FOOTE AND MOHLERI LOW VOLTAGE DISCHARGE 515 



volts is nearly equal to the ionization potential. This fact 

 shows that the initial velocity of the greater portion of the 

 electrons emitted from the cathode could not have been as high 

 as 1.3 volts. At an applied potential of 2 volts, a true potential 

 certainly not greater than 3.3 volts, the two D lines were dis- 

 tinctly visible, and as nearly as could be judged, the ratio of 

 their intensities remained about the same as the applied voltage 

 was decreased to 0.8 volts, when both lines disappeared. Hence 

 our work would indicate that the two D hnes appear together at 

 the resonance potential. 



This observation is not, however, conclusive evidence that 

 the lines were not separately excited by electronic impact. 

 Wood and Mohler^ have shown that in the excitation of sodium 

 vapor by incident radiation, although it is possible to excite the 

 D lines separately, in general both lines appear since the in- 

 fluence of the surrounding vapor may cause a transfer of energy 

 from the excited line to the other component of the doublet. 

 Possibly a similar influence is present in the above observations. 



The fact that the higher frequency line of the thallium doublet 

 determines the value of the energy quantum absorbed, on the 

 other hand, may be analogous to the emission of characteristic 

 X-rays under electronic bombardment. D. L. Webster^ con- 

 cluded that the K group does not appear until the energy of the 

 impacting electrons is greater than that corresponding to the 

 highest frequency of the K group, namely, Ky. At this voltage 

 all of the lines of the K group appear and the ratio of the in- 

 tensity of the lines remains the same when the voltage is further 

 increased. Similarly in the case of collision of the atom of 

 thallium, sodium, etc., with electrons of low velocity, both lines 

 of the doublet may always appear simultaneously, as observed 

 directly for sodium, while the line determining the value of the 

 quantum is the line of higher frequency, as observed directly 

 for thallium. However, if the K group is to be looked upon as a 

 series of lines, the above analogy is not justified. 



1 Phys. Rev. 11: 70. 1918. 



2 Phys. Rev. 7: 599. 1916. 



