CONTEMPORARY ADVANCES IN PHYSICS 



353 



are quantities with which the theories deal. According to Foote, 

 Mohler and Chenault, these relative heights are in fair accordance 

 with the theories. The actual heights, however, depend on the mean 

 duration of the excited states. I do not know whether it has been 

 proved that these last long enough to permit the explanation. 



36 



32 



28- 



IS-4p 



Ui 



>- 

 t 



> 24 



20 



z 

 u 

 <rt 

 o 



I- 

 o 

 I 

 a 



u 16 

 > 



i 12 



_L 



162°C - 



.28 



- 24 



- .20 



.16 



.12 



08 



.04 



.00 



3900 3800 3700 



3600 3500 

 \ IN A.U. 



3400 



3300 3200 



Fig. 4 — Ionization of caesium vapor by light, at wavelengths greater than the 

 critical (3184A). (Aiohler, Foote & Chenault.) 



Since the quanta spent in ionization vanish from the light, the 

 transmitted beam when spread into a spectrum reveals absorption 

 at their frequencies. These absorption-spectra supply all that is 

 known as yet about the process of ionization by light in sodium and 

 in atomic hydrogen and valuable additions to the data for the three 

 heavier alkali metals. 



It will be remembered that the lines of a line-series in an absorption- 

 spectrum occur because the photons of the corresponding frequencies 

 can be absorbed by atoms in a particular initial state (normal or 

 excited) which thereupon pass over into higher states of excitation; that 

 as the lines converge upon the limit of the series, the corresponding 

 terminal states approach that of ionization; that the limiting or 



