292 



AMMONIA AND NITRIC ACID. 



The field at Mcmsegast. 



It is hardly necessary to carry this calculation any 

 further ; for all give the same result, viz. that even on the 

 most unfavourable supposition, a field receives back, by 

 the rain alone, actually more, certainly not less, nitrogenous 

 nutriment, than it loses in the ordinary course of agri- 

 culture. 



This fact may well justify the assertion that a farmer 

 need trouble himself as little about a compensating 

 supply of nitrogen, as of carbon. Both are, in fact, 

 originally constituents of the air, or capable of again 

 becoming air constituents, and are in the circulation of 

 life inseparable from one another. 



From the presence of ammonia and nitric acid in rain- 

 water we are led to infer that a source of nitrogen exists, 

 which without the aid of man, supplies plants with this 

 necessary nutriment. With regard to the other nutritive 

 substances, such as phosphoric acid and potash, which of 

 themselves are not moveable, this restoration from natural 

 sources does not exist. Hence, we might have supposed, 

 that when inquiry was made as to the causes which, in 

 consequence of cultivation, diminish the productive power 

 of land, the reason of such decrease would first and 



