86 Dr. P. D. Foote and Dr. W. F. Meggers on Atomic 



assumption, for a large amount of radiation escapes from the 

 tube. Thus with sodium at 2*2 volts the D-lines are visibly 

 intense, and with caesium at 1*5 volts the infra-red lines 

 /V8943and 8521 may be readily photographed with properly 

 sensitized plates. The ionization of a vapour below its 

 ionization potential is observed when the vapour and electron 

 densities are high. This fact substantiates the present hypo- 

 thesis, for more collisions occur, more radiation is emitted, 

 and more is absorbed when the electron current and the 

 vapour pressure are increased. Thus, as pointed out by 

 Compton, in the operation of the mercury arc at 5 volts and 

 high-current density, electrons are maintained in the 2p 2 ring, 

 partly by electronic impact, but mainly by absorption of 

 radiation of the frequency l'5s — 2p 2 , and a 5-volt impact is 

 sufficient to ionize from the 2p 2 ring. 



An extension of this hypothesis leads to interesting con- 

 clusions in the case of absorption of radiation. The lines of 

 the principal series of the alkali metals are all absorption- 

 lines, the energy absorbed increasing the total energy of the 

 atom by a displacement of the electron from the T5.9 ring to 

 some p ring. If, however, the vapour is excited by electronic 

 impact (or radiation) corresponding to Ve = hv, where 

 v = l'5s—2p, electrons normally in the l"5s ring are driven 

 to the 2p ring, and are no longer capable of absorbing 

 l'5s — mp, but rather series lines which converge at 2p>. 

 Hence, if the excitation is sufficient (i. e., electron current 

 very dense), the principal series should show less absorption, 

 and the 1st and 2nd subordinate series should tend to become 

 the prominent absorption-lines. 



It appears from the above that the general impression of 

 the existence of a single-line spectrum at a certain low 

 voltage, and an abrupt transition to the many-line spectrum 

 as the voltage is increased to the ionization potential is 

 scarcely justified a priori on any theoretical basis. If a 

 mechanical theory is accepted, it is quite conceivable, in spite 

 of Stokes' law, that the lines of higher frequency may be 

 excited by resonance when a fundamental line is produced. 

 We have analogies to this effect in the relation between 

 transverse and longitudinal vibrations of a rod, or in the 

 "tripling" of the frequency of alternating current, etc. 

 Thus, instead of a single line, a single series might appear — ■ 

 the fundamental frequency and its harmonics: a view held 

 by certain spectroscopists. 



On the basis of the Bohr theory various groups of lines 

 may appear as the excitation is increased. There is no 

 experimental evidence on this question. In all previous 



