1921] on Isotopes and Atomic Weights 301 



Quantities were obtainable which were ample for the most accurate 

 determinations by chemical methods, and the atomic weights were 

 found to differ from each other and from ordinary lead by quantities 

 altogether outside possible experimental error. 



Long before this convincing proof was forthcoming the theory of 

 isotopes was discussed with the greatest interest in connection with 

 atomic weights in general. If isotopes occurred among the heavy 

 elements, why should they not be possible among thj lighter non- 

 radioactive ones ? In which case elements with fractional atomic 

 weights might clearly be mixtures, the constituents having atomic 

 weights equal to whole numbers. This explanation was a very attrac- 

 tive one, for the 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 35 ■ 46 

 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 rirst support of the isotope theory among non-radioactive 

 elements was given by the anomalous behaviour of the inactive gas 

 neon when analyzed by Sir J. J. Thomson's method of positive rays. 

 It is of interest to note that the announcement was made in this 

 room by Prof. Thomson himself, and that the first sample of gas 

 to show the effect was supplied by Prof. 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 investi- 

 gate 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 thousands of operations the result was entirely negative. 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 apparent difference of density of • 7 per cent 

 between the lightest and heaviest fractions was obtained after an 

 exceedingly laborious set of operations. When the war interrupted 

 the research, it might be said that several independent lines of 

 reasoning: pointed to the idea that neon was a mixture of isotopes. 



Y 2 



