MONATOMIC GAS: DIFFUSION, VISCOSITY, AND THERMAL CONDUCTJON. 173 
We may also compare (13'24) with the result obtained from the simple mean-f'ree- 
path theory (first introduced by Maxwell* * * § ) with Jeans’ correctionf for the 
persistence of velocities, viz. (in our notation) 
(13'26) D n = 1'34 
P 
This is therefore about 10 per cent, larger than the exact value (13'24). 
(g) The Variation of D 12 with the Relative Concentration of the Component Gases. 
In the course of the development of the kinetic theory perhaps no branch has been 
the subject of more dispute than that dealing with diffusion. The point of greatest 
difference was the effect on the rate of diffusion of the relative concentration of the 
diffusing gases. Meyer’s elementary mean-free-path theory | led to a formula for 
D 12 according to which the coefficient of diffusion should vary with the proportions of 
the mixture over the extreme range indicated by the equation 
(13-27) 
(Dia)»>i = 0 _ TR 
(1^12)^ = 0 
Thus, when the molecular masses are very unequal, the range in the value of D 12 
should be very great. No such large variation is found to exist, however, according 
to the results of experiment. 
Meyer’s theory took no account of the tendency of a molecule to persist in motion 
along its original direction after collision : as Jeans§ has shown in connection with 
viscosity, however, “ persistence of velocities ” is a very important fact, the neglect of 
which leads to grave error in the mean-free-path theory. Iyuenen || has shown that 
when taken account of in the theory of diffusion, it largely removes the discrepancy 
between the small observed variations of D 12 , and the variations theoretically 
calculated by the method referred to. 
An earlier modification of Meyer’s theory by Gross^I may also be mentioned. This 
predicts variations of amount similar to those observed, but generally of the wrong 
sign ; its merits are not such as to demand more than this brief historical reference. 
* Maxwell, ‘Scientific Papers,’ i., p. 377, or ‘Phil. Mag.,’ 1860, January-July. 
t Jeans’ ‘ Dynamical Theory of Gases,’ p. 273. The whole of the chapter on diffusion (ch. xii. in the 
second edition) is of great interest, and a general reference may be made to it for comparison both of 
theory and experiment with the results of this memoir. 
I Meyer, ‘ Kinetic Theory of Gases ’ (English edition), p. 255. 
§ Jeans’ ‘ Dynamical Theory of Gases,’ pp. 276, 292. 
|| Kuenen, ‘ Supp. No. 8 to the Communications from the Leyden Physical Laboratory,’ January, 1913. 
Of. also Jeans’ treatise, ch. xiii. (2nd eel.), p. 328. 
H Gross, ‘Wied. Ann.,’ 40, p. 424, 1890; the disagreement of Gross’s theory with experiment has 
been indicated by Lonius, ‘Ann. d. Phys.,’ 29, p. 664, 1909. 
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