160 THE REALITIES OF MODERN SCIENCE 



that any individual molecule is moving with a higher 

 velocity as that it is moving with a lower velocity than 

 the average. We may therefore deal with the average 

 velocity. 



What is the effect of a molecular impact against 

 the box? If the molecules were inelastic like lumps 

 of putty they would not bound back from the walls 

 of the box. Their kinetic energy would be imparted 

 to the molecules of the walls and they would cease to 

 move, dropping under gravitation to the bottom of 

 the box. The pressure they exert would then decrease 

 to zero. If they are elastic they bounce back. In 

 order, however, that the pressure shall remain constant 

 as time goes on they must maintain the same average 

 velocity, and hence on the average they must bounce 

 back with a velocity numerically equal to that of their 

 approach. Otherwise the phenomenon would be like 

 that taking place if the door of a room were opened 

 while a large number of tennis balls were thrown in. 

 The door might then be closed and the balls left to 

 bounce back and forth. As one strikes the wall, how- 

 ever, it does not bounce with its original velocity, 

 because some of its kinetic energy is converted into 

 molecular motions in itself and in the wall. The 

 temperature of the balls and the walls then rises as 

 the bulk motion of the balls ceases. That the kinetic 

 energy which is subtracted from a moving body by a 

 collision is manifested as heat is well illustrated by the 

 rise in temperature which occurs when a piece of metal, 

 e.g. a nail, is pounded with a hammer. 



If the average velocity of the rapidly moving mole- 

 cules of an inclosed gas is not to change, two conditions 



