DIFFUSION OF GASES 85 



numerous collisions in which if participates. The average speed of the entire 

 group will remain constant, however, as long as the temperature remains 

 unchanged. As a result of this haphazard mutual buffeting, the tennis balls 

 will remain essentially equally distributed throughout the room. 



Suppose now that the double doors connecting the two rooms are thrown 

 open. As a result of their haphazard movement some of the balls close to 

 the door will pass into the empty room. The first ones to do this will travel 

 without interruption until they bump into one of the walls, as their rate of 

 progression will not be impeded by collisions with other molecules. When 

 they hit against one of the walls of the room they will bounce back into its 

 interior along a new pathway. As more and more of the balls pass into the 

 originally empty room their rate of progress becomes slower since the 

 greater their concentration in the room, the greater the number of col- 

 lisions per unit of time. As soon as any appreciable number of tennis balls 

 has invaded the empty room, as a result of their random movements, some 

 will pass back through the doorway into the room which originally contained 

 all of them. As long, however, as the concentration (number per unit 

 volume) of the tennis balls is greater in one room than in the other their 

 random movements will result in more passing through the door into the 

 room in which their lesser concentration prevails, than in the opposite direction. 



In a relatively short time the concentration of tennis balls will have be- 

 come equal in both rooms. In other words they have "diffused" from one 

 room into the other. After equality of concentration has been established 

 the number of balls passing through the door in one direction in any interval 

 of time will be exactly equal to the number moving in the opposite direction. 

 When this condition of dynamic equilibrium is attained, diffusion, ni the 

 sense the word will be used in this discussion, is no longer occurring. 



If, in the hypothetical illustration just described, one room were filled 

 with white tennis balls, and the other with red tennis balls, diffusion would 

 occur simultaneously in both directions. The red balls would "diffuse" toward 

 the room in which their initial concentration was zero while the white balls 

 would "diffuse" in the opposite direction. At equilibrium the concentration 

 of the white balls would be equal throughout the two rooms, and this would 

 likewise be true of the red balls. 



Even after a djaiamic equilibrium has been attained in any system hap- 

 hazard kinetic activity of the molecules continues. This is sometimes referred 

 to as diffusion, but will not be so considered in this book. The term diffusion 

 will be used only to characterize situations in which there is gain in the 

 number of molecules of a certain kind in one part of a system at the expense 

 of other parts. According to this concept diffusion can only occur when 



