632 Dr. F. W. Aston on the 



gas was admitted, on the same plate with approximately the 

 same fields. The very marked strengthening of lines 32 and 

 44 is no doubt due to S and OS. New lines appear at 33 SH, 

 34 SH 2 , 60 COS, 64 S0 2 or S 2 , and 76 OS,. It may be 

 noticed that lines 32, 60, and 76 are accompanied by a taint 

 line one unit higher and a rather stronger line two units 

 higher. In the first case it is certain and in the others 

 probable that these are, at least partly, due to hydrogen 

 addition compounds. It' a higher isotope of sulphur exists, 

 as is suggested by the chemical atomic weight, it seems 

 unlikely that this should have mass 33, for this would have 

 to be present to the amount of 6 per cent., and should give a 

 line at 35 one-thirteenth the strength of 34 (normal SH 2 ). 

 No such line is visible. A sulphur of atomic weight 34 

 present to the extent of 3 per cent, is more likely, but there 

 is hardly enough evidence as yet to warrant its serious 

 consideration. 



Phosphorus (At. Wt. 31-04). Arsenic (At. Wt. 74-96). 



The gases phosphine PH 3 and arsine AsH 3 were used in 

 the experiments on these elements, and the results were of 

 notable similarity. The mass-spectrum of each gas was 

 characterized by a group of four lines . The first and strongest 

 doubtless due to the element itself, the second rather weaker 

 due to the monohydride, the third very faint to the dihydride, 

 and the fourth fairly strong to the trihydride. The spectrum 

 of AsH 3 is shown in Spectrum IX.; that of phosphorus is 

 similar but its lines are weak, and therefore unsuited to 

 reproduction. Both elements appear to have no isotopes, 

 and neither give visible second-order lines. 



Lines of unknown origin. 



During the experiments with mass-spectra, lines have 

 appeared from time to time to which it has been difficult 

 to assign an origin with certainty. Three of these seem 

 worthy of special note. 



On one of the spectra taken with boron trifluoride there 

 appeared a faint but unmistakable line at 5'33. The 

 accuracy of its fractional value seems to insure it being 

 triply-charged 16. If the source of the mass 16 is oxygen, it 

 is somewhat odd that the line never appeared when oxygen 

 was present in much greater quantity, but it may be possible 

 that the loss of three charges only takes place when fluorine 

 is present. 



An even more baffling line is one at 6*50 which has 



