MASS SPECTRA OF THE ELEMENTS — ASTON. 235 



given by singly, doubly, and multiply charged particles, respectively, 

 as lines of the first, second, and higher orders. Thus the molecule 

 of ox} T gen 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 help- 

 ing to differentiate between elementary atoms and compound mole- 

 cules of the same mass. Some very recent results give indications 

 that in certain exceptional cases it may break down, so that inferences 

 made from it must not be taken as being absolutely conclusive. 



OXYGEN (AT. WT. 1G.0 0) 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 ex- 

 tremely 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 C line (12) and the CO line 

 (28) with the above standards shows that the expected whole number 

 relation and additive law hold to the limit of accuracy, i. e., one 

 part in a thousand; and this provides standards C ++ (6), C (12), CO 

 (28), and C0 2 (44). In a similar manner, hydrocarbons give the 

 Cx 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 with this gas have already been fully dealt 

 with (Phil. Mag., April, 1920, p. 449). It has been shown to con- 

 sist of two isotopes of masses 20 and 22, respectively, with the faint 

 possibility of a third of mass 21. Spectrum I on plate 1 shows 

 the singly charged lines of neon, to the left of the C 2 group. It is 

 reproduced here to show the condition of the discharge tube im- 

 mediately before compounds of chlorine were introduced. 



CHLORINE (AT. WT. 35.46). 



Spectra, indicating that this element was a mixture of isotopes, 

 were first obtained by the use of hydrochloric acid gas, but as this 

 was objectionable, on account of its action on mercury, phosgene 

 (COClo) was substituted. Spectra II, III, and IV are reproduced 

 from one of the plates taken with this gas. It will be seen that 



