i46 Dr. S. Chapman on the Partial Separation by 



or non-equivalence of expressions (7) and (8). I£ these are 

 really not equivalent, then the disagreement constitutes an 

 outstanding discrepancy, one solution of which would be the 



addition of the term — (div A) 2 to the magnetic energy 



density. 



I am, 



Yours very truly, 



E. A. BlEDERMANN. 



XIV. On the Partial Separation by Thermal Diffusion of 

 Gases of Equal Molecular Weight, By S. Chapman, 

 M.A., U.Sc* 



IN a recent memoir f I have shown mathematically that a 

 temperature gradient in a mixture of two gases is in 

 general sufficient to produce diffusion, independently of any 

 non-uniformity of composition or of the action of external 

 forces. This phenomenon had apparently escaped the notice 

 of experimental workers on gases, as well as of previous 

 theoretical writers, but in a recent joint note $ with 

 Dr. F. W. Dootson, an account of some experiments has 

 been given which affords satisfactory qualitative confir- 

 mation of the theory§. Further experiments are now in pro- 

 gress with the aim of obtaining a close numerical comparison 

 of observational data with the chief features of the somewhat 

 complex theory of the phenomenon. According to the theory, 

 the amount of the effect is greatest when the gases are mixed 

 in nearly equal proportions by volume, and also is greater 

 the more unequal are the masses and diameters of the gas 

 molecules. It depends, moreover, on the nature of the 

 molecules. It seems to be greatest for rigid elastic spherical 

 molecules, while it vanishes altogether for Maxwellian mole- 

 cules, i. e. for point centres of forces varying inversely as the 

 fifth power of the distance. It also vanishes when the masses 

 and diameters (or laws of inter-action) of the two sets of 

 molecules are alike. 



* Communicated by the Author. 



t Phil. Trans. A. 1916 (unpublished) ; an abstract is given in Proc. 

 Roy. Soc. A. xciii. p. 1 (1916). 



X Phil. Mao-, xxxiii. p. 249 (1917). 



§ Since writing this Note I have received an Inaugural Dissertation 

 (Upsala, 1917) in which Dr. D. Enskog, by a different method of analysis, 

 lias arrived at the same theorelical results as are contained in my recent 

 memoirs (Phil. Trans. A. ccxvi. p. 279, 1915, and A. ocxvii. p. 1, 1916), 

 including those relating to thermal diffusion. 



