Mass-Spectra of Chemical Elements. 621 



so that by the time it had reached suitable working conditions 

 again, all traces of chlorine had disappeared. The tube was 

 run with a mixture of C0 2 and CH 4 , and then about 20 per 

 cent. of argon added. The main constituent of the element was 

 at once evident from a very strong line at 40 (Spectrum VI.) 

 reproduced in the second and third orders at 20 and 13*33 

 (Spectrum Y.). The third order line is exceedingly well 

 placed for measurement, and from it the mass of the singly- 

 charged atom is found to be 40*00 + "02. At first this was 

 thought to be the only constituent, but later a faint companion 

 was seen at 36, which further spectra showed to bear a very 

 definite intensity relation to the 40 line. No evidence drawn 

 from multiple charges is available ini this case owing to the 

 probable presence of OH 2 and ; 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 

 density. 



Nitrogen (At.Wt, 14-01). 



This element shows no abnormal characteristics : its atom 

 cannot be distinguished, on the present apparatus, from CH 2 

 nor its molecule from ( )0. Its second order line on careful 

 measurement appears 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 reference 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 1000 by 

 means of a null instrument. If exposures are made with such 

 potentials applied to the electric plates first in parallel and 

 thenin series, the magnetic field being kept constant, allmasses 

 having an exact two to one relation will be brought into coin- 

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



