LIFE ELEMENTS IN THE SUN 47 



part weak, and only shown on high dispersion plates, while 

 hydrogen is represented by very strong lines, as shown by 

 spectroheliograms of solar prominences. The lines of oxygen 

 are relatively faint; it appears principally as a compound, 

 titanium oxide (Ti02) in sun-spots, although a triple line in the 

 extreme red seems also to be due to it. In the chromosphere, 

 or higher atmosphere of the sun, hydrogen is not in a state of 

 combustion, and the fine hydrogen prominences show radia- 

 tions comparable to those in a vacuum tube.^ 



Nitrogen, the next most important life element, is displayed 

 in the so-called cyanogen bands of the ultra-violet, made visible 

 by high-dispersion photographs. 



Carbon is shown in many lines in green, which are relatively 

 bright near the sun's edge; it is also present in comets, and 

 carbonaceous meteorites (Orgueil, Kold Bokkeveld, etc.) are 

 well known. Graphite occurs in meteoric irons. 



In the solar spectrum so far as studied no lines of the "life 

 elements," phosphorus, sulphur, and chlorine, have been de- 

 tected. On the other hand, the metallic elements which enter 

 into the life compounds, iron, sodium, and calcium, are all 

 represented by strong lines in the solar spectrum, the excep- 

 tion being potassium in which the lines are faint. Of the eight 

 metallic elements which are most abundant in the earth's crust, 

 as well as the non-metallic elements carbon and silicon, six 

 are also among the eight strongest in the solar spectrum. In 

 general, however, the important life elements are very widely 

 distributed in the stellar universe, showing most prominently 

 in the hotter stars, and in the case of hydrogen being uni- 

 versal. 



We have now considered the source of four "life elements," 

 namely, hydrogen, oxygen, nitrogen, and carbon, also the 



1 Hale, George Ellery, letter of March lo, 19 16. 



