Mass Spectra of the Alkali Metals. 439 



The most successful experiment done with the parabola 

 method of analysis was one in which a mixture of sodium 

 and lithium phosphates was employed (this contained traces 

 of potassium salts). By great good fortune a very strong- 

 jet of rays must have been axially directed, and three satis- 

 factory exposures were obtained before the anode "dried up." 

 One of these is reproduced in the accompanying Plate. 

 A strong parabola at 7 and a weak one at 6 demonstrate 

 clearly that lithium is a complex element, as its chemical 

 atomic weight 6*94 leads one to expect. This result, which 

 was announced by the writer and G. P. Thomson in 

 ' Nature,' February 24th, has since been confirmed indepen- 

 dently by Dempster (' Science/ April 15, 1921) using a 

 different method. The several photographs here considered 

 all gave approximately the same rates of intensities, and they 

 corresponded as well as was to be expected with the accepted 

 atomic weight. On the other hand, Gr. P. Thomson's parabolas 

 (which were obtained with a composite anode) and Dempster's 

 electrical measurements suggest a more nearly equal intensity 

 relation which also appears to vary. 



Experiments with the Mass Spectrograph, 



The parabola method of analysis is perfectly satisfactory 

 in the case of so light an element as lithium, but cannot be 

 used for the critical examination of the heavier members 

 of the group ; and so the apparatus for the production of the 

 rays was fitted to the mass spectrograph already described 

 (Phil. Mag. xxxix. p. 612, May 1920). The experimental 

 difficulties became now very serious indeed, for, in addition 

 to those already indicated, there was no means of finding the 

 most suitable voltage to apply to the electrostatic plates. 

 In normal cases this is done by visual inspection of the 

 hydrogen lines, but here it could only be guessed at. Under 

 these conditions it is not a matter for surprise that the 

 photographs, though sufficient for the purpose of detecting- 

 isotopes if present in reasonable proportion, are not suitable 

 for reproduction. 



Sodium gave the brightest effects, and its single line was 

 obtained so intense that the presence of another constituent 

 to the extent of even less than 1 per cent, could probably 

 have been detected. It may therefore be safely regarded as 

 a simple element. 



A mixture of potassium sulphate, potassium bromide, and a 

 little sodium phosphate was now used on the anode, ami after 

 several unsuccessful attempts some fairly satisfactory spectra 



