﻿Elements of the Higher Groups. 1129 



s ; ngle kind. Recently E. A. Milne and Henry Norris 

 Russell * have extended the theory to mixtures of elements. 

 By a comparison of the sun-spot and the solar spectrum, 

 Russell finds that the predictions of the theory with reference 

 to the relative intensity in the hotter and the cooler spectrum 

 of lines associated with ionized and non-ionized atoms are 

 found to be in general agreement with the facts. Russell 

 has also shown that the temperature of the sun and the sun- 

 spot comes out to be much more in accord with the figures 

 obtained from general intensity measurements when mixtures 

 of different elements are considered instead of one single 

 element. 



But discrepancies have also been pointed out by Russell, 

 which suggests the need of some modification. The nature 

 of these discrepancies may be grasped from the following. 



Let I x and I 2 be the ionization- potentials of elements A 

 and B. Then at a definite temperature and pressure the 

 ratio of the degrees of ionization of A and B is given by the 

 equation 



^K! = lo Kl^- lo Hl^J= 5036k T l2 



where x u x 2 are the fractions ionized, I 1? I 2 are expressed 

 in volts. 



If Ii = I 2 , X\ should equal x 2 . 



That this is not the case is shown from the fact that sodium 

 and barium have got practically the same ionization potential 

 (5*11 and 5*12 volts respectively), yet both in the sun and 

 in the sun-spot, barium is a good deal more ionized than 

 sodium. The resonance line of Ba, \ = 5535*93, is absent or 

 very faint both in the solar and the spot spectrum, and it is 

 represented only by the enhanced lines (Ba + , \ = 4934'07, 

 4554*04). which shows that barium is completely ionized 

 not only in the sun but also in the spot. The resonance 

 lines of sodium, 'A. = 5889*97, 5895*94, on the other hand, are 

 very prominent in the solar spectrum, and are greatly in- 

 tensified in the spot, which shows that in the sun a large 

 percentage of sodium is unionized, and in the spot the 

 percentage increases owing to a lowering of temperature. 



AY hat has been said of sodium and barium admits of a 

 widegoing generalization, viz. the alkaline earths are, as a 

 rule, much more strongly ionized than their ionization 

 potential would indicate. The behaviour of the alkalies is- 



* Milne, The Observatory, Sept. 1921 ; Russell, The Astro-physical 

 Journal, March 1922. 



Phil. Mag. S. 6. Vol. 44. No. 264. Dec. 1922. 4 D 



