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XIII. The possibility of separating Isotopes. 

 By S. Chapman, M.A., D.Sc, F.R.Sr 



§ 1. TN an interesting paper bearing the above title f.. 

 A Prof. F. A. Lindemann and Dr. F. W. Aston 

 have described various ways of separating isotopes, i. e... 

 elements of slightly differing atomic weights occupying the 

 same position in the atomic table, and inseparable by chemical 

 means. The object of this note is to draw attention to a 

 further method, which may possibly be of service in this 

 difficult task. 



The method depends upon thermal diffusion, a phenomenon 

 of gases which was independently discovered by Dr. I). 

 Enskog J and myself § by theoretical reasoning, and sub- 

 sequently experimentally verified by Dr. F. W. Dootson||. 

 It is most simply described, by stating that if free com- 

 munication is made between like mixtures of gases in 

 vessels maintained at different temperatures, diffusion will 

 take place until an equilibrium condition is reached in 

 which there is a slight excess of the heavier gas in 

 the cold vessel, and of the lighter gas in the hot vessel. 

 If the molecular masses are equal, but the diameters 

 unequal, the larger molecules will be in excess in the cold 

 vessel. 



The mathematical theory of thermal diffusion is highly 

 complicated, but a first approximation to its results can be 

 expressed fairly simply (Phil. Trans. A. 217, pp. 181-186).. 

 In the present note I shall confine myself to the case of a 

 gas mixture in which the molecular masses m l5 m 2 are nearly 

 equal, as, for instance, in a mixture of neon and the hypo- 

 thetical metaneon (of molecular weights 20 and 22). This 

 case was the one specially considered by the two authors 

 first-named. 



The separating power of thermal diffusion depends on a 

 constant k^ whose value, when m 3 —m. 2 is small (we will 



* Communicated by the Author. 



t F. A. Lindemann & F. W. Aston, Phil. Mag., May 1919. 

 | D. Enskog-, Phys. Zeitschrift, xii. p. 538 (1911) ; Ann. d. Phys.. 

 xxxviii. p. 750 (1912) ; Dissertation, Upsala, 1917. 



§ S. Chapman, Phil. Trans. A. 217, p. 115 (1916) ; Phil. Mag. xxxiv.. 

 . 146 (1917). 

 || S. Chapman & F. W. Dootson, Phil. Mag. xxxiii. p. 248 (19] 7). 



