42 Dr. Foote and Dr. Moliler on Ionization and 



resonance potential, and 7*bl volts for ionization potential. 

 The observed values o£ 2*65 volts and 7*75 volts respectively 

 are in satisfactory agreement with the theoretical values. 



In the case of thallium the observed value of the resonance 

 potential, 1*07 volts, agrees exactly with that computed from 

 the quantum relation hv = eY if v is taken as the frequency of 

 the shorter wave-length member of the first term of the 

 principal doublet series. The resonance potentials of sodium 

 and of potassium are also determined by the first line of the 

 principal series for these elements; but here the doublets are 

 so close together that, as far as experimental confirmation is 

 concerned, either member of the doublet maybe used for the 

 critical frequency v. In thallium, however, the two lines of 

 the doublet are widely separated, one lying at \ = 115 13 A. 

 and the other at \=13014A. The former line corresponds 

 to a resonance potential of: 1*07 volts, and the latter line to 

 0*95 volt. The curves of fig. 3 show no indication of reso- 

 nance collision at multiples of ,( J5 volt. This fact raises 

 the question as to whether the components of the thallium 

 doublet can be separately excited by electronic impact. To 

 determine this spectroscopically would be difficult, since the 

 two lines lie in a region of the spectrum where temperature 

 radiation from the surrounding walls is especially high. 



A spectroscopic study of the radiation emitted when low 

 velocity electrons collide with sodium atoms might prove 

 fruitful. If the D-lines can be excited separately by elec- 

 tronic impact, we would expect one component alone to 

 appear with two volt electrons, and would not expect the 

 other component to appear until the ionizing potential was 

 attained. Wood and Mohler* have shown that in the 

 excitation of sodium vapour by incident radiation the D-lines 

 may be excited separately, although the influence of the 

 surrounding vapour may cause a transfer of energy from 

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

 similar transfer of energy might take place in thallium, 

 sodium, &c«, vapour excited by electronic impact. The fact 

 that the higher frequency line of the thallium doublet de- 

 termines the value of the energy quantum absorbed may be 

 analogous to the emission of characteristic X-rays under 

 electronic bombardment. D. L. Websterf concluded 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 K v . At this voltage all 

 of the lines of the K group appear, and the ratio of the 



* Wood and Moliler, Phys. Rev. xi. p. 70 (1918). 

 t I). L. Webster, Phys. Rev. vii. p. 599 (191(5). 



