44 Dr. Foote and Dr. Mohler on Ionization and 



converging near v — 59000, but a closer determination of this 

 frequency must be made from spectroscopic data as yet not 

 correlated in series. Prof. Saunders suggests that the lines 

 ought to be of high intensity and that it is quite likely one 

 or two of the lines are already known. 



When the ionization potential of thallium was reached, a 

 brilliant green glow filled the ionization-chamber No glow 

 whatever could be detected below this potential in agreement 

 with the assumption that the infra-red line X— 11513 is the 

 single line spectrum. It is interesting to note that in all 

 of this work on ionization and resonance we were unable to 

 operate the apparatus when wireless signals were being 

 transmitted in the wireless laboratories of the Bureau, as the 

 ionization-chamber constitutes an efficient detector. 



Dr. Tate and one of the writers have pointed out in earlier 

 work that the long wave-length limit for the photoelectric 

 effect in a metallic vapour is probably identical with the 

 wave-length limit determining ionization potential. Prof. 

 Kunz * has discussed this point in some detail, using our 

 data on Na, Zn, Cd, and K. Thus, for example, Tate and 

 Foote observed an ionization potential of 5*13 volts for 

 sodium and a resonance potential of 2*12 volts. Metallic 

 sodium is photcelectrically active to radiation of wave-length 

 \=5893, corresponding to 2*12 volts. Accordingly, if 

 sodium vapour were photoelectrically active in the same 

 manner as the solid metal we should have observed ionization 

 when the D-line was emitted at resonance of 2'12 volts. 

 Actually no ionization was detected until the voltage reached 

 5*13 volts. This fact proves that sodium vapour is not photo- 

 electrically active to yellow light, and while not proving 

 definitely that ultra-violet radiation is required to produce 

 the photoelectric effect in the vapour, it is an indication that 

 such is the case. For at 5*13 volts the ultra-violet lines of 

 sodium are excited. Whether the photoelectric effect in 

 sodium vapour can be produced by radiation of wave-length 

 greater than that corresponding to 5*13 volts is a question 

 which must be decided experimentally. It is hoped that 

 work now in progress may offer conclusive evidence. Apply- 

 ing the above reasoning to the present work we may conclude 

 that magnesium vapour is not photoelectrically active to 

 radiation of wave-length 4571, and that very likely the 

 limiting wave-lengths for photoelectric sensibility of mag- 

 nesium and thallium vapours are X= 1622 A. and X = about 

 1700 A. respectively. 



Van der Bijl f has recently discussed the relation between 



* Phys. R. xi.*p. 246 (1918). 

 t Phys. R. x. p. 552 (1917). 



