Temperature Radiation of Gases. 277 



§ 4. Absorption. 



In this connexion, we may consider the puzzling question 

 of reversal of lines. According to KirchhorFs law, we expect 

 that the emission-lines of an element should be reversed when 

 a strong beam of white light is sent through cooler layers 

 o£ the vapour. But this expectation is not always fulfilled. 

 Wood * has found that if a white light be sent through a 

 column of sodium vapour, only the lines of the principal 

 pair-series (1, 5) — ^in,p{), (l,s) — (m, p 2 ), can be obtained as 

 absorption- lines. None of the lines of the diffuse or the 

 sharp series are reversed. Bevan f has extended the method 

 to the other alkali metals, i. e. Potassium, Rubidium, and 

 Caesium, and arrived at identical results. Recently Dobbie 

 and Fox (Proc. Roy. Soc. vol. xcviii. p. 147) studied the 

 absorption-spectra of Hg, Zn, and Gd vapour, and found no 

 absorption up to \ = 3200. But this is due to the fact that the 

 (1, s) — (m,p) lines of these elements lie below 8000 A. IT. 

 In fact, Wood found in 1913 (Phys. Zeit. pp. 191-195) that 

 ordinary Hg vapour absorbs the fundamental line X = 2536 

 (l,M)-\2,pj. _ 



The explanation easily follows from our theory. The 

 condition for absorption is that in the atoms present, there 

 should be a fairly large number with orbits corresponding 

 to the first term of the pulse of radiation to be absorbed. 

 Thus, in order that a line ('2,p) — (m,d) may be absorbed, 

 we must have a sufficient number of atoms with (2, p) 

 orbits. At low temperatures only atoms with (1, s) orbits 

 are present. Hence only the lines corresponding to the 

 combination (1, s)—(m,p) are absorbed. The lines re- 

 presented by the positive combinations (2, p) — (nu d), 

 (3, d) — (m,/), or the negative combinations (2, p) — (m, s), 

 can only occur when atoms with (2, p) or (3, d) orbits are 

 present. This can happen only at high temperature or 

 under electrical stimulus. 



The temperature required for this purpose is very high — 

 much higher than the temperatures used by Wood and 

 Bevan for all elements. In fact, the atoms begin to absorb 

 the lines (2,/>) — (m, d) only when they are hot enough to 

 emit the leading terms of the principal series. A line of 

 the Bergmann series will be^in to be absorbed at even a 

 higher temperature, viz. at the temperature of omission of 

 the diffuse series (3, d) — (4, b) . 



If the views presented here be correct, we may probably 

 obtain the reversal of the lines of the diffuse or the Bergmann 



: ' ? Wood, The Astrophys. Journal, vol. xxix. pp. 97-100. 

 t Bevan, Proc. Roy. Soc. vol. Ixxxiii. pp. 423-428 j vol. Ixxw. 

 pp, 58-76. 



