534 Prof. K. T. Compton on 



impact may be of primary importance in producing ionization 

 in rarified gases, as in Geissler tubes, ionization by cumulative 

 action is of preponderating importance in arcs in monatomic 

 gases and vapours. 



As to the nature of this cumulative action, there are four 

 possibilities which have been suggested. The majority of 

 atoms which are in an abnormal or partially ionized state 

 may be in this state either as a result of a previous impact 

 or by absorption of resonance radiation produced by electron 

 impacts against neighbouring atoms. Then the second stage 

 of complete ionization may be brought about, either by an 

 electron impact against the abnormal atom or by its photo- 

 electric ionization by additional radiation. 



As the result of a tentative formulation of the theory of 

 ionization by direct successive impacts * and experiments on 

 helium f , the writer came to the conclusion that the actual 

 amount of ionization is much larger than can reasonably be 

 accounted for in this way. It can be shown, for example, 

 that if the observed results are to be explained simply by 

 successive impacts, it would be necessary to take the time 

 constant of damping of resonance radiation to be about a 

 million times greater than any value of this constant which 

 has been found lor any substance by direct experiment. 

 Hence, it was concluded that the energy of resonance 

 radiation from neighbouring atoms must contribute to the 

 total energy required for ionization. 



If impurities are present in helium, they will be photo- 

 electrically ionized by the helium resonance radiation. 

 Goucher J and others who have obtained experimental evi- 

 dence of ionization in helium below the ionizing potential 

 have' attributed it to such photoelectric ionization of 

 impurities. 



Professor Horton and Miss Davies § have criticized the 

 writer's proof of the existence of true ionization of helium 

 below the ionizing potentials as inconclusive on two grounds : 

 (1) the probable photoelectric ionization of neon or other 

 impurity, and (2) uncertainty regarding the significance of 

 a measured ratio R, on whose values the conclusions w r ere 

 based. The following evidence, however, proves these 

 criticisms to be unfounded : — 



(1) Although earlier experiments were made with helium, 

 in which a trace of neon was spectroscopically detectable, 

 later experiments in which similar ionization was obtained 



* Phil. Mao-, loc. cit. t Phil. Mag-, loc. tit. 



X Proc. Phys. Soc. xxxiii. p. 13 (1920). 

 § Loc. cit. 



