the Wave-Length of the Soft 7 Rays from Radium B. 867 



changes by well-marked steps. For example, on the formula 

 given bv him, the reflexion angles from rocksalt of the 

 strongest line of the spectrum from an element of nucleus 

 charge 81 is 12"41°, for 82, 12-07°, for 83, 11-77°. In order 

 to make a mistake of one unit in the nucleus charge, an 

 error of 2 per cent, is necessary in measuring the angle of 

 reflexion of the reference line, while the experimental deter- 

 mination of the reflexion angles of the 7 rays from radium B 

 is believed to be correct within 0'3 per cent. 



It thus appears that the nucleus charge of radium B is 

 the same as that of lead, for the atomic number of radium B, 

 deduced by Moseley's formula from the 7-ray spectrum, is 

 that to be expected for lead, and the strong lines of the 7-ray 

 spectrum of radium Bseem to be coincident with those of lead. 

 According to radioactive calculation, the atomic weight of 

 radium B is 211, while that of lead is 207. Provided the 

 difference in atomic mass has not a large influence on the 

 vibration frequencies of the outer distribution of electrons, 

 it is to be anticipated that the ordinary light spectra of 

 radium B and lead should be nearly identical, while we 

 already know that these two elements have apparently 

 identical chemical properties. 



These results confirm in an unexpected way the correctness 

 of this deduction of Soddy and Fajans, and also give a definite 

 verifi cation of the hypothesis that two elements of different 

 atomic weights may have identical spectra and identical 

 chemical properties. A similar result has been recorded by 

 Sir J. J. Thomson and Aston in their work indicating that 

 neon consists of a mixture of two gases of atomic weights 

 about 20 and 22. The theory of the nucleus atom affords a 

 simple explanation of such a result ; for the chemical and 

 physical properties are for the most part determined by the 

 charge on the nucleus, and are practically independent of the 

 mass of the nucleus. The properties of radioactivity and 

 gravitation belong mainly to the nucleus. The fact that 

 radium B is radioactive while lead is not, shows that the 

 constitution of the nucleus is different in the two cases, and 

 this is borne out by the known difference in atomic weights. 



Taking the nucleus charge of radium B as 82, the nucleus 

 charge of all the elements in the uranium-radium family can 

 be deduced at once from the generalization already referred 

 to. The numbers are given in the following table : an a 

 radiation gives a decrease of 2 in the nucleus charge, a @ 

 radiation an increase of 1. 



