PRESSURE IN GASES 43 



may be more nearly perfect. With this pump gases may 

 be rarefied to one-millionth of one atmosphere ( 50). 



52. Compression. When a force greater than the 

 pressure of the atmosphere is^ exerted upon any body 

 its molecules may be crowded nearer together; 

 the substance is then said to be compressed. In 

 general, solids bear almost no compression, and 

 liquids only a little ; but gases, whose molecules 

 are commonly far apart, may be compressed into 

 a small fraction of their usual volume. 



i 



53. Compressed Air. We have learned that 

 gases are elastic ( 23) ; moreover, they are 

 perfectly elastic. That is, when force has been 

 used to compress a gas, the gas will exert the 

 same amount of force in trying to return to its 

 former volume. It is owing to this fact that j [ 

 compressed air is so much used as a motive force. 

 Energy may be stored by forcing air into strong 

 tanks under heavy pressure ; the tanks are then carried 

 about, and work may be done by the force which the 

 air exerts when it is allowed to escape. Compressed-air 

 engines are run by this means. 



Experiment 39. Fill a bicycle tire with air by means of a 



a ft cycle pump (Fig. 27). Does it 



-Q |,'^ become harder to work the pump 



' as the tire becomes filled ? Why ? 



2g Press upon the tire from time 



to time with the finger. Does it 



become harder to dent the tire ? Is the tire more strongly elastic 

 when well filled ? 



Do the same things with a rubber football. 



