercially on a large scale. By 

 means of powerful pistons air is compressed to one third or 

 one fourth its original volume, passed through a coil of 

 pipe surrounded with cold water, and then allowed to 

 escape into large refrigerating vaults, which thereby have 

 their temperatures noticeably lowered, and can be used for 

 the permanent storage of meats, fruits, and other perishable 

 material. In summer, when the atmospheric temperature is 

 high, the storage and preservation of foods is of vital impor- 

 tance to factories and cold storage houses, and but for the low 

 temperature obtainable by the expansion of compressed gases, 

 much of our food supply would be lost to -use. 



92. Unexpected Transformations. If the pressure on a gas 

 is greatly increased, a sudden transformation sometimes 

 occurs and the gas becomes a liquid. Then, if the pressure 

 is reduced, a second transformation occurs, and the liquid 

 evaporates or returns to its original form as a gas. 



In Section 23 we saw that a fall of temperature caused 

 water vapor to condense or liquefy. If temperature alone were 

 considered, most gases could not be liquefied, because the tem- 

 perature at which the average gas liquefies is so low as to be 

 out of the range of possibility; it has been calculated, for 

 example, that a temperature of 252 C. below zero would have 

 to be obtained in order to liquefy hydrogen. 



Some gases can be easily transformed into liquids by pres- 

 CL. GEN. sci. 7 



