382 PALAEOZOIC SYSTEM OF ROCKS. 



Tyndall lias shown * that the property of opacity to dark heat in the 

 case of the atmosphere is due wholly to the small quantity of carbonic 

 acid and aqueous vapor present ; that oxygen and nitrogen are trans- 

 parent to dark heat, and, therefore, if the atmosphere consisted only of 

 these two gases, it would not be heated by radiation from the earth, and 

 the ground would lose all its heat by radiation during the night, and 

 become intensely cold like space. In other words, the blanket put about 

 the earth to keep it w r arm is woven of carbonic acid and aqueous vapor. 



Now, we have seen that during the Coal period the quantity of car- 

 bonic acid and aqueous vapor in the air was far greater than now. 

 The atmosphere was then a double blanket, and therefore kept the 

 young earth much warmer. We believe that Prof. T. S. Hunt f was 

 the first to apply this discovery of Tyndall to the explanation of the 

 climate of the Coal period. E. B. Hunt had previously attributed it to 

 greater density of the air (Dana, Manual, p. 353) ; but this is a wholly 

 different principle.]; 



Thus the physical geography explains the humidity and uniformity, 

 and the greater humidity and the carbonic acid explain the greater mean 

 temperature. But there is still the carbonic acid to be accounted for. 



The more highly -carbonated condition of the atmosphere must be 

 attributed to the original constitution of the air. All carbonic-acid- 

 producing causes, such as animal respiration, combustion, general decay 

 of organic matter, volcanoes, carbonated springs, etc., only return to the 

 air what has been previously taken from it. There can be no doubt 

 that all the carbon in the world, whether in the form of organic matter, 

 or of coal, or of bitumen, or of carbonates, existed once as carbonic acid 

 in the air, and has been progressively withdrawn. First immense quan- 

 tities were withdrawn and fixed as carbonates, especially as carbonate of 

 lime (limestone), and the air correspondingly purified. Again, immense 

 quantities were withdrawn by the luxuriant vegetation of the Coal pe- 

 riod, and fixed as coal. In this latter method of withdrawal the oxygen 

 of the carbonic acid is returned, and the oxygenation of the air is in- 

 creased. We shall see hereafter that the process of purification did not 

 cease with the Coal period ; for large quantities were again withdrawn 

 and laid down as coal and lignite in the Jurassic, the Cretaceous, and 

 Tertiary periods. There can be no doubt that this progressive purifica- 

 tion of the air, by the withdrawal of superabundant carbonic acid and 

 returning the pure oxygen, fitted it for the purposes of higher and 

 higher animals. 



* Proceedings of the Royal Society, vol. xi, p. 100 ; American Journal, second series 

 vol. xxxvi, p. 99. 



f Chemical and Geological Essays, p. 42. 



\ According to Buff, Archives des Sciences, vol. lvii, p. 293, the opacity to dark heat 

 of carbonic acid and aqueous vapor has been exaggerated by Tyndall. 



