Elements in the Sun. 81i 



elements of which no lines are found either in the Fraunhofer 

 or tin 1 flash spectrum are totally absent from the sun. But 

 this view is most unsatisfactory, and can only he regarded as 

 a stop-gap. There is, a priori, no reason why, in the sun, 

 certain elements should be preferred to the exclusion of 

 others. On the contrary, it seems natural to infer that the 

 sun is composed of the same elements as the earth, and 

 contains all the 92 elements known to the chemists on the 

 earth. 



It therefore becomes increasingly necessary to investigate 

 why certain elements should entirely fail to be recorded on 

 the Fraunhofer or the flash spectrum. It may be supposed 

 that certain elements fail to be recorded because, on account 

 of their heavy atomic weight, they are practically confined 

 to the photosphere. But it is not merely a question of 

 atomic weight, for in the list of missing elements we find 

 light elements like boron and nitrogen side by side with 

 a heav}- element like thallium. 



The view which is urged in the present paper is that the 

 varying records of different elements in the Fraunhofer spectrum 

 mat/ be regarded as arising from the varying response of these 

 elements with regard to the stimulus existing in the sun. The 

 stimulus existing in tlie sun is the same for all elements, viz., 

 that arising from a temperature of about 7500° K., but owing 

 to different internal structure, elements will respond in a varying 

 degree to this stimulus. The manner in which we can quanti- 

 tatively estimate the effect of the stimulus has been sketched 

 in papers A and C*. In paper A, the effect of the stimulus 

 on the alkaline earths Ca, Sr, and Ba was estimated. It 

 was shown that while on the photosphere 30, 40, and 

 57 per cent, of the atoms are respectively ionized, the 

 percentage of ionization increases with height and becomes 

 practically complete for Ca at a pressure of 10 ~ 6 atm., 

 for Sr at a pressure of 10~ 3 , and for Ba at a pressure of 

 10~ 2 atm. In this connexion it may prove interesting to 

 compare the results with the following remarks of Fowler: — 



u We find further, that while many of 'the metals are 

 represented by both arc and enhanced lines, there are some 

 which are identified only, or mainly, by their enhanced lines 

 alone. Thus, although Oa shows both classes of lines 

 strongly, Sr and Ba practically show enhanced lines alone." 



Fowler ascribes the different behaviour of Ca on the one 



* Paper A — " Ionization in the Solar Chromosphere," Phil. Mag. Oct. 

 1920 ; Paper C — " On the Temperature Radiation of Gases " (to appear 

 shortly). 



3G2 



