MASS SPECTRA OF THE ELEMENTS ASTON. 237 



available in this case, owing to the probable presence of OIL and C ; 

 but the above intensity relation and the absence of the line from 

 spectra, taken just before argon was introduced, make it extremely 

 likely that it is a true isotope. The presence of about 3 per cent 

 would account for the fractional atomic weight determined from the 

 density. 



NITROGEN ( AT. WT. 1 4 .0 1 ) . 



This element shows no abnormal characteristics ; its atom can not 

 be distinguished, on the present apparatus, from CH 2 , nor its mole- 

 cule from CO. Its second-order line, on careful measurement, ap- 

 pears to be exactly 7, so it is evidently a pure element, as its chemical 

 combining weight would lead one to expect. 



HYDROGEN (AT. WT. 1.008) AND HELIUM (AT. WT. 3.99). 



The determination of masses so far removed as these from the refer- 

 ence lines offers peculiar difficulties, but as the lines were expected to 

 approximate to the terms of the geometrical progression 1, 2, 4, 8, etc., 

 the higher terms of which are known, a special method was adopted 

 by which a two to one relation could be tested with some exactness. Two 

 sets of accumulators were selected, each giving very nearly the same 

 potential of about 250 volts. The potentials were then made exactly 

 equal by means of a subsidiary cell and a current-divider, the equality 

 being tested to well within 1 in 1,000 by means of a null instrument. 

 If exposures are made with such potentials applied to the electric 

 plates first in parallel and then in series, the magnetic field being kept 

 constant, all masses having an exact 2 to 1 relation will be brought 

 into coincidence on the plate. (Phil. Mag., April, 1920, p. 453.) Such 

 coincidences can not be detected on the same spectrum photographi- 

 cally ; 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 contain- 

 ing hydrogen and helium at potentials of 250, 500-)- 12, and 500—12 

 volts, respectively. The hydrogen molecule line was found symmetri- 

 cally 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 sym- 

 metrical (Spectrum VII, £>), nor was it when the hydrogen molecule 

 was bracketed by two helium lines (d). Both results show in an un- 

 mistakable manner that the mass of He is less than twice that of H 2 . 



