816 Dr. Megh Nad Saba on 



7774:'4:3, and 7775*62, lying in the extreme red, with the 

 three emission lines of oxygen, having the series formula 



(2, s)-{m, Pl ), (2, s)-(m,p 2 ), (2, s )-(m,p 3 ). 



In the sun, therefore, oxygen exists in the atomic state. 



The heat of decomposition of the oxygen molecule is not 

 yet known with certainty. From Langmuir's * observation 

 that at 2400° K. and 10 -1 atmospheric pressure oxj^gen is 

 completely dissociated. I have calculated provisionally the 

 heat of decomposition to be less than 50,000 calories, 

 which is less than the corresponding value for hydrogen 

 (84,000 cal.). Since the equation of chemical equilibrium 

 is almost the same in both cases f, it follows without any 

 calculation that oxygen should be completely decomposed 

 into atoms in the sun. The ionization potential of oxygen 

 is probably large, and no lines due to ionized oxygen seem 

 to occur in the sun. 



The lines of + and ++ have been qualitatively studied 

 by Lunt, Fowler J, and Brooksbank, but no series-formulse 

 liave yet been obtained for these lines. But Fowler's 

 identification of certain of these lines in the Bo-class of 

 stars and in Wolf-Rayet stars respectively seem to indicate 

 that. the first step ionization of oxygen is reached at an 

 approximate temperature of 20,000° K. and second step 

 ionization at probably not less than 30,000° K. 



Magnesium* 



The wave-lengths of the strongest lines of the emission- 

 spectrum of Mg, viz. lines (1, S) - (2, P), (1, S) - (2, p,), 

 lie beyond the range of atmospheric absorption, so that we 

 have to fall upon the next strongest lines, the triplet 



(2,p)-(3,^)> 1 = 1,2*3, \ = 3838'34, 3832-46, 3825-51, 



which are very prominent in the flash spectrum, and reach, 

 according to 'Mitchell, a level of about 7000° km. The 

 height reached by the line (1, S)-(2, P), \ = 2852, if it 

 were available for observation, would probably be somewhat 

 higher, say about 9000° km. The brief is strengthened 

 from an examination of the table of ionization given below. 



* Langmuir, Journ. Chem. Soc. vol. xxxiv. pp. 864, 1030 (1912). 



t Vide Hydrogen in the Sun, paper A, p. 483. 



X Fowler and Brooksbank, Month. Not. Koy. Astr. Soc. April 1917. 



