1068 
Physics. -— “The diffusion-coeffictent of gases and the viscosity of 
gas-miatures.” By Prof. J. P. Kurnen. (Communications from 
the Physical Laboratory at Leiden. Suppl. 38). 
In two previous communications’) on the same subject it was 
shown to what cause the difference between the results of the two 
current theories of gas-diffusion is to be ascribed. Whereas MaxweLr’s 
theory (STRFAN, LANGEVIN, CHAPMAN ®)) leads to the result, that the 
coefficient of diffusion is independent of the proportion in which two 
gases are mixed, O. E. Meyer’s theory which uses the method of 
the molecular free path gives a coefficient which changes with the 
composition of the mixture. For two gases whose molecules have 
, the coefficient of diffusion lies between two 
limiting values which are to each other in the proportion m,:m,. 
The coefficient of diffusion for carbon-dioxide (J/,—44) with a 
trace of hydrogen (J/, = 2) would be 22 times larger than that of 
hydrogen with a trace of carbon-dioxide; between those limiting 
values D would diminish regularly with the composition. 
The cause of this difference between the two results is due to 
the circumstance, that in O. EK. Mnygr’s theory no account is taken 
of the ‘persistence’ of molecular motion which was introduced into 
masses 7, and m 
the kinetic theory by Jrars®). The persistence consists in that a 
molecule, when colliding with another, retains on the average a 
component of velocity in the direction of its motion before the 
collision, so that after a collision all directions are not, as used to 
be generally assumed, equally probable. The manner in which 
Juans corrects O. E. Mryrr’s formula for the persistence is incorrect, 
however, and does not lead to a better result, because he takes for 
the persistence the value derived by him for a single gas, whereas 
the persistence in a mixture should have been introduced. In the 
previous papers cited above it was shown, that by doing the latter, 
the formula is modified in such a way, that the strong contradiction 
with the result of the other theory disappears. 
Qualitatively the matter may be put as follows. Let us first 
consider earbon-dioxide with a trace of hydrogen; the theory shows 
that in this mixture D is determined by the mobility and by the 
free path of the hydrogen-molecules which are both relatively large, 
1) J, P. Kuenen. These Proc. 15. p. 1152. 1913. 16. p. 1162. 1914. Comm. 
Leiden, Suppl. 28, 36. 
2) S. CraPuaN, Phil. Trans. 211 A p. 483. 1912; this article which was not 
mentioned in the previous papers may be specially referred to on this occasion. 
5) J. H. Jeans Dyn. Theory of gases. 1904. p. 236. seq. 
