62 THE ORIGIN AND EVOLUTION OF LIFE 



Nitrogen comes next in importance to hydrogen and oxygen 

 as structural material^ and when combined with carbon and 

 sulphur gives the plant and animal world one of the chief 

 organic food constituents, protein. It was present on the 

 primordial earth, not only in the atmosphere but also in the 

 gases and waters emitted by volcanoes. Combined with hy- 

 drogen it forms various radicles of a basic character {e. g., NHo 

 in amino-acids, NHj in ammonium compounds) ; combined with 

 oxygen it yields acidic radicles, such as NO3 in nitrates. It 

 combines with carbon in — C ^ N radicles and in ^ C — NH2 

 and = C = NH forms, the latter being particularly important 

 in protoplasmic chemistry.- This life element forms the basis 

 of all explosives, it also confers the necessary instability upon 

 the molecules of protoplasm because it is loath to combine 

 with and easy to dissociate from most other elements. Thus 

 we find nitrogen playing an important part in the physiology 

 of the most primitive organisms known, the nitrifying bacteria. 



Carbon also exists at or near the surface of cooling stars 

 which are becoming red.'^ It unites vigorously with oxygen, 

 tearing it away from neighboring elements, while its tendency 

 to unite with hydrogen is less marked. At lower heats the 

 carbon compounds are remarkably stable, but they are by no 

 means able to resist great heats; thus Barrel^ observes that a 

 chemist would immediately put his finger on the element car- 

 bon as that which is needed to endow organic substance with 

 complexity of form and function, and its selection in the origin 

 of plant life was by no means fortuitous. Including the arti- 

 ficial products, the known carbon compounds exceed 100,000, 

 while there are thousands of compounds of C, H, and O, and 

 hundreds of C and H.^ Carbon is so dominant in living mat- 



^ Henderson, Lawrence J., 1913, p. 241. - W. J. Gies. 



^ Henderson, Lawrence J., 1913, p. 55. * Joseph Barrell, letter of March 20, 1916. 



5 Henderson, Lawrence J., 1913, pp. 193, 194. 



