RECENT ADVANCES IN SCIENCE 



21 



Values apparently differing from those of Franck, Knipping, 

 and Kruger have been obtained by other workers. Thus Bishop 

 as well as Davis and Goucher found, somewhat earlier, that 

 ionisation sets in at a potential of about 1 1 volts (already 

 discovered by Franck and Hertz), and also that a second ionisa- 

 tion potential exists at 15-8 volts. Further, Horton and Miss 

 Davies {Proc. Roy. Soc, 1920, 97, A, 23) have recently found 

 the following potential stages : 



First, a radiation or resonance potential at a minimum 

 electron velocity equivalent to 10-5 volts. This is presumably 

 the same as the lo-i volt inferred by Franck and his collabora- 

 tors as a possible resonance potential of the molecule. Horton 

 ascribes this effect to radiation from the atom, by displacement 

 of an electron from one orbit to another. 



Secondly, a further type of radiation resonance is produced 

 at 13-9 volts. Horton ascribes this to radiation from the 

 molecule. This is presumably the same as the resonance poten- 

 tial observed by Franck at 13-6 volts. 



Thirdly, Horton and Miss Davies find that ionisation of the 

 gas occurs at 14-4 volts. This value does not appear to have 

 any counterpart in the observations of Franck. It may be 

 pointed out that 14-4 volts corresponds to the wave length 

 84/i/x, which lies fairl3^ close to the accepted value of the dis- 

 persional wave length, 86 to Sy/j^/n, obtained from refractive 

 index data. Horton ascribes this stage to ionisation of the 

 atom, viz. H -> H+ + 0. It will be recalled that the value 

 calculated for this process on Franck's data is 13-3 volts. The 

 results are discordant. 



Finally, Horton and Miss Davies have observed a second 

 type of ionisation at 16-9 volts. It is ascribed to ionisation of 

 the molecule, namely H2 -> H + H^ + 0. This is presumably 

 the same potential as that observed by Franck and his colla- 

 borators at 17-1 volts, to which the same chemical significance 

 is attached. Horton and Miss Davies do not appear to have 

 observed the 15-8 volt potential of Bishop, Davis and Goucher. 



Although the various values quoted show a rough general 

 agreement, the discrepancies seem to be beyond the experi- 

 mental error involved in any actual determination. This 

 naturally raises the question as regards the interpretation of 

 the experimental results. It is impossible at the present time 

 to make any definite statement in this connection, but reference 

 may be made to a paper by van der Bijl {Physical Rev., 191 7, 

 10, 546), who concludes that three causes operate to affect the 

 value ascribed to a true ionisation potential. It is evident that 

 this aspect of the problem requires further investigation by 

 physicists. If reliable and concordant values could be obtained 

 for hydrogen and other gases, it would represent a considerable 



