Mass-Spectra of Chemical Elements 619 



After several possible systems had been discussed it was 

 decided, for the present, to adopt the rather clumsy but 

 definite and elastic one of using the chemical symbol of 

 the mixed element with an index corresponding to its mass : 

 e.g., Ne 22 , Kr 84 . This system is made reasonable by the 

 fact that the masses of constituents of mixed elements 

 have all so far proved whole numbers on the scale used. 



In cases of particles carrying more than one charge 

 it will be convenient to borrow the nomenclature of optics 

 and refer to the lines given by singly, doubly, and multiply 

 charged particles respectively as lines of the first, second, 

 and higher orders. Thus the molecule of oxygen gives a 

 first order line at 32, and its atom first and second order 

 lines at 16 and 8. 



The empirical rule that molecules only give first order 

 lines (J. J. Thomson, 'Rays of Positive Electricity/ p. 54) 

 is very useful in helping to differentiate between elementary 

 atoms and compound molecules of the same mass. Some 

 very recent results give indications that in certain ex- 

 ceptional cases it may break down, so that inferences made 

 from it must not be taken as being absolutely conclusive. 



Oxygen (At. Wt. 16-00) and Carbon (At. Wt. 12-00). 



On a mass-spectrum all measurements are relative, and so 

 any known element could be taken as a standard. Oxygen 

 is naturally selected. Its molecule, singly-charged atom, 

 and doubly-charged atom give reference lines at 32, 16, 

 and 8 respectively. The extremely exact integral relation 

 between the atomic weights of oxygen and carbon is itself 

 strong evidence that both are " pure " elements, and so 

 far no evidence appears to have arisen to throw any doubt 

 on this point. Direct comparison of the line (12) 

 and the CO line (28) with the above standards shows that 

 the expected whole number relation and additive law hold 

 to the Lmit of accuracy, i. e. one part in a thousand : 

 and this provides standards C ++ (6), C (12), CO (2.8), and 

 C0 2 (4:4). In a similar manner, hydrocarbons give the 

 Gj and C 2 groups already mentioned (Phil. Mag. April 1920, 

 pp. 452, 453) ; so that a fairly complete scale of reference 

 is immediately available. 



Neon (At. Wt. 20-20). 



The results obtained w.ith this gas have already been fully 

 dealt with (Phil. Mag. April 1920, p. 449). It has been 

 shown to consist of tw 7 o isotopes of masses 20 and 22 re- 

 spectively, with the faint possibility of a fhird of mass 21. 



2 S 2 



