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LXXV. Mote on the possibility of separating Mercury into its 

 Isotopic Forms by Centrifuginq. By J. H. J. Poole, 



M.A* 



IT appears certain from the recent work of Dr. F. W. 

 Aston on the mass spectra of the elements that mercury 

 is really a mixture of several isotopes of varying atomic 

 weight. Dr. Aston in a letter to ' Nature ' (Dec. 9, 1920) 

 states that his most recent results have shown that it consists 

 of at least six isotopes with atomic weights of 197, 198, 199, 

 200, 202, and 204. The exact resolution of the four isotopes 

 between 197 and 200 has not yet been definitely deter- 

 mined, but the existence of some such isotopes and also the 

 two of atomic weight 202 and 204 may be taken as definitely 

 established. The maximum difference between the isotopes 

 of mercury would amount to 7 units, but as we are ignorant 

 as to the proportions of the various isotopes present it is 

 impossible to deal with the question of their separation when 

 centrifuged in a rigid manner. I propose to assume that 

 the mercury can be treated as a nearly equal mixture of two 

 isotopes only which differ in atomic weights by 4 units. 

 This seems to be a fairly justifiable assumption to make, as 

 the mean 'atomic weight of the two heaviest isotopes whose 

 exact atomic weight is most accurately known, exceeds 

 that of ordinary mercury by over 2 units. 



As regards the equilibrium state of a mixture of two 

 liquid isotopes in either a gravitational or centrifugal field 

 of force, the case is not very clear. Drs. Lindemann and 

 Aston have dealt with the subject as regards gaseous 

 isotopes in a paper in the Philosophical Magazine for May 

 1919. Their application of the results obtained to the case 

 of two liquids is, however, not so plain. The following 

 discussion will perhaps throw some light on the problem. 



Let us assume that the two isotopes differ only in mass, 

 i.e., the molecular volumes and all other properties are the 

 same for the two. Further, let us assume that mercury is 

 incompressible, so that the total number of molecules per c.c. 

 is constant. Consider first the case of a column of mercury 

 in an ordinary gravitational field. It is obvious from 

 symmetry that if equal volumes of both isotopes are present 

 on the whole, then at the central section of the column the 

 number of molecules per c.c. of each isotope will be the 

 same. Let us accordingly take this point as our origin and 

 consider the equilibrium of a layer of mercury at a depth oc 

 below this. 



* Communicated by the Author. 



