190 Lord Kelvin on 



Q g is the quantity of the G gas entering across GGG and 

 leaving across RRR per sec. of time per sq. cm. of area ; 

 Q r is the quantity of the R gas entering across RRR and 

 leaving across GGG per sec. of time per sq. cm. of area ; the 

 unit quantity of either gas being that which occupies a cubic 

 centimetre in its entry tube. The equations 



where g and r are the proportions of the G gas at R and of 

 the R gas at G, define the average diffusivities of the two 

 gases in the circumstances in which they exist in the different 

 parts of the length a between the end-plates. This statement 

 is cautiously worded to avoid assuming either equal values of 

 the diffusivities of the two gases or equality of the diffusivity 

 of either gas throughout the space between the end-plates. 

 So far as I know difference of diffusivity of the two gases 

 has not been hitherto suggested by any writer on the subject. 

 What is really given by Loschmidt/s experiments, § 43 below, 

 is the arithmetic mean of the two diffusivities D g and D r . 



§ 40. In 1877 0. E. Meyer expressed the opinion on theo- 

 retical grounds, which seem to me perfectly valid, that the 

 inter-diffusivity of two gases varies according to the pro- 

 portions of the two gases in the mixture. In the 1899 edition 

 of his ' Kinetic Theory of Gases'* he recalls attention to this 

 view and quotes results of various experimenters, Loschmidt, 

 Obermayer, Waitz, seeming to support it, but, as he says, not 

 quite conclusively. On the other hand, Maxwell's theory 

 (§ 41 below) gives inter-diffusivity as independent of the 

 proportions of the two gases ; and only a single expression 

 for diffusivity, which seems to imply that the two diffusivities 

 are equal according to his theory. The subject is of extreme 

 difficulty and of extreme interest, theoretical and practical ; 

 and thorough experimental investigation is greatly to be 

 desired. 



§ 41. In 1873 Maxwell | gave, as a result of a theoretical 

 investigation, the following formula which expresses the inter- 

 diffusivity (D 12 ) of two gases independently of the proportion 

 of the two gases in any part of the mixture : each gas being- 

 supposed to consist of spherical Boscovich atoms mutually 

 acting according to the law, force zero for all distances 

 exceeding the sum of the radii (denoted by s l2 ) and infinite 

 repulsion when the distance between their centres is infinitely 



* Baynes' translation, p. 264. 



t " On Loschmidt's Experiments on Diffusion in relation to the Kinetic 

 Theory of Gases/' Nature, Aug. 1873 ; Scientific Papers, vol. ii. pp. 343- 

 350. 



