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XI. The Mass Spectra of Chemical Elements. (Part 3.) 

 By F. W. Aston, M.A., JJ.Sc, Clerk Maxwell Student 

 of the University of Cambridge* . 



[Plate VII.] 



IN two previous communications (Phil. Mag. xxxix. May 

 1920, p. 611, and xl. Nov. 1920, p. 628) the mass- 

 spectra of a number of elements were described. The 

 following paper is an account of further experiments made 

 with the same apparatus. 



The particular method of generating positive rays employed 

 (I. c. p. 612) by means of a large discharge tube is only 

 suitable for investigating elements which have themselves a 

 reasonably high vapour pressure, or are capable of forming 

 stable compounds which possess that necessary property. 

 To the first group belong the elementary gases, to the second 

 such elements as carbon or boron . 



Unfortunately the majority of the elements, including 

 all the metals except mercury, do not satisfy either of these 

 conditions to anv great extent. It is therefore natural 

 that as elements less and less suitable were employed the 

 work grew progressively more and more difficult and the- 

 results obtained either inconclusive or entirely negative. 



Selenium (At. Wt. 79-2). Tellurium (At. Wt. 1275). 



The compounds used in the experiments on these elements 

 were selenium hydride, made by passing a stream of hydrogen 

 through boiling selenium, and tellurium methyl, a sample 

 of which was very kindly supplied by Mr. Vernon, of the 

 Chemical Department. Complete failure resulted in both 

 cases. There was, indeed, on one spectrum an exceedingly 

 faint line at 79, but no shred of reliable evidence could be 

 found to ascribe it to an isotope of Se. In the case of 

 tellurium no trace of any line near 127 could be discovered. 

 The failure is unfortunate in the case of Te on account of its- 

 well-known anomalous position with respect to I in the 

 periodic table ; in the case of Se particularly so for the 

 following reasons : — If the accepted atomic weight is even 

 approximately correct this element must have one isotope at 

 least, of atomic weight greater than 78. But the numbers 

 79, 80, 81, 82, 83, 84, are already filled by isotopes of Br 

 and Kr, so that it is extremely probable that one of the iso- 

 topes of Se has an atomic weight identical with one of an. 



* Communicated by the Author. 



