DIFFUSION. 



o/ Ghoet acconling to the Kinetic Theory. 



80 many of the phenomena of gases are found to be explained in a c<>n 

 MM- by the kinetic theory of gases, that we may describe \\ith 

 probability of correctness the kind of motion which constitute! 

 in gave*. We shall therefore consider gaseous diffusion in the light 

 of the kinetic theory before we consider diffusion in liquids. 



A gat, according to the kinetic theory, is a collection of particles or mole- 

 cule* which are in rapid motion, and which, when they encounter each other, 

 before pretty much as elastic bodies, such as billiard balls, would do if no 

 energy were lost in their collisions. Each molecule travels but a very small 

 distance between one encounter and another, so that it is every now and then 

 Altering its velocity both in direction and magnitude, and that in an exceed- 

 ingly irregular manner. 



The result is that the velocity of any molecule may be considered as com- 

 pounded of two velocities, one of which, called the velocity of the medium, 

 in the same for all the molecules, while the other, called the velocity of agi- 

 tation, is irregular both in magnitude and in direction, though the average 

 magnitude of the velocity may be calculated, and any one direction is just as 

 likely as any other. 



The result of this motion is, that if in any part of the medium the 

 molecules are more numerous than in a neighbouring region, more molecules 

 will pass from the first region to the second than in the reverse direction, 

 and for this reason the density of the gas will tend to become equal in all 

 puts 6f the vessel containing it, except in so far as the molecules may be 

 crowded towards one direction by the action of an external force such as 

 gravity. Since the motion of the molecules is very swift, the process of 

 equalization of density in a gas is a very rapid one, its velocity of propaga- 

 tion through the gas being that of sound. 



Let us now consider two gases in the same vessel, the proportion of the 

 gases being different in different parts of the vessel, but the pressure being 

 everywhere the same. The agitation of the molecules will still cause more 

 molecules of the first gas to pass from places where that gas is dense to 

 places where it is rare than in the opposite direction, but since the second 

 gas is dense where the first one is rare, its molecules will be for the most 

 part travelling in the opposite direction. Hence the molecules of the two 



