812 Dr. Meo-h Nad Saha on 



h 



hand, and Sr and Ba on the other hand, to their differences 

 in atomic weight, but according to the view presented in 

 paper A, this is mainly due to the varying values of the 

 ionization potential. The author's belief is that in the sun 

 and the stars, the attraction due to gravity is largely com- 

 pensated 'by selective radiation pressure, and atomic weight 

 is of much less consequence than can be supposed. 



The method sketched in paper A has been extended to the 

 alkali metals, and a few other elements. It will be seen 

 that the theory accounts in a most gratifying manner for the 

 varying behaviour of sodium lines in the Fraunhofer spec- 

 trum, and its intensification in the sun-spot spectrum, for the 

 faint occurrence of the potassium lines; and for the complete 

 absence of the lines of Cs and Eb, and for the varying' 

 behaviour of the lines of Mg and Mg + , though, on this last 

 point, the results are not so satisfactory. There is very 

 little doubt that if proper data be available, the method can 

 be extended to the explanation of all the details of the 

 Fraunhofer spectrum. 



For the explanation of the method, the reader is referred 

 to Sections 2 and 3 of paper A. The temperature of the 

 photosphere has been taken to be 7500° K., the pressure 

 1 to 10 _1 atm., while for the high-level chromosphere 

 a temperature of 6000° K. has been used. 



1. The Alkali Elements in the Sun. 



(a) Sodium. 



The following table shows the ionization of sodium in 

 per cents, under varying conditions of temperature and 

 pressure : — 



Ionization Potential =5' 12 volts = l'17x 10 5 calories. 



Pressure I. 10 _1 . 10 -2 . 10~ 3 . 10 -4 . 10 -5 . 



Temp. 



5000 6 19 53 89 985 



6000 21 56 90 98S I 



7000 46 85 98 i l 



7500 60 72 99 | Complete 



8000 72 96 I ' 



9000 87 98-5 



Ionization. 



10000 97 



