48 On the Separation of the Isotopes of Mercury. 



mean free path of the molecules is more than ten times as 

 great as the diameter of the opening. Knudsen, however, by 

 using a 2-4 mm. wide opening and working at 59°*8, noted a 

 deviation of only about 8 per cent, from what was calculated 

 from the rule of the molecular flow, and it was to be expected 

 that also by using essentially larger openings or a smaller 

 mean free path the partial separation of mercury would be 

 successful. 



By 'heating the bulb A to 105°, corresponding to a mean 

 free path of the mercury molecules of about 0*3 mm. at E, 

 the product condensed in B showed the density 0*999987. 



In this connexion we might mention that diffusion methods 

 have been repeatedly employed in order to attain separation 

 of the isotopes. Aston * was the first to use this method 

 and got, in the case of neon, considerable changes in density. 

 By means of a similar method, Stern and Yolmerf have tried 

 to separate the elements hydrogen and oxygen, yet without 

 result, in accordance with their nature as pure elements. A 

 trial to separate the isotopes of uranium through diffusion in 

 aqueous solution was equally unsuccessful ±. 



Summary. 



1. A partial separation of the isotopes of mercury based 

 on the two methods following was successful. 



(a) Evaporation method (" ideal distillation ") based on 



the difference in the evaporation velocities of 

 isotopes. The distillate was found richer, the 

 remainder poorer in the lighter isotope, than the 

 initial substance. 



(b) Effusion method. A fraction of the mercury vapour 



penetrates through narrow openings into the con- 

 densation space, where the lighter isotope is found 

 in a relatively larger amount than in ordinary 

 mercury. 



2. The results of the experiments agree with the theory, 

 according to which the evaporation — as well as the effusion 

 velocity of the isotopes — is inversely proportional to the 

 square root of their molecular weights ; they are further 

 in conformity with Aston^s results obtained by means of his 

 mass spectrograph. 



* Brit. Assoc. Report, 1915 ; Phil. Mag-, xxxix. p. 450 (1920). 

 Compare also the following attempts: — I). Harkins, 'Nature,' cv. p. 2' J 

 (1920) ; E. Kolilweiler, Zeitschr. f. Phys. Chem. xcv. p. 95 (1920) ; i.ml 

 II. Grimm (K. Fa'ans, Radioaktimtat, 3 Aufl., footnote p. 112). 



t Ann. d. Fhysik, lix. p. 225 (1919). 



% Hevesy & Putnoky, Phys. Zeitschr. xiv. p. 63 (1913). 



