Ionization by Cumulative Action. 535 



in absolutely pure helium are quoted in the writer's original 

 paper. The degree of parity may be judged by the fact 

 that, in this helium, arc currents as large as an ampere were 

 obtained at the minimum radiating voltage witlwut observing 

 any spectral line of an impurity. Every line in the visible 

 line spectrum of helium (excepting the enhanced lines) was 

 observed and, in addition, more than two hundred lines of 

 the band spectrum. It is impossible that such a degree of 

 ionization and such an intense helium spectrum could be 

 attributed to ionization of impurities which were present in 

 too small quantity to be detected in the spectrum. 



(2) The criticism of Horton and Miss Davies to the effect 

 that the writer's measurements of R (which is the ratio of 

 the electrometer deflexions observed with the foil and gauze 

 ends, respectively, of a hollow cylinder presented toward the 

 filament) do not give an exact estimate of the proportion of 

 the observed effect due to ionization or radiation separately, 

 is well taken. The uncertainty is pointed out as due to the 

 presence of a photoelectric emission from the sides and back 

 of the cylinder,* as well as from its front, due to resonance 

 radiation passed on from atom to atom through the gas. 

 Such photoelectric emission from the sides was neglected in 

 the writer's calculations. Yet the observed variation in R 

 is entirely too large to be accounted for in this way. For 

 instance, at 25 mm. pressure, it would require a photoelectric 

 emission from the sides of the cylinder twelve times larger 

 than that from the foil end to account in this way for the 

 observed value of R. But the area of the sides is only four 

 times that of the face, and they are much less favourably 

 placed as regards the electric field and their accessibility to 

 the radiation. For these reasons the writer stated that the 

 absolute values of R are of little importance, but that the 

 variation of R pointed conclusively to ionization. 



For these reasons the writer cannot admit that Professor 

 Horton and Miss Davies have been the first to definitely 

 prove the existence ol ionization between the minimum 

 radiating and ionizing voltages in helium, or the essential 

 part played by resonance radiation in producing this ioniza- 

 tion. But the work of Horton and Miss Davies strikingly 

 proves that radiation may play an even more important role 

 in cumulative ionization than had previously been suggested, 

 being: active not onlv in maintaining atoms in an abnormal 

 state, but also in photoelectrically ionizing them when in 

 this condition. 



It is reasonable to suppose that cumulative ionization 

 becomes an increasingly important factor in ionization as 



