ATMOSPHERIC FACTOR IN EVOLUTION. 29 1 



presumption is that during- the process of coohng-, most of the 

 available oxygen would unite with, and oxidise other elements, and 

 that most of the carbon would unite with as much oxygen as it 

 could get and so form carbon dioxide. 



We thus see that, on theoretical grounds, there is good reason 

 for believing that, at the beginning, there was abundance of 

 carbon dioxide in the air, but little or no free oxygen. But we 

 have something more definite to go upon. Let us, for a minute, 

 leave the consideration of the carbon in the air at that time, and 

 turn our attention to the large accumulations of carbon at the 

 present time in the bowels of the earth, mainly in the form of coal. 

 Where has all this carbon come from? It has gradually been 

 withdrawn, in the past ages, from the carbon in the air by the 

 action of plants. 



Let us assume that at the beginning all the oxyg^en now present 

 in the air was then present but attached to carbon in the form of 

 carbon dioxide, and that the other constituents of the air were the 

 same and in the same proportions as at present. On that assump- 

 tion, one-fifth of the atmosphere would be composed of carbon 

 dioxide. Now the weight of the air is equal to about 30 feet of 

 water. Therefore the weight of the carbon dioxide would have 

 been equal to about one-fifth of 30, that is about 6 feet of water. 

 Now take carbon dioxide as equal to one atom of carbon and two 

 of oxygen ; therefore the weight of the carbon in carbon dioxide 

 is equal to ^ of the whole weight of the carbon dioxide, that is 

 ij-of 6 feet of water, i.e. jj or in feet of water. 



That is, if all the assumed quantity of carbon dioxide in the 

 air at fhe beginning were withdrawn from it, and converted into 

 carbon, it would form a uniform seam of coal all over the world 

 of about if feet in thickness. 



Now let us try to estimate, it can only be roughly, the weight 

 of the world's coal. First let us take that of Great Britain and 

 Ireland. The following are figures taken from the Report of a 

 Royal Commission on Coal, and founded on investigations made 

 in the years 1866-1871. The quantities represent the aggregate 

 yield of all seams over i ft. in thickness : — 



Coal in exposed coal-fields within 4,000 feet = 90,2o6,240,387 

 tons. The quantity estimated as lying below the 4,000 ft. limit 

 not yet exposed in the central and northern counties of England 

 was 56,248,000,000 tons, and of the non-available coal there is 

 estimated between 4,000 and 6,000 ft. 26,341,659,067 tons, and 

 between 6,000 and 10,000 ft. 15,302,741,333 tons; giving a total 

 non-available of 41,644,390,400 tons, as compared with the total 

 available of 146,454,240,387 tons. 



What we are trying to estimate is the total quantity of coal in 

 the bowels of the earth ; whether available or non-available is all 

 the same. We therefore get a grand total of 188,098 million 

 tons estimated as existing in Great Britain and Ireland in 1877; 

 and at that time the output had been going on for some time at 

 the rate of 125 million tons per year. We thus see that before 

 coal mining started in those islands, there must have been little 

 under 200,000 million tons. Now if this quantity of coal were 



