380 Scientific Intelligence. 



make it a very complete and satisfactory one, not only for the 

 use of chemists engaged in research, but also for technical 

 chemists. ■ h. l. w. 



5. Separation of Isotopes. — The lecture by F. AY. Aston 

 before the Royal Institution as reported in Nature 107, 334, 

 1921, contains an account of all the elements with their isotopes 

 which have so far been examined by the positive ray spectro- 

 graph. The great interest which attaches to the sweeping sim- 

 plications which have been made in our ideas of mass, by the now 

 well established view that all atoms are built of primordial atoms 

 of positive and negative electricity, has stimulated efforts to 

 secure evidence as to the existence of various isotopes, by other 

 lines of attack. 



A very laborious series of operations on fractional diffusion 

 through pipe clay has shown that neon may be separated into two 

 components differing in density by 0.7 of one per cent. 



The spectra of the three isotopes of lead have been shown to 

 have small differences in the wave lengths of the principal lines 

 and the same is probably true of the spectra of ordinary thallium 

 and that extracted from pitchblende. 



Following Aston 's announcement of the discovery that chlor- 

 ine consists of a mixture of two isotopes of atomic weights 35 and 

 37, Merton and Hartley suggested that as chlorine gas would 

 probably consist of three molecules in the ratio of 9 :6 :1, if a 

 beam of white light were allowed to traverse a column of chlorine 

 the light might be differently absorbed by the different molecules, 

 so that the emergent radiations would have different intensities 

 in different wave lengths. It was accordingly proposed to allow 

 the light which had traversed the filter to enter a vessel contain- 

 ing a mixture of hydrogen and chlorine which combine under the 

 influence of light of these wave lengths. It was calculated that 

 the rates of reaction for the different HCT molecules would be in 

 the ratio 1 : (10) 9 : (10) 24 . If this were true the hydrochloric acid 

 formed would consist almost entirely of HCL T provided the reac- 

 tion were allowed to proceed for a sufficient length of time. 

 This experiment has now been carried out with great care {Phil. 

 Mag. 43, 430, 1922) but it was found impossible to say whether 

 a real separation had been effected or not. The failure may have 

 been due either to secondary reactions or to the fact that the dif- 

 ference in the absorption spectra of the two isotopes was insuf- 

 ficient for the purpose. 



Bronsted and Hevesy have attempted the separation of mer- 

 cury isotopes by methods depending upon the different 

 molecular velocities appearing in consequence of the dif- 

 ferent masses of the isotopes. When the liquid is allowed to 

 evaporate, the rate at which the two molecules leave the liquid 

 should be inversely as the square roots of the masses. If a 



