CONTEMPORARY ADVANCES IN PHYSICS 



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families classified into identical classes with identical binding-energies. 

 There is an evident difference; the atoms in the latter case are ionized 

 by radiation poured upon them from without, in the latter by processes 

 which occur within their own nuclei. (Whether in the latter case a 

 wave-train does actually leave a nucleus, and enjoy a real existence 

 during the brief time before it reaches the circumnuclear electron which 

 it is destined to eject, is a question to which it is not easy to give a 



Fig. 7. Beta ray spectra of radioactinium and actinium X 



(After O. Hahn and L. Meitner, ZS. f. Physik. The three upper pictures repre- 

 sent portions of the beta-ray spectrum photographed respectively a few hours, 

 6 days, and 20 days after the preparation of a pure sample of RdAc, in which the 

 daughter-substance AcX was steadily growing; the lowest, the corresponding 

 portion of the spectrum of a sample of AcX with its descendants. The lines which 

 diminish in intensity from top to bottom are those of RdAc, those which increase 

 belong to AcX and its descendants (note especially the lines marked a and b).) 



sensible answer!) But the difference does not affect the energies of the 

 ejected electrons; only their number, for, as seems natural enough, 

 the beta-rays expelled from atoms of which the nuclei are emitting 

 gamma-rays are much more abundant than those which an equal 

 amount of gamma-radiation extracts from atoms on which it falls 

 from without. Corresponding electron-groups have the same energy. 



