126 JAMES CLERK MAXWELL 



constant. Now suppose passengers commence to 

 jump across from one train to the other. Kuch man 

 carries with him his own momentum, which is in the 

 opposite direction to that of the train into which he 

 jumps ; the result is that the momentum of each 

 train is reduced by the process; the velocities of the 

 two decrease ; it appears as though a frictional force 

 were acting between the two. Maxwell suggests that 

 a similar process will account for the apparent 

 viscosity of gases. 



Consider two streams of gas, moving in opposite 

 directions one over the other ; it is found that in 

 each case the layers of gas near the separating sur- 

 face move more slowly than those in the interior of 

 the streams ; there is apparently a frictional force 

 l>etween the two streams along this surface, tending 

 to reduce their relative velocity. Maxwell's explana- 

 tion of this is that at the common surface particles 

 from the one stream enter the other, and carry with 

 them their own momentum: thus near this surface 

 the momentum of each stream is reduced, just as the 

 momentum of the trains is reduced by the people 

 jumping across. Internal friction or viscosity is due 

 to the diffusion of momentum across this common 

 surface. The cttect does not penetrate far into the 

 gas, for the particles soon acquire the velocity of the 

 stream to which they have come. 



Now, the rate at which the momentum is diffused 

 will measure the frictional force, and will depend on 

 the mean free path of the particles. If this is consider- 

 able, so that on the average a particle ran penetrate a 

 considerable distance into the serond <MS before a 



