Elements in the Sun. 819 



According to the Tetrode-Sackur formala, 



C N =-l-6 + |logM='119 J 



and On, = -'05*. 



Taking #=*03 in Langmuir's experiment, we can calculate 

 U from the above formula ; we obtain 



U = T50x 10 5 calories, in round numbers. 



Taking T = 7000° 0., we can now calculate the value of 



O 7 



loe— - ., P. This becomes 1'390. If P = l atm., almost 



98 per cent, of the nitrogen ought to be completely broken 

 up into atoms. For P = 10 atms., the proportion is 8o per 

 cent. 



Thus the calculation does not seem to favour the sugges- 

 tion that a certain percentage of nitrogen occurs in the sun 

 in the molecular state. But the data used for calculation 

 are of the roughest nature, and nothing definitely can be said 

 until better data are available. 



Helium, 



Ir is well known that none of the Helium lines occur in the 

 Fraunhofer spectrum, though occasional reversals of the D d 

 have been observed in the spectra of the penumbra of sun- 

 spots f . But in the flash spectrum, helium lines are very 

 prominent, the D 3 reaching a height of 7500 km. in the 

 chromosphere. These facts may be explained in the fol- 

 lowing manner : — 



The lines by means of which helium is detected belong to 

 the series-combination (2, p) — (m, d) , or (2, s) — (m, p). 

 These lines cannot be absorbed by the ordinary He-atoms, 

 which possess (I, s) orbits, but by such atoms as. possess the 

 (2,p) or (2,5) orbits. These orbits can only be produced 

 under very great stimulus. It is known from the 'Harvard 

 Annals'' (vol. xxviii. 91) that the absorption lines of 

 helium disappear below stars of the Ao class. Hence 

 a temperature of about 11,000° K. (temperature of the 

 Ao class) is required for producing a sufficient number of 

 He-atoms with (2, p) orbits, when the pressure is one 

 atmosphere. These conditions are not attained on the 

 photospheric level ; but at great heights, owing to 



* Laski, Phys. Zeits. xx. p. 269 (1919). 

 t Pringsheim, Physik der Sonne, p. 123. 



