26 Prof. Maxwell on the Process of Diffusion of two or 

 The effect of these collisions is therefore to produce a force 



on particles of the first system, and 



on particles of the second system. 



The effect of the collisions of those particles of the second 

 system which strike into the stratum, is to produce a force 



- ite^^ + H; 



on the first system, and 



on the second. 



The whole effect of these collisions is therefore to produce a 

 resultant force 



on the first system, 



on the second, and so on. 



Prop. XVIII. To find the mechanical effect of a difference in 

 the mean velocity of translation of two systems of moving particles. 



Let Vj, V 2 be the mean velocities of translation of the two 



M M 



systems respectively, then A } * (V t — Vj) is the mean mo- 

 mentum lost by a particle of the first, and gained by a particle 

 of the second at collision. The number of such collisions in unit 

 of volume is 



Nj B/3 2 v v or N 2 Cpi v 9 ; 



therefore the whole effect of the collisions is to produce a force 



^N'^'lT+M,^'-^ • • • < 36 > 

 on the first system, and an equal and opposite force 



= +N a C PlBs5 »- 4 (V 1 -V 9 ) . . . (37) 



on unit of volume of the second system. 



