Elements in the Sun. 813 



The table shows that under the solar conditions, GO percent, 

 of sodium atoms are ionized in the photosphere, and ioniza- 

 tion is practically complete at a level where the pressure 

 falls to 10~ 3 atm. The result is in very good agreement 

 with observational facts, for according to Mitchell the Y>i 

 ami Do lines reach a level of only 1200 kins. Over this 

 height, only ionized Na atoms are present, the chief emission 

 line- of which lie, according to Goldstein *, in the remote 

 ultra-violet, and so escape detection. 



Taking the temperature of the spot =5000° K.f, we see 

 from the tables that only 6 to 19 per cent, of the atoms are 

 ionized. So over the spot, there is a great increase in the 

 proportion of unionized Na-atoms, and we should expect 

 a much stronger absorption of the D { and D 2 lines. The 

 following table (taken from Kayser's Handbuch tier Spektro- 

 skopie, vol. vi. p. 114) shows that this is actually the 

 case : — 



Intensity Intensity in 



Line. Series-Description. in the Sun. Sun-spot Spectrum. 



V l -5896-15 (1, s)-(2,p 2 ) 



D 2 -589019 (1, s)-{2,p 1 ) 



5682-90 (2,p 2 )-(3, d) 



568S-26 (2, Pl )-(3,d) 



6154-41 (2,jo 2 )-(3, s) 



6161-95 (2,jp 1 )-(3, s) 



(b) Potassium. 



The identification of Potassium is rather doubtful. 

 Rowland has identified only two faint Fraunhofer lines 

 \ = 404736, 401:4-29, with the emission lines (1, «<?) — (3, p x ), 

 (1, s) —(3, p 2 ) of potassium. The following table shows 

 that owing to the low value of the ionization potential, 

 potassium is highly ionized throughout the whole of the 

 solar atmosphere. 



* Goldstein, Ann. d. Pht/sik, vol. xxvii. pp. 773-796; Schillinger, 

 Wien. Ber. p. 608 (1919) ; Nelthorpe. Astropliysical Journal, Jan. 1915. 



t Emden, Gas-kugeln, p. 443. See also numerous papers on the 

 spectra of sun-spots by Fowler, Hale, and others. 



20 



60 



30 



90 



5 



12 



6 



12 



2 



8 



3 



9 



