MONATOMIC GAS: DIFFUSION, VISCOSITY, AND THERMAL CONDUCTION. 117 
The experimental data which were determined in order to test the point confirmed 
neither conclusion ; a variation in I) ]2 was observed, but it was so much smaller than 
that suggested by Meyer’s formula that the experimental values of I) 12 were in closer 
numerical agreement with a constant expression than with Meyer’s highly variable 
result. Recently Ktjenen # has modified Meyer’s theory by taking into account 
the tendency of a molecule to persist in its original direction after collision, a 
consideration the importance of which was brought into prominence by JEANSt 
in connection with the elastic-sphere theory of viscosity. As a result of this revision 
Meyer’s theory is brought much more closely into accord with experiment (§13 (i )), 
though the discrepancies still exceed those between the observed values of D ]2 and 
a suitable constant mean. 
In §13 (i) the observations of D 13 relating to the above variations are compared 
also with the results of the present theory, which affords a formula similar to that 
of Langeyin but multiplied by a correction factor which is a function of the relative 
proportions of the component gases. The agreement with experiment, while not 
exact, is perhaps as close as the degree of experimental errors, and the uncertainty 
as to the best molecular model, entitle us to expect. 
The present theory of diffusion is compared also, in two particular cases, with 
exact results obtained theoretically by other writers. These cases are (a) that of 
a gas formed of two sets of molecules which are identical in their dynamical 
properties (as in the problem of the self diffusion of a gas), and (b) that of a gas 
in which the molecules of one kind are infinitesimal in size and mass compared with 
those of the other kind (as in electronic diffusion in gases). These problems have 
been dealt with respectively by Pidduck if and Lorentz§ ; the corresponding special 
forms of the general expressions here deduced are in satisfactory accordance with 
their results (§13 (e), {/)). 
Certain other phenomena connected with diffusion are also discussed which do not 
seem to have been investigated in any detail hitherto. In §§10, 14, 15 it is shown 
that diffusion may be produced by a gradient of pressure or temperature independently 
of the presence of a concentration gradient or of external forces; in §§14, 15 the 
amount of this effect is considered, with numerical illustrations relating to particular 
pairs of gases. Conversely, the absence of diffusion in a composite gas in which the 
temperature is non-uniform (the pressure being uniform and there being no external 
forces) implies the existence of a corresponding variation in the relative concentration 
of the two gases. This latter variation (cf. § 16) appears to be so large in amount 
* Kuenen, ‘ Supp. No. 8 to the Communications from the Leyden Physical Laboratory,’ January, 1913 ; 
also ‘Amsterdam Acad. Proc.,’ 16, p. 1162, 1914. 
t Jeans, ‘ Dynamical Theory of Gases’ (2nd ed.), pp. 276, 292, 328. 
I Pidduck, ‘Proc. Lond. Math. Soc.’ (2), 15, p. 89, 1915, 
§ Lorentz, ‘Archives N^erlandaises,’ 10, p. 336, 1905 ; ‘ Theory of Electrons,’p. 268. More general 
results obtained by Jeans, using Lorentz’s method, are given in his ‘Dynamical 'theory of Gases,’ 
2nd ed., §§ 314, et seq . Cf . also Pidduck, loc . cit ., p. 112. 
S 2 
