PRECONCEIVED NOTIONS. 

 The field at Mdusegast. 



277 



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 ordi- 

 nary course of agriculture. 



This fact may well justify the assertion that a far- 

 mer 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 movable, this res- 

 toration 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 



