ON FALLOWING. 173 



" one of 1 the principal speculative reasons for the 

 " defence of summer fallows. 



" Nitrous salts are produced during the expo- 

 " sure of soils containing vegetable and animal 

 " remains, and in greatest abundance in hot 

 " weather 5 but it is probably by the combination 

 " of azote from these remains, with oxygene in 

 " the atmosphere, that the acid is formed, and 

 " at the expense of an element which otherwise 

 " would have formed ammonia, the compounds 

 " of which, as is evident from what was stated in 

 " the last lecture, are much more efficacious 

 " than the nitrous compounds in assisting vege- 

 " tation. 



" When weeds are buried in the soil, by their 

 " gradual decomposition they furnish a certain 

 " quantity of soluble matter j but it may be 

 " doubted whether there is as much useful ma- 

 " nure in the land at the end of a clean fallow, 

 " as at the time the vegetables clothing the sur- 

 " face were first ploughed in. Carbonic acid gas 

 " is formed during the whole time by the action 

 " of the vegetable matter upon the oxygene of 

 " the air, and the greater part of it is lost to the 

 " soil in which it is formed, and dissipated in the 

 f atmosphere." 



The reasoning here offered by Sir Humphry 

 Davy, affords further proof of the possibility of 



