﻿Ionization oj Abnormal Helium Atoms. 1141 



particular state of the helium atom is referred to by Kannen- 

 stine as the " metastable " state, and his results are consistent 

 with the view that the particular state of the helium atom 

 which is metastable is the one into which the atom is thrown 

 as the result of an encounter with an electron having 20*4 

 volts energy. It will he shown later, however, that the 

 results obtaned by Kannenstine can be explained without 

 the necessity of supposing one of the abnormal states of the 

 helium atom to be endowed with any greater stability than 

 other abnormal states. 



The suooestion that a metastable state of the helium atom 

 exists was first made, on theoretical grounds, by Franck and 

 Reiche*, and the results of certain experiments made by 

 Franck and Knipping were interpreted by these experi- 

 menters as confirming the suggestion f- There is, however, 

 an essential difference between the term "metastable" as 

 used by Kannenstine and as used by Franck, Reiche, and 

 Knipping ; for the former uses the term in the sense that the 

 helium atom remains in that particular condition for a small, 

 but measurable, interval of time, whereas Franck and his 

 collaborators use the term to denote inability of the helium 

 atom to revert from that particular state to the normal state 

 without the assistance of external agencies. 



Evidence on this point is provided by the results of 

 certain experiments performed by the authors, and described 

 by them in the Philosophical Magazine for November 

 1921. The conclusions to which these experiments led,, 

 while agreeing with those of Franck and Knipping in some 

 respects, differed from them in regard to the production 

 of radiation in pure helium bombarded by 20'4 volts elec- 

 trons ; for while Franck and Knipping concluded that 

 radiation is not produced at all iii these circumstances, the 

 authors concluded that an emission of radiation undoubtedly 

 takes place, but to a smaller extent than at 21*2 volts, the 

 second critical electron energy in helium. The results 

 obtained by the authors in the experiments referred to, 

 indicated that the abnormal atoms produced by 20'4 volts 

 electronic-atomic encounters reverted to the normal con- 

 dition less readily than the abnormal atoms produced as a 

 result of 21*2 volts encounters. 



In the paper already referred to, the authors describe 

 experiments in which they attempted to detect ionization 



* J. Franck and O. Reiche, Zeits. f. Phys. vol. i. p. 154 (1920). 

 t J. Franck and P. Knipping, Zeits. f. 'P?n,s. vol. i. p. 320 (1920). 



