818 Dr. Megh Nad Saha on 



radiant particles. Though the upper layers of the chromo- 

 sphere favour relatively stronger ionization, yet the density is 

 so small that such lines fail to be recorded. They are recorded 

 only from the lower regions where the density is sufficiently 

 great, and the stimulus is not much weakened. 



Nitrogen in the Sun. 



The emission spectrum of Nitrogen is a rather difficult 

 matter to deal with. Under the stimulus in which most of 

 the other elements can be made to emit their characteristic 

 line spectrum or even their enhanced lines, nitrogen gives 

 only several classes of band or molecular spectra. The line 

 spectra seem to require a very great stimulus for their 

 production. But the existing knowledge on the subject is 

 too meagre to allow the discussion of the line emission of 

 atomic nitrogen from the present standpoint According to 

 a recent paper, the ionization potential of N is 17-18 volts, 

 but this must be made up of the energy of dissociation of the 

 molecule plus the energy of ionization of the atom. 



If the so-called cyanogen band having its head/ at 

 \ = 3883 * be really due to molecular nitrogen, the con- 

 clusion follows that in the sun nitrogen occurs in the 

 molecular state. There may be a fair proportion of atomic 

 nitrogen, but the stimulus is not sufficiently strong to make 

 it emit the lines we are familiar with. All these charac- 

 teristics are in very good accord with the chemical inertness 

 of nitrogen, which is. again due to the high value of the 

 heat of dissociation of the N 2 molecule. The fact is only 

 qualitatively known. According to Langmuir f, less than 

 5 per cent, of nitrogen is dissociated at 3500° K. and at 

 atmospheric pressure. From this we may calculate the heat 

 of dissociation in the following manner. The reaction takes 

 place according to the scheme 



Tsr 2 ^— >• N + N-U, 



and if x be the fraction dissociated, P the partical pressure, 

 we have 



Now 2C = 2C N -CV 



* This is the opinion of Runge and Grotian (Phys. Zeits. vol. xv. 1914). 

 Fowler, on the contrary, believes that the band is due to CN (cyanogen). 

 See also Barratt, Proc. Roy. Soc. Lond. vol. xcvii. 



f Langmuir, Journal of the American Chem. Soc. vol. xxxiv. p. 860 

 (1919). 



