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XLYII. An Attempt to Separate the Isotopes of Chlorine. By 

 H. Hartley, M.A., A. 0. Ponder, B.A., E. J. Bowen, 

 B.A., and T. R. Merton, B.Sc, F.R.S* 



AS the isotopic elements are chemically identical, and 

 therefore inseparable by the ordinary chemical 

 methods of separation, there remain only physical methods 

 of fractionation depending on small differences in the 

 physical properties of the atoms, which, as Lindemann 

 and Aston have shown (Phil, Mag. vol. xxxvii. p. 523, 

 1919), are likely to be excessively slow. It has been 

 found, however, that the spectra of isotopes are not iden- 

 tical (Aronberg, Astrophys. Journ. vol. xlvii. p. 96, 1918 ; 

 Merton, Proc. Roy. Soc. A. vol. xcvi. p. 388, 1920, and 

 A. vol. c. p. 84, 1921), but that there are small differ- 

 ences in the wave-lengths of the principal lines in the 

 spectra of three isotopes of lead, and probably also in 

 the spectra of ordinary thallium and of the thallium 

 extracted from pitchblende. In these cases the difference 

 was established in the case of some of the strongest spectrum 

 lines only; but since it is probable that such differences are 

 to be found throughout the spectrum, it is of interest to con- 

 sider whether they can be made of use in effecting a separation 

 of a mixture of isotopes. Aston's (Phil. Mag. vol. xxxix. 

 p. 620, 1920) announcement that chlorine consists of a mix- 

 ture of two isotopes of atomic weights 35 and 37, with possibly 

 a trace of another of atomic weight 39, has presented a case 

 in which it appeared that a difference in the spectra might 

 he turned to account (Merton and Hartley, 'Nature,' vol. cv. 

 p. 104, 1920). Neglecting the possible isotope of atomic 

 weight 39, it is evident that ordinary chlorine must consist 

 •of a mixture of the isotopes Cl 35 and CI37 in the ratio of 

 very nearly 3:1. In the gaseous condition chlorine will 

 thus consist, on probability considerations, of the molecules 

 CI35CI35, OJ35CI37, and CU37O37 in the ratio 9:6:1. If we 

 suppose that white light traverses a column of chlorine 

 of such a length that the radiation absorbed by CI37CI37 is 

 reduced in intensity by a factor 10 -3 , the corresponding- 

 factors in the case of CI35OI37 and CI35CI35 will be 10 ~ 18 and 

 10 ~ 27 respectively. If, now, the light after passing through 

 this column of chlorine enters a A^essel containing a mixture 

 ■of hydrogen and chlorine, which combine under the influence 

 of the light absorbed by the chlorine, it would appear to 

 .follow that the initial rates of reaction for the molecules 



* Communicated by the Authors. 



