Geological History of the Atmosphere. 313 



acid then than it is now. Further, as the dissociation of 

 carbonic acid by vegetation not only has for its result the 

 fixation of carbon in the vegetable tissues, but also involves 

 the liberation of oxygen, it is obvious that the deposition of 

 the earth's supply of coal implies that a corresponding 

 quantity of oxygen has become permanently free, and there- 

 fore the atmosphere must be to that extent richer in oxygen 

 than it was before the coal was deposited. But, on the other 

 hand, large quantities of oxygen are consumed and large 

 quantities of carbonic acid are produced by respiration and 

 combustion, and, also, very large quantities of carbonic acid 

 are observed to issue from the earth in volcanic districts ; so 

 it is not at all surprising to find that some chemists have been 

 of the opinion that the proportion of carbonic acid in the 

 atmosphere is increasing, and that the proportion of oxygen 

 is decreasing. In fact there has been, and probably there 

 still is, a very great divergence in the opinions held on this 

 question. And it is a question of the very greatest interest 

 and importance to chemical and geological science, for even 

 comparatively slight changes in the composition of the 

 atmosphere, especially as regards the amounts of oxygen and 

 carbonic acid present, would greatly affect the development 

 of animal and vegetable life ; while if such changes were at 

 all considerable they would exert a powerful influence on 

 many other phenomena, such as the weathering and denu- 

 dation of rocks, the solution and precipitation of carbonates 

 and hydrated oxides, and also the colouring of rocks and 

 soils by iron compounds, which are very sensitive to oxidizing 

 and reducing influences, and vary in colour accordingly. 



Further, the very existence of such a large quantity of 

 oxygen in the free condition as we find present in the atmo*- 

 sphere, notwithstanding its very strong affinities for other 

 elements, presents a sufficiently striking scientific problem. 

 No doubt a ready solution of it is sometimes found in the 

 great abundance of oxygen on the earth generally, but this 

 solution is by no means a satisfactory one. It would, no 

 doubt, be satisfactory and sufficient if the materials composing 

 the earth were all thoroughly oxidized. If that were the 

 case, then the free oxygen of the atmosphere would be simply 

 the extra or surplus oxygen that was left over after the 

 requirements of the other elements were satisfied. But, 

 unfortunately for this view, the earth, or at any rate the crust 

 of the earth, is very far from being thoroughly oxidized. 

 Not only do we find coal and other carbonaceous matter 

 diffused throughout sedimentary rocks, but there are also 

 found large quantities of sulphur, sulphide of liron and other 



