748 Prof. F. Horton and Miss A. C. Davies on 



about 20 volts is not the production of. radiation only, but 

 that ionization or! the gas also occurs, and becomes of 

 increasing importance as the gas pressure is increased. He 

 attributes this ionization to the impacts of electrons with 

 atoms which have absorbed radiant energy of the resonance 

 radiation from neighbouring atoms. Now it must be observed 

 that if the process of the handing on of resonance radiation 

 from atom to atom of the gas occurs (and experiments to be 

 described in the present paper show that this phenomenon 

 does take place), it of necessity means that the area of the 

 collecting electrode at which a photoelectric effect can take 

 place is considerably greater when this radiation is being 

 produced inside the apparatus, than when one end only of 

 the collecting electrode is being exposed to the radiation 

 from the mercury arc. This means that the value of the 

 ratio R obtained from the experiment with the mercury arc, 

 or from the geometrical calculation, is not applicable to the 

 case when resonance radiation is produced from the helium 

 atoms. It is easily seen that the production of this radiation 

 involves a decrease in the value of R, but without further 

 evidence it is not possible to decide whether R should vary 

 with the gas pressure for a pure radiation effect, 



Compton's conclusion that his results which were obtained 

 with helium containing a trace of neon cannot be due to the 

 presence of impurities is also open to criticism. The expe- 

 riments of Horton and Bailey have shown that the ionization 

 of an impurity by helium radiation may be considerable even 

 when the amount of the impurity present is so small as 

 to produce no measurable effect as the result of collisions 

 between its atoms and electrons. Moreover, ionization of a 

 trace of impurity by the helium radiation would lead to a 

 variation in the value of R with variation of the gas pressure 

 similar to that observed by Compton, if the amount of the 

 impurity present increased as the pressure of the helium was 

 raised, as it would do in the case of neon present in the 

 helium used. The amount of ionization of impurity produced 

 by a given quantity of helium radiation will become greater 

 as the small amount of impurity present increases, and, since 

 the ionization of this impurity involves the loss of a corre- 

 sponding amount of radiation available for photoelectric 

 action, the ratio R must decrease as the gas pressure in- 

 creases. For these and for other reasons the authors cannot 

 regard these experiments of Compton as affording conclusive 

 proof of the ionization of helium by electrons with less 

 than 25 volts energy, as the result of impacts on abnormal 

 atoms. 



