Constitution of Atmospheric Neon. 455 



mass a little too low. The above figures therefore can be 

 accepted as fairly conclusive evidence that Atmospheric Neon 

 contains two isotopes of atomic weights 20*00 and 22"00 

 respectively to an accuracy of about one-tenth per cent. 



In order to give the accepted density the quantities 

 required are 90 per cent, and 10 per cent., which is in good 

 agreement with the estimated intensity of the lines. 



Possibility of a Third Isotope. 



On the clearest spectra obtained with neon there are 

 distinct indications of a line corresponding to an isotope of 

 mass 21. This line is extremely faint, so that if this con- 

 stituent exists its proportion would be very small, probably 

 well under 1 per cent., and it would not affect the density 

 appreciably. Attempts to bring this line out more distinctly 

 by longer exposures have not succeeded owing to the fogging 

 from the strong neighbouring lines, but it is intended to 

 return to this point when further improvements of the 

 method give hope of more conclusive results. This matter 

 is interesting in connexion with the suggestion by Emerson 

 Reynolds already quoted*. 



In conclusion the author wisbes to express his thanks to 

 M. Georges Claude, who kindly supplied the neon used, and 

 also to the Government Grant Committee for some of the 

 apparatus employed. 



Summary. 



A brief account is given of facts which lead to the idea 

 that atmospheric neon may be a mixture of isotopes. 



The results of attempts of separation are summarized. 



The several lines of evidence adduced from the parabolas 

 obtained by Thomson's method of Positive Ray analysis are 

 considered and shown to be consistent with the above theory 

 but hardly conclusive. 



Mass-spectra obtained by means of the Positive Ray 

 Spectrograph are produced. Measurements from these are 

 given which prove conclusively that neon contains two 

 isotopes having atomic weights 2000 and 22'00 respectively 

 to an accuracy of about one-tenth per cent., their proportions 

 being therefore 9.0 per cent, and 10 per cent, by volume. 



The possibility of a third constituent is indicated. 



Cavendish Laboratory, 

 December 1919. 



* Though at the time this was made isotopes were not thought of, 

 and the modern idea of atomic members has since precluded the 

 1 "litv of three distinct elements. 



