Mr. J. C. Maxwell on the Dynamical Theory of Gases. 199 



rents arise so as to mix the strata, the composition of the gas 

 will be made more uniform throughout. 



The result at which we have arrived as to the final distribu- 

 tion of gases, when left to themselves, is independent of the law 

 of force between the molecules. 



Diffusion of Gases. 



If the motion of the gases is slow, we may still neglect the 

 tangential pressures. The equation then becomes for the first 

 system of molecules 



Pi^ + ^^^p^K-zO+X^, . . (78) 



and for the second, 



In all cases of quiet diffusion we may neglect the first term 

 of each equation. If we then put^ 1 +jt? 2 =j9, and p ± + p 2 =p, we 

 find by adding, 



i= x " <*» 



If we also put p^+p^u^pu, then the volumes transferred in 

 opposite directions across a plane moving with velocity u will be 

 equal, so that 



Here p^^— u) is the volume of the first gas transferred in 

 unit of time across unit of area of the plane reduced to pressure 

 unity, and at the actual temperature; and p 2 (u— w 2 ) is the equal 

 volume of the second gas transferred across the same area in the 

 opposite direction. 



The external force X has very little effect on the quiet diffu* 

 sion of gases in vessels of moderate size. We may therefore 

 leave it out in our definition of the coefficient of diffusion of two 



When two gases not acted on by gravity are placed in differ- 

 ent parts of a vessel at equal pressures and temperatures, there 

 will be mechanical equilibrium from the first, and u will always 

 be zero. This will also be approximately true of heavy gases, 

 provided the denser gas is placed below the lighter. Mr. 

 Graham has described, in his paper on the Mobility of Gases*, 

 experiments which were made un'der these conditions. A ver- 



* Philosophical Transactions, 1863. [Phil. Mag. December 1863.] 



