ORIGIN OF ORGANIC COMPOUNDS 141 



gravitational field could not hold at the temperatures then 

 prevailing. 



In the final fifth stage of the formation of the planet the 

 primaeval atmosphere of the Earth still kept some remnants 

 of its original hydrogen, water, ammonia, methane and hydro- 

 gen sulphide. It was thus highly reducing in character. Only 

 hydrogen and traces of inert gases were continually escaping 

 from the atmosphere of the Earth into interplanetary space 

 while the other gases of the primaeval atmosphere were 

 almost completely held by the gravitational force of the Earth 

 at the temperatures then prevailing. The amount of water 

 on the surface of the Earth at the period under discussion 

 must have been considerably less than it is now. According 

 to Urey the total amount of water present on the primaeval 

 Earth was only lo per cent of that in the present-day oceans. 

 The rest of the ^vater arose during the development of the 

 lithosphere, being derived from the hydrates of silicates and, 

 in general, from the condensed water of the interior of the 

 Earth. ^^^ 



In just the same way the amount of methane in the 

 primaeval atmosphere of the Earth w^as very small because 

 the greater part of this gas had escaped during the earlier 

 stages in the development of the planet. As we have seen, 

 carbon was still present on the Earth in the form of metallic 

 carbides and graphite. During the formation of the litho- 

 sphere, however, the carbides reacted with the constitutional 

 water of the interior of the Earth to form methane and other 

 hydrocarbons. These separated out from the lithosphere 

 and accumulated in the atmosphere where they were noAV 

 retained by the force of gravity. There thus occurred at this 

 time the same reactions leading to the abiogenic formation 

 of hydrocarbons which ^ve can even now see taking place 

 to a small extent. 



In just the same way the amount of ammonia in the 

 primaeval atmosphere of the Earth was constantly augmented 

 at the expense of ammonium salts and, even more, of nitrides 

 of metals. The probable formation of nitrides at some period 

 in the formation of the Earth is supported by the geological 

 discovery of nitrides of iron in the deep layers of the crust 

 of the Earth (A. Gautier^") and in volcanic lavas (A. Brun). 



