68 



HEAT 



BICYCLE' 



FIG. 58. 

 Bicycle Pump. 

 When the 

 handle i 

 raised, air en- 

 ters around 

 the piston. 



are working against the energy of the molecules. There- 

 fore the compression of a gas liberates heat. This is 

 illustrated in the compression pumps (Fig. 58) 

 used to fill the tires of bicycles and automo- 

 biles. They become hot. 



Since compressing a gas heats it, expansion 

 cools it. The air rising from the earth in con- 

 vection currents ( 67) is cooled as it ascends. 

 This is because its volume increases as the 

 atmospheric pressure becomes smaller. The 

 heat needed to increase the volume of the 

 rising air is taken from the air itself. In the 

 balloon ascent described in 41 the temperature at 7 

 miles height was 60 F. 



To change a gas into a liquid we can (1) force the molecules together 

 by increasing the pressure; or (2) make the motion of the molecules 

 smaller by lowering the temperature (Fig. 

 59); or (3) combine both methods. 



The liquefying of air requires both a 

 very low temperature and a great pressure. 

 The method consists, first, in compressing 

 the air greatly in long tubes; then in re- 

 moving the heat produced by the com- 

 pression; and, finally, in allowing some of 

 the compressed air to expand. The heat 

 needed to produce the expansion comes 

 from the air that is still under pressure. The removal of this heat 

 cools the air until it condenses in drops of liquid. 



72. Kindling Temperature. All of us know that 

 fires must be " started " by some hot or burning body. 

 The temperature at which a substance begins to burn is 

 called its kindling temperature. A piece of iron picture 



Sulphur Dioxide 



"Gas 



FIG. 59. 



Liquefying Gaseous Sulphur 

 Dioxide by Cooling It. 



