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limited period ; and the only counteractive influence, of the 

 least moment, which is known to be in operation, is the action 

 of vegetation. 



But passing from the relations which exist between the 

 plant and the animal, having a reference to their effects upon 

 the air in which they reside, we must proceed to consider 

 some of the phenomena of their nutrition. 



As a general statement, there is one marked distinction 

 between the materials for the nutrition of the beings belong- 

 ing to these two great kingdoms of nature ; — the plant can 

 appropriate only such principles as are strictly inorganic ; 

 the food of the animal must consist of organized substances, 

 or the organic principles derived from such. Any exceptions 

 which might be made to this rule are quite unimportant for 

 our present purpose ; — thus, it may be said, that water and 

 chloride of sodium, (common salt), and perhaps other salts, 

 though quite inorganic, are strictly alimentary to the animal; 

 and amongst plants, those which are parasitic, as the mistle- 

 toe or the dodder, derive supplies from the juices of the 

 plants on which they grow, which contain organic principles ; 

 and the whole of the fungi are probably likewise an excep- 

 tion to the rule, and their action upon the air is likewise 

 anomalous. 



Passing by these limited and unimportant exceptions, 

 the statement I have made is virtually correct. Animals, 

 therefore, are incapable of adding to the total amount of 

 existing organic matter; as they increase in size or num- 

 bers, it is only by appropriating to themselves matter already 

 possessing the form of organic compounds. But by the 



years, it might prove incompatible with health, or even destructive to life ; and 

 this, perhaps, before Chemistry eould detect any departure from the previous 

 condition. The endemic miasmata causing fever, and those more migratory 

 derangements of the air, giving rise to epidemic diseases, are quite imperceptible 

 by the most delicate chemical analysis. 



