84 Dr. P. D. Foote and Dr. W. F. Meggers on Atomic, 



many times less. Thus an electronic impact of 2*7 volts 

 could not excite the visually intense line A, 6973 (2p 1 — 5d) 

 although the quantum involved requires but 1'8 volts. The 

 energy (4*3 x 10~ 12 erg) is sufficient to produce this line 

 (2*9 X 10 -12 erg), but is not sufficient to displace the electron 

 to the 5d ring (5*1 X 10~ 12 erg) — a condition necessary for its 

 excitation. 



The above conception is in agreement with fluorescence 

 phenomena observed in sodium vapour. Thus D-light is 

 absorbed by sodium vapour, and for each quantum absorbed 

 an electron is displaced from the l m os to the 2p ring in an 

 atom. The electron in falling back to the 1*55 ring emit& 

 the D-line (l'5.s — 2/V), which may be observed at right angles 

 to the beam of incident radiation. Recently, Strutt* has 

 found that the sodium line X3303 (l'5s— 3jm) similarly 

 stimulates both l'5s — 3pj and l'5s — 2pi. Thus, after absorb- 

 ing a quantum of frequency l'5s— 3jP§, an electron is displaced 1 

 to the 3pi orbit. In returning to equilibrium it may fall 

 directly to the l\5s ring, in which case the line X3303 is 

 emitted, or it may fall to the 2p* ring and then to the l'5s ring,. 

 the second step involving an emission of the D-lines. This 

 leaves a quantum of frequency 2p* — 3p± (A 7519) looked for 

 but not observed by Strutt. However, this line represents 

 a very improbable type of orbital transition, as is evidenced 

 by the fact that the frequency is never observed in any 

 sodium spectrum. A more probable method of falling from 

 3pi to 2/>i is first to the 3d ring and then to the 2p ring, 

 giving rise to the lines X8196, 8184, 9048, and 9085, or 

 from the 3p$ ring to the 2" 5s ring and then to the 2p± ring. 

 In both cases the quantum relations are correct and the lines 

 are well known, but, lying in the infra-red, they would not 

 have been observed by Strutt. 



The fact that an atom can absorb radiation of frequency 

 1*5* — mp (absorption has been observed f for m = 2 to 60 in 

 the case of sodium), thus resulting in displacement of an 

 electron to the mp ring, for each absorbing atom suggests 

 that similar displacements may be produced by electronic 

 collision. On the other hand, the failure to detect such dis- 

 placements is not an argument against the Bohr theory. 

 It is possible that the impacting electron must have the 

 correct velocity as well as energy to produce any displace- 

 ment at all. Thus, while the mass of an electron happens to 

 be such that when its velocity is that obtained by falling 

 through. 1*45 or 3*88 volts, a collision with a ca?sium atom is 



* Proc. Roy. Soc. xcvi. p. L'S2 (1919). 



f Wood and Fortrat, Astroph. Journ. xliii. p. 73 (1916). 



