630 Dr. F. W. Aston on the 



reduced the lines 48 and 49, and so they must be attributed 

 to boron compounds. At the same time line 47 remained 

 very strong, and was evidently due to a compound Si 28 F, so 

 that silicon has a predominant constituent 28. This con- 

 clusion is further supported by the presence of very strong 

 lines at 66, Si 28 F 2 , and 85, Si 28 F 3 . 



The chemical atomic weight shows that this cannot be its 

 only constituent. Lines at 29, 48, 67. and 86 all suggest a 

 silicon of atomic weight 29. Practically conclusive proof of 

 this is given in Spectrum V., which shows its second-order 

 line unmistakably at 14*50. The only other reasonable 

 origin of this line, namely second-order B 10 F, is eliminated 

 by the fact that there is no trace of a line at 10 in this 

 spectrum. 



The evidence of a silicon of atomic weight 30 is of a much 

 more doubtful character. Its presence is suggested by the 

 lines 30, 49, 68, and 87, but the possibility of hydrogen 

 compounds makes this evidence somewhat untrustworthy, and 

 no proof can be drawn from a second-order line 15, as this is 

 normally present and is due to CH 3 . On the other hand, if 

 we accept a mean atomic weight as high as 28*3, the relative 

 intensity of the lines due to compounds of Si 28 and Si 29 in- 

 dicates the probable presence of an isotope of higher mass. 

 These considerations taken with the complete absence of any 

 definite evidence to the contrary make the possibility of 

 Si 30 worth taking into account. 



Molecular lines of the Second Order. 



The work of Sir J. J. Thomson on multiply-charged positive 

 rays showed very definitely that molecules carrying more 

 than one charge were at least exceedingly rare (' Rays of 

 Positive Electricity/ p. 54), for not a single case was observed 

 which could not be explained on other grounds. Up to the 

 time of the experiments with the fluorine compounds the 

 same could be said of the results with the mass-spectrograph. 

 This absence of multiply-charged molecular lines, though 

 there is no particular theoretical reason for it, has' been used 

 as confirmatory evidence on the elementary nature of doubtful 

 lines. 



The spectra obtained with BF 3 show lines for which there 

 appears no possibility of explanation except that of doubly- 

 charged compound molecules. The two most obvious of 

 these may be seen on Spectrum III. and at the extreme left- 

 hand end of Spectrum IV. They correspond to masses 

 2350 and 24*50, the first being quite a strong line. Were 



