278 Problems of Temperature Radiation of Gases. 



series of the alkali metals by heating the absorbing column 

 of vapour to about 2000° to 3000° K. The most favourable 

 element to start with is csesium, which has the lowest 

 ionization-potential of all elements. 



In many cases confusion may arise about the proper 

 identification of the (l,s) — (?n, p) terms. Thus, what are 

 usually called the Principal series of helium and parhelium 

 (viz. the series beginning with the line 20,587 for parhelium 

 and 10,834 for helium) do not really correspond to the 

 combination (1, s) — (in,p), but to the combination (2, s) 



— (m,p). The (1,5) term for helium is still unknown, and 

 the series (1, s) — (??i, p) ue ^ ar down in the ultraviolet*. 

 Hence, according to our theory, none of the lines belonging 

 to the combinations (2, s) — (m, p), (2, p) — (m } d) can be 

 absorbed, by a layer of helium gas. 



But if by heating or some other means we can convert a 

 good proportion to the states (2, 5) or (2,p), then and then 

 only can these lines appear as absorption-lines. But at a 

 pressure of 10 _1 atm. helium becomes incandescent, i. e. 

 emits the lines (1, s) — (m,p), and absorbs the lines (2,jb) 



— (m,.d) at probably not less than 11,000° or 12,000° K, 

 i. e. only in stars of the B-class. 



But instead of a high temperature we may think of other 

 means. The spark produces mechanically the very same 

 conditions which can be realized at very high temperatures. 

 This is exactly what Paschen f has done. He found that 

 the lines of the combination (2, s)—(m, p) for helium and 

 parhelium cannot be absorbed by an ordinary layer of the 

 helium gas. But when a spark is sent through the absorbing 

 layer the lines are strongly absorbed, the absorbed energy 

 being again re-emitted in all directions. 



The paper thus suggests more problems than it attempts 

 to solve. A critical examination and further development 

 of the hypothesis advanced here requires an overhauling of 

 the whole data on the line-radiation of gases- — such as are 

 contained in Kay ser's Handbuch der Spektroshopie and further 

 works. But this programme requires much more time and 

 more extensive study, both practical and theoretical. 



University College of Science, 



Calcutta, § 



May 25, 1920. 



* It is quite possible that some of the (1, s) — (?n,p) lines for helium 

 and parhelium may be identical with the lines discovered by Lyman in 

 the ultra-violet, and some with the lines discovered by Richardson and 

 Bazzoni in the region of 300 to 400 A.U. by the photo-electric method 

 (vide Richardson and Bazzoni, Phil. Mag. 1918). 



t Paschen, Ann. d. Physik. vol. xlv. p. 625 (1914). 



