The Mass-Spectra of Chemical Elements. 629 



quite unmanageable and the tube ceased to work. Just 

 before it did, however, it yielded two very valuable spectra 

 which confirmed the isotopic nature of boron. These are 

 reproduced side by side as they were taken (Spectra I. &IL). 

 The lines at 10 and 11 are undoubtedly both first-order lines 

 of boron. The hypothesis that these might be due to neon 

 liberated by the action mentioned is not tenable, both on 

 account of their relative intensities and the absence of strong 

 neon first-order lines. Even if it were, it could not explain 

 the presence of the well-defined lines at 5 and 5'5 which had 

 never been obtained before at all, and which must be second- 

 order lines of boron. This element therefore has at least two 

 isotopes 10 and 11. The relative photographic intensity of 

 the lines 5 and .V5 does not agree well with an atomic 

 weight as high as 10"9, and the discrepancy might be ex- 

 plained by the presence of a third isotope at 12 ; which 

 would be masked by carbon, for it has not yet been found 

 practicable to eliminate carbon from the discharge. But 

 Spectrum IV. contradicts this suggestion for, as will be shown 

 later, the line at 49 is mainly if not wholly due to B n F 2 , so 

 that there should also be a line at 50 for B 12 F 2 . The line at 

 49 is very strong, but at 50 any small effect there may be 

 can safely be ascribed to the fourth order of mercury. The 

 evidence is clearly against the presence of a third isotope of 

 boron. 



The exceedingly accurate whole-number value for the 

 atomic weight of fluorine suggests the probability of this 

 element being simple. This conclusion is borne out by the 

 strong line at 19'00 with second-order line at 9*50. The 

 accompanying line at 20, very faint in Spectrum II., is no 

 doubt HF, though it may be also Ne 20 or second-order A 40 . 

 As there is no evidence whatever to the contrary, fluorine is 

 taken to be a simple element with an atomic weight 19. 



Having adopted these values for boron and fluorine, we 

 may now apply them to Spectra III. and IV. taken with 

 boron trifluoride. Consider first the group of three very 

 strong lines 47, 48, and 49. The last two are to be expected 

 as being due to B 10 F 2 and B n F 2 respectively, but since there 

 is no evidence of a boron 9 or a fluorine 18, line 47 cannot 

 be due to a compound of these elements. But line 47 only 

 appeared when BF 3 was introduced, and so must be due to 

 silicon fluoride formed by the action of the fluorine on the 

 glass walls and the silica anticathode. 



To test this the BF ; > was washed out and replaced by SiF 4 , 

 which has been made by the action of sulphuric acid on 

 calcium fluoride and silica in the usual way. This greatly 



