1867. J HUNT — THE CHEMISTRY OF THE EARTH. 231 



purifying the primitive atmosphere was long since pointed out by 

 Brongniart, and our great stores of fossil fuel have been derived 

 from the decomposition, by the ancient vegetation, of the excess of 

 carbonic acid of the early atmosphere, which through this agency was 

 exchanged 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 proposed to account 

 for the warmer climate of earlier times, but are at best unsatisfac- 

 tory, and it appears to me that the true solution of the problem 

 may be found in the constitution of the early atmosphere, when 

 considered in the light of Dr. Tyndall's beautiful researches 

 on radiant heat. He has found that the presence of a few hun- 

 dredths of carbonic acid gas in the atmosphere, 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 beneath such an atmosphere would 

 become like a vast orchard-house, in which the conditions of climate 

 necessary to a luxuriant 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 surface 

 involves the simultaneous production of carbonate of magnesia. 

 This, being more soluble than the gypsum, is not always now 

 found associated with it; but we have indirect evidence that it 

 was formed and subsequently carried away, in the case of many 

 gypsum deposits, whose thickness indicates a long continuance ^f 

 the process under conditions much more perfect and complete than 

 we can attain under our present atmosphere. While studying 

 this reaction I was led to inquire whether the carbonic acid of the 

 earlier periods might not have favoured the formation of gypsum ; 

 and I found, by repeating the experiments in an artificial atmos- 

 phere impregnated with carbonic acid, that such was really the 

 case. We may thence conclude that the peculiar composition of 

 the primeval atmosphere was the essential condition under which 

 the great deposits of gypsum, generally associated with magnesian 

 limestones, were formed. 



The reactions of the atmosphere which we have considered, 

 would have the effect of breaking down and disintegrating the 

 surface of the primeval globe, covering it everywhere with beds of 



