G86 Velocities of Photo-Electrons emitted from Matter. 



/ i = G'55xlO- 27 erg sec, e=4-75 x lO" 10 E.S.U., and tbe 

 critical wave-length to be \ 1350, we get the ionizing 

 potential V ( = /tnje) to be 9 '2 volts. The most recent and 

 accurate work on the ionizing potential is that of Franck and 

 Hertz *, who found that the ionizing potential for oxygen is 

 9'0 volts, which agrees well with the value calculated from 

 photo-electric experiments. 



One point arises in connexion with the ionizing potential. 

 It might be expected that less work is required to remove 

 si n electron from a molecule of a substance when solid or 

 liquid than when gaseous. The forces tending to pull the 

 escaping electron back into the parent molecule may be 

 considerably reduced by forces originating in adjacent 

 molecules, and hence the ionizing potential may depend on 

 the physical state of a substance. The spontaneous ionization 

 of a gas can be regarded as the dissociation of uncharged 

 molecules into positive and negative ions. This dissociation 

 should increase rapidly with the temperature in the same 

 way as that of a dissociating vapour. Sir J. J. Thomson 

 has calculated the amount of dissociation — in this case, the 

 spontaneous ionization — in a vapour whose ionizing potential 

 is about 3 volts, and finds that such a vapour should be 

 highly conducting at temperatures of the order of 500° C. 

 According to Dunoyer, the natural conductivity of sodium 

 vapour is of the order of that of ordinary gases, and the 

 inference from this is that the ionizing potential of sodium 

 vapour is considerably higher than that derived from photo- 

 electric experiments on solid sodium (2*2 volts). The 

 author f found that the longest wave-length which produced 

 any photo-electric effect in solid anthracene was about 

 X2100, corresponding to V =5'9 volts. Now, Stark has 

 obtained intense ionization in anthracene vapour illuminated 

 by the ultra-violet light from a mercury arc in quartz glass. 

 The shortest wave-length emitted by such a lamp is X 1849, 

 and the strong ionization observed renders it unlikely that 

 X 1849 was the only active line. Hence the ionizing 

 potentials for solid and gaseous anthracene, calculated from 

 photo-electric experiments, can only differ at the most by 

 •7 volt. Experiments which bear on this point are now 

 being made on the photo-electric effect of a substance in 

 different physical states. 



I wish to thank Prof. Sir J. J. Thomson for his interest 

 and suggestions in the course of this work. 



* Franck & Hertz, Verh. d. Dents//. Phys. Ges. p. 34 (1913). 

 t Hughes, Phil. Ma-, xxiv. p. 3S0 (1912). 



