Geological History of the Atmosphere. 407 



rocks, and that probably an essential condition to their 

 formation is the absence of free oxygen and of water. When 

 we also remember the large proportion of metallic iron or 

 iron carbide that is usually found in meteorites, and that 

 among the other constituents there are found sulphides, 

 nitrides, and even phosphides — the last being a class of com- 

 pound that has not been found on the earth at all in a properly 

 telluric form — we can easily see that meteorites are very 

 imperfectly oxidized, and probably never contained free 

 oxygen. 



If, therefore, the earth is analogous to the other members of 

 the solar system with which we are best acquainted, viz. the 

 sun itself and the meteorites which have fallen on the earth, 

 it must obviously be very imperfectly oxidized — the oxygen 

 present must be much less than that required for the complete 

 oxidation of the other elements. It does not necessarily 

 follow from this that there never was any free oxygen in the 

 earth's atmosphere in primitive times ; but it follows that 

 there probably was a time, and possibly a long time, when 

 there was none. On this hypothesis our present supply of 

 free oxygen has been all produced by the action of sunlight 

 on vegetation ; and if we hold that the primitive atmosphere 

 of the earth contained free hydrogen or hydrocarbon gases, 

 then the amount of oxygen liberated by vegetation must have 

 been even greater than that of our present free oxygen, for a 

 large quantity may have been used up in oxidizing the 

 hydrogen and hydrocarbon gases to water and carbonic acid. 



The most important evidence on our general subject is 

 probably that derived from the study of the amount of car- 

 bonaceous and other oxidizable matter due to vegetation which 

 is found on the earth. Our information on this point is very 

 imperfect as yet, but it indicates that a high estimate — an 

 estimate which makes it equivalent to the free oxygen of the 

 atmosphere — is quite possible, and even probable. We may 

 expect that our information will become much more exact in 

 course of time, and that it may yet enable us to give a decisive 

 answer to the question before us ; but we can say in the 

 meantime that the available evidence indicates that the theory 

 is quite credible that there was a time when there was no 

 free oxygen on the earth. 



[Note added Sejrt. 25th. — With reference to the argument 

 on page 401 I may add that Ebelmen in his essay on the 

 Decomposition of Silicates (published 1845) considers that 

 the amount of iron pyrites existing on the earth is probably 

 more than equivalent to the total free oxygen of the atmo- 

 sphere.] 



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