the Weights of Atoms. 187 



degree the mass or shape or elastic quality of the molecules, 

 and they do not disturb the equilibrium of the gas or alter 

 the motion of any one of its particles ; they are merely to 

 give us a means of tracing ideally the history of any one 

 molecule, or set of molecules, moving about and colliding 

 with other molecules according to the kinetic nature of a gas. 



§ 37. Whatever may have been the initial distribution of 

 the greens and reds, it is clear that ultimately there must be 

 a regular transition from all greens at the plane GGG and 

 all reds at the plane RRR, according to the law 



-*-r tL -m-» (Jj ^™" %Xj / — \ 



g=JSf-; r = N . . . . (1), 



where g and r denote respectively the number of green 

 molecules and of red molecules per unit volume at distance x 

 from the plane RRR. In this condition of statistical equili- 

 brium, the total number of molecules crossing any inter- 

 mediate parallel plane from the direction GGG towards RRR 

 will be equal to the number crossing from RRR towards 

 GGG in the same time ; but a larger number of green mole- 

 cules will cross towards RRR than towards GGG, and, by 

 an equal difference, a larger number of red molecules will 

 cross towards GGG than towards RRR. If we denote this 

 difference per unit area per unit time by QN, we have for 

 what I call the material diffusivity (called by Maxwell, '*' co- 

 efficient of diffusion"), 



D=Qa (2). 



We may regard this equation as the definition of diffusivity. 

 Remark that Q is of dimensions LT~ l , because it is a number 

 per unit of area per unit of time (which is of dimensions 

 L~ 2 T~ l ) divided by N t a number per unit of bulk (dimen- 

 sions L~ 3 ). Hence the dimensions of a diffusivity are LPT' 1 ; 

 and practically we reckon it in square centimetres per second. 



§ 38. Hitherto we have supposed the G and the R particles 

 to be of exactly the same quality in every respect, and the 

 diffusivity which we have denoted by D is the inter-diffusivity 

 of the molecules of a homogeneous gas. But we may suppose 

 G and R to be molecules of different qualities; and assem- 

 blages of G molecules and of R molecules to be two different 

 gases. Everything described above will apply to the inter- 

 diffusions of these two gases ; except that the two differences 

 which are equal when the red and green molecules are of 

 the same quality are now not equal or, at all events, must 



