PHASES OF THE EARTH'S DEVELOPMENT. 301 



required. From the decomposition of the nitrides and phosphides 

 below the surface, ammonia and phosphine were escaping into the ocean 

 and into the air. The conditions then during long periods of time were 

 especially favorable for marine life, and as sand and mud accumulated 

 on the rocky surface of the earth, for land plants; the absence of a thick 

 soil being more than compensated for by the abundance of plant food, 

 notably of carbon dioxide and ammonia. 



The statement may be found in excellent modern text-books of 

 chemistry that ammonia is always formed by the decomposition of 

 plants and animals, accompanied by the further statement that ammonia 

 is a requisite for plant food. No plants — no ammonia; no ammonia — 

 no plants. If this were true, the beginning of plant life would indeed 

 have been a struggle for existence; that it is not true is shown above. 

 This decomposition of nitrides has ceased practically on the actual 

 surface of the earth at the present day because the nitrides have all been 

 decomposed; yet it may be mentioned that specimens of rock freshly 

 quarried in Sweden were recently found to give off ammonia when wet 

 with water, showing the presence of nitrides. Below the actual earth's 

 surface it is probable that nitrides still exist in large quantity, for 

 ammonia is one of the constituents of volcanic gases; to believe that vol- 

 canic ammonia is a product of plant or animal decomposition is difficult; 

 to suppose it formed by the action of steam on nitrides in the earth's 

 interior is simple. 



Much the same may be said of the presence of carbides. While they 

 no longer exist on the surface, there is no doubt of their existence 

 in the interior of the earth, and the volcanic gases contain their decom- 

 position products. In this connection the theory — first put forward by 

 Mendeleeff and since supported by Moissan — of the origin of petroleum, 

 may be mentioned. These writers favor the hypothesis that it was 

 formed by the decomposition of carbides by water under pressure; and 

 while the evidence at hand perhaps favors the belief that the petroleum 

 of the more important oil fields owes its origin to decomposition of the 

 lower forms of marine animal life, yet there can be no doubt that 

 petroleum may be formed by carbide decomposition, and it seems prob- 

 able that natural gas is in part at least a result of the same action. 



