64 THE ORIGIN AND EVOLUTION OF LIFE 



Potassium is of service to life in building up complex com- 

 pounds from which the potassium cannot be dissociated as a 

 free ion; it is thus one of the building stones of living 

 matter.^ 



Magnesium is fourth in order of activity among the metallic 

 elements. It is essential to chlorophyll, the green coloring 

 matter of plants, which in the presence of sunshine is able 



Fig. 8. The Sun, Showing Sun-Spots and Calcium Vapor. 



Calcium, a life element essential to all plants and animals, and especially abundant in 

 the bones and teeth of vertebrates, is also a constituent of the solar atmosphere, as 

 shown by these two photographs of the sun, both displaying the same view and the 

 same group of sun-spots. The one at the left, made by calcium rays alone with the 

 spectro-heliographji shows in addition the clouds of calcium vapor which are not 

 evident in the photograph at the right. From the Mount Wilson Observatory. 



• An instrument devised by Professor George E. Hale for taking photographs of the sun by the light of a 

 single ray of the spectrum (calcium, hydrogen, etc.). 



to dissociate oxygen from the carbon of carbon dioxide and 

 from the hydrogen of water. It is also found in the skeletons 

 of many invertebrates and in the coralline algae, and is an im- 

 portant factor in inhibiting or restraining many biochemical 

 processes. 



Calcium is third in order of activity among the metallic 

 elements. According to Loeb- it plays an important part in 



1 Op. ciL, p. .72. 2 Op. ciL, 1906, p. 94. 



