STRUCTURE OF AIR MOLECULAR THEORY 61 



particles are forced together. The force with which air or 

 any other gas resists compression makes it appear that in 

 some way the particles resist being pressed together and 

 are ready to fly apart again, as they do, in fact, when the 

 pressure is removed and the gas expands. 



61. Gas pressures. If you strike a blow upon a punching 

 bag, it is pushed to one side and immediately returns, but 

 if you hit it again as it starts to return, and continue to 

 strike one blow after another very rapidly, the bag will re- 

 main pushed to one side as long as you continue to strike. 

 The molecules are believed to be doing something very 

 similar to this. They not only strike against each other 

 but if the gas is inclosed they also strike against the walls 

 of the containing vessel. Since they are so small, each blow 

 is very insignificant indeed, but there are so many of them 

 that they strike innumerable blows every second upon each 

 square inch of the containing surface and the sum of these 

 blows is very great. The result of all these blows is that 

 the gas exerts pressure against the walls which confine it, 

 for instance, against the inside of the rubber tire. 



62. Heat, molecular movement, and gas mixtures. As the 

 temperature of a gas increases, the speed with which its 

 molecules move increases. Since they move faster, they strike 

 harder when they hit the walls of the tire or other container, 

 and the pressure is greater. This agrees with our observa- 

 tion, for we find that pressure exerted by a confined gas 

 always increases with the temperature. 



According to this theory, when two or more gases are 

 mixed, their molecules are intermingled, but each kind of 

 molecule remains unchanged. If some means of sorting the 

 two kinds of molecules is available, the two gases may 

 be recovered unchanged. A gas like the air may be made 

 up of a mixture of molecules of several different kinds of 

 gases, just as a swarm of gnats may be made up of several 

 kinds of gnats. 



