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LXXII. The Mass-Spectra of Chemical Elements. (Part 2.) 

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

 of the University of Cambridge * . 



[Plate XIV.] 



IN a previous paper (Phil. Mag. xxxix. May 1920, p. 611) 

 the apparatus for obtaining mass-spectra was fully de- 

 scribed and the results of analysis of eleven different elements 

 tabulated. The following paper deals with the analyses of 

 some additional elements for which the same apparatus and 

 method was used. 



Boron (At. W. 11-90). Fluorine (At. W. 19*00). 

 Silicon (At. W. 28-3). 



It will be convenient to treat of these three elements 

 together. The atomic weights of boron and fluorine have 

 both been recently redetermined by Smith and Van Haagen 

 (Carnegie Inst. Washington Publ. No. 267, 1918), with the 

 above results. On the atomic weight of silicon there is some 

 divergence of opinion. The international value is quoted 

 above, but Baxter, Weatherell, and Holmes make it nearer 28*1 

 (Journ. Am. Chem. Soc. vol.xlii. p. 1194, June 1920). 



After a failure to obtain the boron lines with some very 

 impure boron hydride, a sample of boron trifluoride was pre- 

 pared from boric acid and potassium borofluoride, and this 

 gave good results. Following the usual practice, it was 

 mixed with a considerable quantity of C0 2 before intro- 

 duction into the discharge-tube. Very complex and inter- 

 esting spectra were at once obtained, and it was remarked 

 that this gas possessed an extraordinary power of resurrecting 

 the spectra of gases previously used in the apparatus. Thus 

 the characteristic first and second order lines of krypton 

 were plainly visible, although the tube had been washed out 

 and run many times since that gas had been used. This 

 property of liberating gases which have been driven into the 

 surf ace of the discharge-bulb is doubtless due to the chemical 

 action of the fluorine, liberated during the discharge, on the 

 silica anticathode and the glass walls. After running some 

 time the corrosion of the anticathode was indeed quite visible 

 as a white frost over the hottest part. 



After several successful series of spectra had been secured, 

 the percentage of boron trifluoride in the gas admitted was 

 increased as far as possible, until the discharge became 



* Communicated by the Author. 



