ON FALLOWING. I7S 



" one of 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 ; 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 ; 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 

 " atmosphere.** 



The reasoning here offered by Sir Humphry 

 Davy, affords further proof of the possibility of 



