1917] on The Complexity of the Chemical Elements 137 



determination of the atomic numbers in every case. From now on, 

 this number, which represents the + charg-e on the nucleus, rather 

 than the atomic weight, becomes the natural constant which deter- 

 mines chemical character, light and X-ray spectra, and, in fact, all 

 the properties of matter, except those that depend directly on the 

 nucleus — mass and weight on the one hand, and radioactive properties 

 on the other. 



What, then, were the isotopes on this scheme ? Obviously they 

 were elements with the same atomic number, the same nett charge on 

 the nucleus, but with a differently constituted nucleus. Take the 

 very ordinary sequence in the disintegration series, one a- and two 

 /?-rays being successively expelled in any order. Two + and two 

 - charges have been expelled, the nett charge of the nucleus remains 

 the same, the chemical character and spectrum the same as that of 

 the first parent, but the mass is reduced 4 units because a helium 

 atom, or rather nucleus, has been expelled as an a-particle. The 

 mass depends on the gross number of + charges in the nucleus, 

 chemical properties on the difference between the gross numbers of 

 + and - charges. But the radioactive properties depend not only 

 on the gross number of charges but on the constitution of the 

 nucleus. We can have isotopes with identity of atomic weight, as 

 well as of chemical character, which are different in their stability 

 and mode of breaking up. Hence we can infer that this finer degree 

 of isotopy may also exist among the stable elements, in which case it 

 would be completely beyond our present means to detect. But when 

 transmutation becomes possible such a difference would Ije at once 

 revealed. 



The case is not one entirely of academic interest, because it is 

 probal)le that the reconciliation of the conflicting views of the geolo- 

 gists and chemists, who concluded that lead was not the ultimate 

 product of thorium, and those who by atomic weight demonstrations 

 on the lead have shown that it is, depends probably on this point. 



As has long been known, thorium-C, an isotope of bismuth, disin- 

 tegrates dually. For 35 per cent of the atoms disintegrating, an 

 a-ray is expelled followed by a /?-ray. For the remaining G5 per cent 

 the /5-ray is first expelled and is followed by the a-ray. The two 

 products are both isotopes of lead, and both have the same atomic 

 weight, but they are not the same. More energy is expelled in the 

 changes of the 65 per cent fraction than in those of the 35 per cent. 

 Unless they are both completely stable a difference of period of 

 change is to be anticipated. 



The same thing is true for radium-C, but here all but a very 

 minute proportion of the atoms disintegrating follow the mode fol- 

 lowed by the 65 per cent in the case of thorium-C. The product in 

 this case, radium-D, which, of course, is also an isotope of lead, with 

 atomic weight 210, is not permanently stable, though it has a fairly 

 long period, 24 years. The other product is not known to change 



