772 Radiation and Ionization of Helium. 



that Kemble, from theoretical considerations, was led to the 

 view that the application of the selection principle of Bohr 

 to atoms reverting to the normal state, would have to be 

 abandoned. 



The investigations described in this paper have shown 

 that abnormal atoms of the crossed system (those produced 

 by the absorption of 21*2 volts energy) revert to the normal 

 condition mere readily than abnormal atoms of the coplanar 

 system (those produced by the absorption of 20'4 volts 

 energy), but as we have shown that the limitations of the 

 selection principle of possible transitions do not apply to 

 the fundamental displacements of the outer electron of the 

 helium atom, no conclusion can be drawn from this fact 

 regarding the identification of the normal orbit of this 

 electron with the one system of orbits rather than with 

 the other. 



Summary. 



The Introduction contains an outline of the development of 

 ideas concerning the arrangement of the two electrons in the 

 helium atom, starting from the authors' determination of the 

 minimum radiation voltage and the minimum ionization 

 voltage for normal helium. In this connexion the theoretical 

 deductions of Bohr, Lande, Franck and Reiche, and Kemble 

 are discussed, and the experimental work of K. T. Compton 

 and of Franck and Knipping is considered and criticised. 



In view of the fact that these experimenters claim that their 

 results prove the occurrence of certain effects, which, in both 

 cases, are not in agreement with the results of the authors' 

 previous work with low pressures of helium, and which, 

 moreover, are not reconcilable with each other, the main 

 points wherein the deductions of these workers extend beyond 

 the authors' former conclusions have been directly investi- 

 gated, in a specially designed apparatus, over a wider range 

 of pressures than that used before. The experiments have 

 shown that vn pure helium : — 



1. Radiation is produced by the impacts of electrons with 



20*4 volts energy, in confirmation of the earlier 

 conclusion of the authors, but in disagreement 

 with the most recent conclusion of Franck and 

 Knipping. 



2. A second type of radiation is produced at 21*2 volts, 



in agreement with the conclusions of Franck and 

 Knipping. 



