6i8 



NATURE 



[July 15, 1920 



which have different weights. This conclusion 

 has been recently confirmed in a most convincing- 

 manner by the production in quantity of specimens 

 of lead from radio-active and other sources, which, 

 though perfectly pure and chemically indistin- 

 guishable, give atomic weights differing by 

 amounts quite outside the possible experimental 

 error.* Elements differing in mass but chemically 

 identical and therefore occupying the same posi- 

 tion in the periodic table have been called "iso- 

 topes " by Prof. Soddy. ^ 



At about the same period as the theory of iso- 

 topes was being developed by the radio chemists 

 at the heavy end of the periodic table an ex- 

 tremely interesting discovery was made by Sir 

 J. J. Thomson, which carried the attack into the 

 region of the lighter and non-radio-active 

 elements. This was that, when positive rays from 

 gases containing the element neon were analysed 

 by electric and magnetic fields, results were ob- 

 tained which indicated atomic weights roughly 20 



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



and 22 respectively, the accepted atomic weight 

 being 202. This naturally led to the expectation 

 that neon might be a mixture of isotopes, but 

 the weight 22 might possibly be due to other 

 causes, and the method of analysis did not give 

 suflficient accuracy to distinguish between 200 and 

 20-2 with certainty. Attempts were made to effect 

 partial separation first by fractionation over char- 

 coal cooled in liquid air, the results of which were 

 absolutely negative, and then by diffusion, which 

 in 1913 gave positive results, an apparent change 

 in density of 07 per cent, between the lightest 

 and heaviest fractions being attained after many 

 thousands of operations. When the war inter- 

 rupted the research, 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 con- 

 viction necessary in such an important develop- 

 ment. 



By the time work was started again the isotope 

 theory had been generally accepted so far as the 

 NO. 2646, VOL. 105] 



radio-active elements were concerned, and a good 

 deal of theoretical speculation had been made as 

 to its applicability to the elements generally. As 

 separation by diffusion is at the best extremely 

 slow and laborious, attention was again turned 

 to positive rays in the hope of increasing the 

 accuracy of measurements to the required degree. 

 This was done by means of the arrangement 

 illustrated in Fig. i. Positive rays are sorted 

 into an extremely thin ribbon by means of parallel 

 slits S^ S2, and are then spread into an electric 

 spectrum by means of the charged plates P^ P2. 

 A portion of this spectrum deflected through an 

 angle Q is selected by the diaphragm D and passed 

 between the circular poles of a powerful electro- 

 magnet O the field of which is such as to bend 

 the rays back again through an angle ^ more 

 than twice as great as B. The result of this is 

 that rays having a constant mass (or more cor- 

 rectly constant m/e) will converge to a focus F, 

 and that if a photographic plate is placed at GF 

 as indicated, a spectrum de- 

 '« pendent 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 spectrum produced a 

 mass-spectrum. 



Fig. 2 shows a number of 



typical mass-spectra obtained 



by this means. The number 



above the lines indicates the 



masses they correspond to on 



the scale = i6. It will be 



noticed that the displacement 



]lo the right with increasing 



mass is roughly linear. The 



I measurements of mass made 



.'ire not absolute, but relative 



to lines the mass of which is 



known. 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 (12), CH (13), CH^ 



(14), CH3 (15), CH4 or O (16), and the Cj 



group 24-30 containing the very strong line 



28 C2H4 or CO. In spectrum i. the presence 



of neon is indicated by the lines 20 and 22 



situated between these groups. Comparative 



measurements show that these lines are 2000, 



2200, with an accuracy of one-tenth per cent., 



which removes the last doubt as to the isotopic 



nature of neon. 



The next element investigated was chlorine ; this 

 is characterised by four strong lines 35, 36, 37, 38, 

 and fainter ones at 39, 40 ; there is no trace of 

 a line at 35-46, the accepted atomic weight. From 

 reasoning which cannot be given here in detail it 

 seems certain that chlorine is a complex element, 

 and consists of isotopes of atomic weights 35 and 



