STRUCTURE OF AIR: MOLECULAR THEORY 53 



50. 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 remain 

 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 they also 

 strike against the walls of the containing vessel if the gas is 

 inclosed. Since they are so small, each blow is very insignifi- 

 cant indeed, but there are so many of them that they strike 

 innumerable blows every second upon each square inch of the 

 containing surface. Each of these blows is very small, but the 

 sum of them is very great. The result of all these small blows 

 is that the gas exerts pressure against the walls which confine 

 it, for instance, against the inside of the rubber tire. 



51. Heat and molecular movement. As the temperature of 

 a gas increases, the speed with which its molecules move in- 

 creases. 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 observation, for we find that 

 pressure exerted by a confined gas always increases with the 

 temperature. 



52. Gas mixtures. 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. 



53. Liquids and solids. It may be more difficult to believe 

 that liquids and solids are composed of molecules, but it is 

 believed to be the case. It is not very difficult when we recall 

 the fact that gases can be condensed into liquids, as when 

 steam becomes liquid water or in the formation of liquid air. 



