May 12, 1921] 



NATURE 



335 



curious jumble of whole numbers and fractions in 

 the atomic weights when referred to oxygen as 16 

 has always been a serious stumbling-block in the 

 way of any simple theory of atom-building. The 

 accurately determined atomic weight of chlorine, 

 .^546, has certainly nothing to recommend it. It 

 is reminiscent of the number of square yards in 

 a square rod, pole, or perch ; but the idea of 

 Nature working on the same lines as the British 

 weights and measures is eminently unattractive. 



The first support of the isotope theory among 

 non-radio-active elements was given by the ano- 

 malous behaviour of the inactive gas neon when 

 analysed by Sir J. J. Thomson's method of posi- 

 tive rays. It is of interest to note that the an- 

 nouncement was made in this room by 

 Sir J. J. Thomson himself, and that the first 

 sample of gas to show the effect was supplied by 

 Sir James Dewar. This peculiarity was that 

 whereas all elements previously examined gave 

 single, or apparently single, parabolas, that given 

 by neon was definitely double. The brighter curve 

 corresponded roughly to an atomic weight of 20, 

 the fainter companion to one of 22, the atomic 

 weight of neon being 20-20. In consequence of 

 reasoning adduced from the characteristics of the 

 line 22, the discoverer was of the opinion that it 

 could not be attributed to any compound, and 

 that therefore it represented a hitherto unknown 

 elementary constituent of neon. 

 This agreed very well with the 

 idea of isotopes which had just 

 been promulgated, so that it was 

 of great importance to investigate 

 the point as fully as possible. 



The first line of attack was an attempt at 

 separation by repeated fractionation over charcoal 

 cooled with liquid air, but, even after many thou- 

 sands of operations, the result was entirely nega- 

 tive. It is some satisfaction to know that this 

 result was inevitable, as Prof. Lindemann has 

 recently shown on thermodynamical grounds. 

 Fractional diffusion through pipeclay was more 

 effective, and gave a positive result. An ap- 

 parent difference of density of 07 per cent, 

 between the lightest and heaviest fractions was 

 obtained after an exceedingly laborious set of 

 operations. When the war interrupted the re- 

 search, it might be said that several independent 

 lines of reasoning pointed to the idea that neon 

 was a mixture of isotopes, but that none of them 

 could be said to carry the conviction necessary in 

 such an important development. 



When the work was recommenced, attention 

 was again turned towards positive rays, for it was 

 clear that if an analysis could be made with such 

 accuracy that it could be demonstrated with cer- 

 tainty that neither of the two atomic weights so 

 determined agreed with the accepted chemical 

 fieure, the matter could be regarded as settled. 

 This could not be done with the parabolas already 

 obtained, but the accuracy of measurement was 

 raised to the required degree by means of the 

 arrangement illustrated in Fig. i. Positive rays 

 are sorted out into a thin ribbon by nieans of the 

 \ NO. 2689, VOL. 107] 



two parallel slits Sj S2, and are then spread into 

 an electric spectrum by means of the charged 

 plates Pj Pg. A portion of this spectrum deflected 

 through an angle B is selected by the diaphragm 

 D and passed between the circular poles of a 

 powerful electromagnet O the field of which is 

 such as to bend the rays back again through an 

 angle ^ more than twice as great as Q. The result 

 of this is that rays having a constant mass (or, 

 more correctly, constant m/e) will converge to a 

 focus F, and if a photographic plate is placed at 

 GF, as indicated, a spectrum dependent on mass 

 alone will be obtained. On account of its analogy 

 to optical apparatus, the instrument has been 

 called a positive-ray spectrograph, and the spec- 

 trum produced a mass-spectrum. 



Fig. 2 shows a number of typical mass-spectra 

 obtained by this means. The numbers above the 

 lines indicate the masses they correspond to on 

 the scale 0=i6. It will be noticed that the dis- 

 placement to the right with increasing mass is 



Fig. I. — Diagram of positive-ray spectrograph. 



roughly linear. The measurements of mass made 

 are not absolute, but relative to lines which corre- 

 spond to known masses. Such lines, due to 

 hydrogen, carbon, oxygen, and their compounds, 

 are generally present as impurities, or purposely 

 added, for pure gases are not suitable for the 

 smooth working of the discharge tube. The two 

 principal groups of these reference lines are the 

 C, group due to C(i2), CH(i3), CH2(i4), CH3(i5), 

 CH4 or O (16), and the Co group (24 to 30) con- 

 taining the very strong line C2H4 or CO (28). 

 These groups will be seen in several of the 

 spectra reproduced, and they give, with the CO2 

 line (44), a very good scale of reference. 



It must be remembered that the ratio of mass 

 to charge is the real quantity measured by the 

 position of the lines. Many of the particles are 

 capable of carrying more than one charge. A 

 particle carrying two charges will appear as 

 having half its real mass, one carrying three 

 charges as if its mass were one-third, and so on. 

 Lines due to these are called lines of the secottd 



