Geological History of the Atmosphere. 81) 



present, we saw that they would bo converted into water and 

 carbonic aeid through the agency of vegetation. There is 

 even a possibility that they may have been directly absorbed 

 bv ancient forms of vegetation, and in the subsequent heating 

 by volcanic action of the vegetable remains in presence of 

 rocks containing oxide of iron, carbonic oxide, carbonic acid 

 and water would be produced. Carbonic acid, whether pro- 

 duced in this way or present from the first in the atmosphere, 

 would also be absorbed by vegetation, this process of absorption 

 being accompanied by the decomposition of the carbonic acid 

 and the production of free oxygen, though it should be noted 

 that there would not be permanent free oxygen in the atmo- 

 sphere as long as free hydrogen existed. The free oxygen 

 produced from carbonic acid by vegetable growth being in 

 the nascent condition, would immediately combine with the 

 hydrogen present in the atmosphere producing water, as 

 inferred from Dr. Phipson's experiment which was referred 

 to in a previous article. It is possible, no doubt, that the 

 hydrogen which disappeared in the course of the experiment 

 was absorbed directly by the plant; but its oxidation by 

 nascent oxygen seems a highly probable explanation of the 

 effect observed; and in any case it is obvious that we could 

 not expect free oxygen and free hydrogen to exist long- 

 together in the atmosphere in considerable quantities. After 

 all the free hydrogen had disappeared, permanent free 

 oxygen, i. e. free oxygen as a permanent constituent of the 

 atmosphere, would begin to appear, and the quantity would 



gradually increase until it reached a limit determined prin- 



. . . 



cipally by the amount of oxidizable matter within reach and 



the readiness of this oxidizable matter to combine with 

 oxygen under prevailing conditions. On the other hand. 

 the quantity of carbonic acid in the atmosphere would pro- 

 bably diminish steadily during the early geological epochs, 

 until a certain limiting percentage determined by a variety of 

 conditions was reached. Large quantities of carbonic acid 

 are annually produced and added to the atmosphere through 

 the operation of various agencies or reactions, and large 

 quantities are removed or decomposed, principally through 

 the growth of vegetation and the weathering of rock- ; 

 that is to say, through the decomposition of silicates bj 

 carbonic acid and water, resulting in the formation of car- 

 bonates and free silica, or of rocks containing a higher pro- 

 portion of silica than the original rocks. This latter process 

 probably operated with very great effect in early geological 

 epoch-, when the proportion of carbonic acid in the atmo- 

 sphere was greater than it is now; and we also infer that 



