in the Neighbourhood of Edinburgh. 263 



And, with regard to a difference in the gaseous constitution of the atmosphere, 

 when compared with its present state, I consider that such a difference might, to 

 some little extent, have subsisted ; — yet so little, as not to be capable of any geolo- 

 gical proof whatever. 



The argument for the air having been greatly charged with carbonic acid during 

 the time when carboniferous strata were deposited, was first advanced by M. 

 Adolphe Brongniart. Mr De la Beche, in adopting this view, has undertaken 

 a sort of origin and history of this atmospheric diffusion. 



Mr De la Beche's view is as follows: In a very early state of the globe, 

 cracks or volcanic fissures in the earth's crust, formed the vents through which a 

 large amount of carbon combined with oxygen, and, forming carbonic acid, made 

 its constant escape into the atmosphere. The waters were so hot, that they could 

 only absorb such portions of carbonic acid as were enabled to form compounds with 

 the mineral matter rising above the level of the waters, and were sufficiently cool for 

 the purpose. And hence it is argued, that, in consequence of the obstacles which 

 would thus arise to the formation of carbonate of lime, at least in any very large 

 proportion, — in consequence also of the elevated temperature of the water, — and 

 in consequence of the difficulty of procuring disseminated oxygen, circumstances 

 would arise unfavourable to the existence of numerous marine polypi, and shell-bear- 

 ing animals, which require carbonate of lime in comparatively large proportions; — 

 and thus, any carbonic acid thrown from the interior of the earth upon its surface, 

 would, for the most part, remain in the atmosphere. 



But, upon the earth becoming less heated, these conditions would necessarily change. 

 The waters, in consequence of their being cooled down, would be enabled to take up 

 more carbonic acid, which, when emitted from volcanic fissures, they would inter- 

 cept. The carbonic acid, thus absorbed, would act chemically on many substances, 

 and, among other actions, take up lime in solution, which, without this addition of 

 carbonic acid, would have been insoluble in these waters ; and hence, during this 

 cooled down state of ancient seas, numerous marine animals would be called into ex- 

 istence, such as the shell-bearing Mollusca, Encrinites, and Corals, which would 

 appropriate to themselves carbon ; while an equal volume of oxygen would be 

 liberated for their support, or would be thrown into the atmosphere. — De la Beche's 

 Theoretical Geology, pp. 298, 308, 313. 



This is Mr De la Beche's very rational theory. The question, however, still 

 remains, whether the amount of oxygen thus liberated, would, after much of it had 

 been arrested for the support of marine animals, form a residue capable, when thrown 

 into the atmosphere, of contributing in any material degree to its purification. The 

 theory shews little more than that any additional impurity would be checked, 1st, by 

 the sea being so cooled down as to admit of an increased absorption of carbonic acid ; 

 and, 2d, by the requisition of carbon for the creation of marine animals which had 

 been called into existence, whence the formation of calcareous deposits, produced by 

 such animal agency. 



M. Adolphe Brongniart has conceived that the atmosphere, after it had re- 



