42 THE CHEMISTRY OF THE PRIMEVAL EARTH. [IV. 



with great changes in the organic life of the globe. The air 

 was doubtless at first unfit for the respiration of warm-blooded 

 animals, and we find the higher forms of life coming gradually 

 into existence as we approach the present period of a purer air. 

 Calculations lead us to conclude that the amount of carbon 

 thus removed in the form of carbonic acid has been so enor- 

 mous, that we must suppose the earlier forms of air-breathing 

 animals to have been peculiarly adapted to live in an atmos- 

 lihi-iv which would probably be too impure to support modern 

 reptilian life. The agency of plants in purifying the primitive 

 atmosphere was long since pointed out by Brongniart, and our 

 great stores of fossil fuel have been derived from the decompo- 

 sition, by the ancient vegetation, of the excess of carbonic acid 

 of the early atmosphere, which through this agency was ex- 

 changed for oxygen gas. In this connection the vegetation of 

 former periods presents the curious phenomenon of plants allied 

 to those now growing beneath the tropics flourishing within 

 the polar circles. Many ingenious hypotheses have been pro- 

 posed to account for the warmer climate of earlier times, but 

 are at best unsatisfactory, and it appears to me that the true 

 .solution of the problem maybe found in the constitution of the 

 atmosphere, when considered in the light of Dr. Tyndall's 

 beautiful researches on radiant heat. He has found that the 

 presence of a few hundredths of carbonic-acid gas in the atmos- 

 phere, while offering almost no obstacle to the passage of the 

 solar rays, would suffice to prevent almost entirely the loss, by 

 radiation, of obscure heat, so that the surface of the land be- 

 neath such an atmosphere would become like a vast orchard- 

 house, iii which the conditions of climate necessary to a luxu- 

 riant vegetation would be extended even to the polar regions. 



This peculiar condition of the early atmosphere cannot fail to 

 have influenced in many other ways the processes going on at 

 the earth's surface.* To take a single example : one of the 

 processes by which gypsum may be produced at the earth's 

 '0 involves the simultaneous production of bicarbonate of 

 This, being more soluble than the gypsum, is not 

 See Appendix to this paper. 



