SOME PROPERTIES OF GASES 165 



109. Boyle's Law. // the temperature remains con- 

 stant, the pressure of a gas varies inversely as the volume. 

 If a cubic foot of gas is forced to occupy half as much 

 space, the pressure will be doubled. If the pressure is 

 increased from 10 pounds to 30 pounds, the volume of 

 the gas will be one-third of what it was. We have learned 

 that the pressure of a gas is due to the blows struck by 

 an enormous number of molecules moving at high speed. 

 To decrease the volume one-half is to double the density, 

 and when double the number of molecules strike against 

 the same area of the surface of the containing vessel the 

 pressure is doubled. By calculation it has been found 

 that air molecules under normal conditions move at the 

 rate of about 445 meters (1390 feet) per second, while 

 hydrogen molecules have the enormous speed of 1700 

 meters (5500 feet) per second. The speed of a cannon- 

 ball is seldom greater than 800 meters per second. Since 

 molecules move at such high speed, it is easy to under- 

 stand why gases produce pressure and move almost 

 instantly into the space left by the rising piston of 

 an air pump, and why any gas always fills completely 

 the vessel containing it. 



110. Compressed Gases. *- Gases are perfectly elastic 

 and can be compressed or made more dense; that is, a 

 given weight of gas can be made to occupy less space if 

 pressure is put upon it. According to Boyle's law, if 

 two cubic feet of gas are made to occupy only one cubic 

 foot of space, the pressure of the gas on the sides of the 

 containing vessel will be doubled if the temperature is 

 constant. 



The discovery of this property of gases has been very 

 useful. Compressed air is one of the best springs that 

 can be used. It would not be very comfortable to ride 



