622 Dr. F. W. Aston on the 



coincidences cannot be detected on the same spectrum photo- 

 graphically ; but if we first add and then subtract a small 

 potential from one of the large potentials, two lines will be 

 obtained which closely bracket the third. To take an actual 

 instance — with a constant current in the magnet of 0*2 

 ampere, three exposures were made with a gas containing 

 hydrogen and helium at potentials of 250, 500 + 12, and 

 500 — 12 volts respectively. The hydrogen molecule line was 

 found symmetrically bracketed by a pair of atomic lines 

 (Spectrum VII. a and c), showing that the mass of the 

 molecule is exactly double the mass of the atom within 

 experimental error. When after a suitable increase of the 

 magnetic field the same procedure was applied to the helium 

 line and that of the hydrogen molecule, the bracket was no 

 longer symmetrical (Spectrum VII. 6), nor was it when the 

 hydrogen molecule was bracketed by two helium lines (d). 

 Both results show in an unmistakable manner that the mass of 

 He is less than twice that of H 2 . In the same way He was 

 compared with ++ , and H 3 , obtained from KOH by Sir J. J. 

 Thomson's bombardment method, with C + + . 



The method has some definite advantages and some dis- 

 advantages. It is not proposed to discuss these in detail at 

 present. The values obtained by its use can be checked in 

 the ordinary way by comparing He with C ++ and H 3 with 

 He, these pairs being close enough together for the purpose. 

 The following table gives the range of values obtained from 

 the most reliable plates :-- 



Line Method. Mass assumed. Mass deduced. 



r bracket 0+ + = 8 3*994— 3'996 



He 



I direct C + + = 6 4-005—4-010 



r bracket C++ =6 3025—3-027 



3 ■'"'"' I direct He = 4 3-021—3030 



H 2 bracket He = 4 2-012—2-018 



From these figures it is safe to conclude that hydrogen is 

 a "pure" element and that its atomic weight, determined 

 with such consistency and accuracy by chemical methods 

 (1*008), is the true mass of its atom. 



The above results incidentally appear to settle the nature 

 of the molecule H 3 beyond doubt. 



