CONTRIBUTIONS TO SCIENCE. 565 



Suppose two trains to be moving in opposite directions, side 

 by side on parallel lines, and suppose that as they pass each 

 other a number of passengers from each train jump into the 

 other. It is clear that each passenger will carry with him the 

 momentum he possesses into the other train which is moving 

 in the opposite direction, and will consequently diminish the 

 momentum and therefore the velocity of that train. If a 

 continuous interchange of passengers, backwards and for- 

 wards, between the trains were to take place, the trains 

 would ultimately be brought to rest relatively to one an- 

 other. This is an illustration of the diffusion of momentum, 

 and was originally given by Balfour Stewart. Now, suppose 

 two streams of gas to be passing each other tangentially. 

 There will be a continuous interchange of molecules by 

 diffusion between the two streams, and the molecules carry- 

 ing with them the momentum they possess, the effect of the 

 diffusion will be to tend to bring the two streams to relative 

 rest. Diffusion of momentum consequently introduces a 

 tangential action between layers of gas which are moving 

 relatively to one another, and therefore causes a resistance 

 to any " continuous change of form, depending on the rate 

 at which that change is effected," that is, it confers upon 

 the gas the property of viscosity. Thus, the so-called vis- 

 cosity of gases received from Maxwell its complete explana- 

 tion in accordance with the kinetic theory. 



The viscosity of a gas depending on the rate of 

 diffusion of momentum, and therefore on the rate of 

 diffusion of matter, is proportional, like the latter, to the 

 average velocity of the molecules. If the gas be very rare, 

 each particle will meet with fewer collisions, and consequently 

 its course will be less interrupted than when the gas is 

 denser, so that Maxwell found that between two strata 

 of gas at a given distance apart the rate of diffusion was 

 the same whatever the density. He thus " arrived at the 

 startling result that the co-efficient of internal friction is 

 independent of the density of any particular kind of gas." 

 The experimental verification of this result occupied a con- 

 siderable portion of Maxwell's leisure time in 1865. The 



