84 DIFFUSION 



Diffusion of Gases. — If a small vial of bromine gas is broken under 

 a bell jar which has previously been evacuated of air the entire jar quickly 

 becomes filled with the brownish vapor of bromine. The distribution of the 

 bromine gas throughout the bell jar has been accomplished by the kinetic 

 activity of the bromine molecules, and is a simple example of the process of 

 diffusion. If the vial of bromine be broken under a bell jar which has not 

 been evacuated of air, the time required for the bromine gas to completely 

 occupy the bell jar will be longer than when diffusion of the gas occurs into 

 a vacuum. Under such conditions the freedom of movement of the bromine 

 molecules is greatly impeded by the presence of molecules of the gases of the 

 air, and the diffusion process is retarded. If the pressure of the air within 

 the bell jar be increased to two atmospheres (which is equivalent to doubling 

 the concentration of all the gases in the jar), the rate of diffusion of the 

 bromine gas through the air would be still less than when the jar was oc- 

 cupied by air at atmospheric pressure. 



Many other simple examples of the diffusion of gases might be cited. If 

 a bottle of ammonia, ether, peppermint oil, or of any other readily volatile 

 substance with a characteristic odor be opened indoors, within a very short 

 time the distinctive odor of that substance can be detected in all parts of 

 the room. Such a dispersal of gas molecules is accomplished at least in part 

 by diffusion, although air currents often assist in speeding up such a distribu- 

 tion of molecules. Except in the rare case of diffusion into a vacuum diffusing 

 molecules move between the molecules of other substances. 



The following somewhat fanciful analogy may aid in a visualization of 

 the kinetics of the diffusion process in gases. Suppose two adjoining rooms 

 to be connected by a closed double door. Imagine also that one of these 

 rooms contains a large number of tennis balls travelling in various directions 

 along straight pathways at different rates of speed. The average distances 

 between the tennis balls are supposed to be relatively great in proportion to 

 their diameters. Each tennis ball represents a molecule. The individual balls 

 will be constantly bumping into each other and into the walls of the room. 

 Because of the large number of balls present innumerable collisions will occur 

 every second. Each time a ball strikes a wall of the room it will bounce off 

 along a different linear pathway. Similarly, whenever two balls collide, each 

 will be deflected out of its course along a different route, to which it will 

 hold undeviatingly until it is again deflected from its path by another collision. 

 The course of each ball will thus be a zigzag progression through space, each 

 short segment of its path being terminated by a collision which changes its 

 direction. All of the balls do not move at the same speed at any given time, 

 but the speed of each fluctuates from moment to moment as a result of the 



