the Wave-Length of the Soft y Rays from Radium B. 865 



properties of the radioelements has been advanced by Fajans 

 and Soddy. From the point o.f view of the nucleus theory 

 of the atom, their conclusions may be expressed by the 

 simple relation that the expulsion of an a particle (carrying 

 two positive charges) from an atom lowers its nucleus charge 

 by two units, and the expulsion of a j3 particle (carrying one 

 negative charge) raises its nucleus charge by one unit. 

 Soddy has pointed out that the products radium B, actinium 

 B, thorium B, and radium D are the " isotopes " of lead, i. e. 

 they show identical chemical properties with those of lead, 

 from which they are inseparable by chemical methods. If 

 this view is correct, the atoms of these elements should have 

 the same nucleus charge, although they may differ slightly 

 in atomic weight. 



If radium B has the same nucleus charge as lead, it must 

 give an X-ray spectra almost identical with that of lead. It 

 should, however, be pointed out that a very small variation 

 in the frequency of the vibrations may be possible if the 

 nuclear masses are different. In his recent paper (loc. cit.) 

 Moseley has not determined the X-ray spectra of lead, but he 

 kindly pointed out to us that on his results its atomic number 

 or nucleus charge should be 82. He found gold had the 

 nucleus charge 79 ; the two intervening elements, mercury 

 and thallium, should have a nucleus charge of 80 and 81 

 respectively. From the relations found by him, it followed 

 by calculation that the strongest line of lead should be 

 reflected at 12*07° from rocksalt. The strongest line from 

 radium B found by us was 12'05° — a very close agreement. 



As it was possible, however, that there might be a small 

 error in comparing the reflexion angles of rocksalt with 

 different crystals and with such different experimental 

 arrangements, it was decided to test by a straightforward 

 method whether the X-ray spectra of radium B and lead were 

 identical within the limits of experimental error. For this 

 purpose it was arranged that the 7-ray spectra of radium B 

 and of lead should be compared, using the same apparatus 

 and under as nearly as possible identical conditions. H. 

 Richardson, working in this laboratory, has found that the 

 j3 rays expelled from radium B and radium C excite strongly 

 the characteristic " L *? type of radiation when they fall on 

 heavy elements. In order to take advantage of this result 

 the /§ rays from an emanation tube of the kind already de- 

 scribed were used to excite the characteristic radiation in a 

 strip of lead, 1 mm. thick and 5 mm. broad, which was then 

 used as the source in place of the tube itself, the rest of the 

 apparatus being disposed much as before. The arrangement 



