Radiation and Ionization of Helium. 767 



radiation passed round from the side tube *. It is difficult 

 to conceive how such a small increase of radiation as is 

 caused by increasing the energy of the electrons from the 

 lower filament to 20*4 volts, could produce ionization to 

 the extent indicated by the rise in the dotted carve after this 

 point, so that it seems probable that it is the abnormal atoms 

 resulting from 20*4 volts electron impacts which are involved 

 in the production of ionization and not the 20'4 volts radia- 

 tion itself. If the abnormal atoms produced as a result of 

 the 20'4 volts electron impacts revert to the normal condition 

 less readily than the abnormal atoms produced as a result of 

 21*2 volts electron impacts, it is reasonable to suppose that 

 the ratio of the number of helium atoms in the main tube 

 which are in the abnormal condition at any instant, to the 

 number returning to the normal condition and emitting 

 radiation, is considerably greater in the case of the 20*4 volts 

 radiation from the lower filament, than it is in the case of 

 the radiation passed round from the side filament, because 

 this latter includes 21'2 volts radiation as well as 20*4 volts 

 radiation, and may, in fact, with the value of Y 5 used, consist 

 almost entirely of 21*2 volts radiation. Thus it is possible 

 that of the number of 20*4 volts abnormal atoms present in 

 the main tube in this experiment, when Y x has values between 

 204 volts and 21*2 volts, the greater proportion originates 

 from the collisions of the electrons from the lower filament 

 with helium atoms. If such is the case, the number of atoms 

 ionizable by helium radiation must be considerably increased 

 when the energy of the electrons from the lower filament 

 reaches 204 volts, and the increase of ionization indicated 

 by the bend at this point in the dotted curve is thus 

 explained. Oar experiments therefore tend to support the 

 view that the 20*4 volts abnormal atoms do not revert to 

 the normal condition so readily as the 21* 2 volts abnormal 

 atoms, and that the increase of ionization indicated by the 

 bends in the two curves is due to the ionization of 20*4 volts 

 abnormal atoms by the 21'2 volts radiation. A special 

 experiment was made to investigate the extent to which the 

 20*4 volts radiation was able to ionize helium atoms which 

 had absorbed 20'4 volts energy and which had not yet 

 returned to the normal condition. This experiment indicated 

 that the effect did occur to a slight extent, but the amount 

 of ionization so produced was very small compared with 

 that produced when 20*4 volts abnormal helium atoms were 

 exposed to 21*2 volts radiation. 



* It must "be observed that in the figure the current scale is different 

 for the two curves. 



