96 GENERAL PROPERTIES OF GASES 



sure we have reduced the volume one half ; similarly, if the 

 pressure is increased threefold, the volume will be reduced to 

 one third of its original volume. (See Laboratory Manual.) 



The pressure P of a gas at constant temperature is in- 

 versely proportional to the volume V of the gas. This can 

 be conveniently stated in another way as follows : If P and 

 P' denote the pressures corresponding to the volumes V and 



y, then 



PV= P'V 



90. Heat due to Compression. We saw in Section 89 

 that whenever the pressure exerted upon a gas is increased, 

 the volume of the gas is decreased ; whenever the pressure 

 upon a gas is decreased, the volume of the gas.is 

 increased. If the pressure is changed very slowly, 

 the change in the temperature of the gas is im- 

 perceptible ; if, however, the pressure is removed 

 suddenly, the temperature falls rapidly, or if the 

 pressure is applied suddenly, the temperature rises 

 rapidly. When bicycle tires are being inflated, the 

 pump b.ecomes hot because of the compression of 

 the air. The heat due to compression can be 

 shown very strikingly by means of the fire syringe. 

 Dip a piece of absorbent cotton in carbon bisul- 

 phide, and shake the cotton in a tube (Fig. 55) so 

 that a small amount of the carbon bisulphide 



Vfeheatof g ets mto tne tube tnen i nsert: the piston and 

 compression compress the air suddenly. So high temperature 



is sufficient . , , 111 , , . r * 



to ignite the 1S developed by the sudden compression of the air 

 carbon bi- that a flash f rom the carbon bisulphide is produced. 



sulphide. 



The amount of heat resulting from compres- 

 sion is surprisingly large ; for example, if a mass of gas at 

 o C. is suddenly compressed to one half its original volume, 

 its temperature rises 87 C. 



