of Radiation by Electron Impacts in Helium. 567 



collisions of various speeds, etc. It is probable that the low- 

 voltage arc in helium, which is now being studied, offers a 

 better opportunity to put the theory in suitable form for tests 

 of a more quantitative nature than can be applied to the 

 experiments of the present paper. 



Can the Observed Ionization between 20'2 and 25*5 volts be 

 due to Impurities in the Helium f 



Franck and Hertz have accounted for ionization between 

 the resonance and ionization potentials by a photoelectric 

 effect of the 20-volt radiation on gaseous impurities. The 

 only impurity detected in these experiments was a slight 

 trace of neon, except for the possible presence of minute 

 quantities of water-vapour, which were shown to be too 

 small to affect the results. Neon is the only substance whicli 

 could complicate the results without its effect being easily 

 separable from that due to helium, since its ionization potential, 

 19*5 volts, is so near the resonance potential of helium that 

 the two effects may merge into one another. Unfortunately, 

 neon is that impurity most difficult to remove from helium. 

 The helium used in this experiment was originally supposed 

 to be very pure. It was subsequently, in the course of this 

 and other researches, purified by several hundred fractionations 

 from coconut charcoal in liquid air. No neon lines could be 

 observed in its spectrum except with a very intense arc main- 

 tained by a thermionic current from a tungsten electrode. 



There are three considerations which seem to disprove the 

 appreciable influence of neon in the above experiments : — 

 (1) There is no evidence of radiation or of inelastic impacts 

 due to collisions with neon atoms at velocities below 20 volts. 

 If neon were present in sufficient quantity to account for th^ 

 observed ionization, we should expect to detect its presence 

 by the effect of inelastic collisions at its resonance potential 

 below 20 volts, especially at higher gas-pressures where the 

 chances of collisions with neon atoms are greatly enhanced 

 by the enormously increased paths of the electrons resulting 

 from the elastic impact with helium. (2) Since the ionizing 

 potential of neon is a little less than the resonance potential 

 of helium, we should expect the first effects detected as V a is 

 increased to be due entirely to ionization of neon, and 

 therefore to find the value of I( near to unity for the first 

 measurable currents, increasing when V a passes the reso- 

 nance potential of helium. Such a variation of R lias not 

 been observed. (3) No variation in the Jas-pressure should 

 alter the proportion of the effect due to ionization of neon in 

 comparison with that due to radiation from helium so long 



