ON THE FOOD OF PLANTS. 129 



•' of vitality, begin to pass through a series of 

 " changes, which ends in their complete destruc- 

 " tion, in the entire separation and dissipation 

 " of the parts. Animal matters are the soonest 

 " destroyed by the operation of air, heat^ and 

 *' LIGHT. Vegetable substances yield more slow- 

 " ly, hut finally obey the same laws. The periods 

 *' of the application of manures, from decom- 

 " posing animal and vegetable substances, de- 

 " pend upon the knowledge of these principles.** 

 Now, notwithstanding the manner of applying 

 dung which I recommend, that of spreading it 

 over the surface, and there permitting it to re- 

 main, before it be ploughed in, twelve months or 

 more, is directly opposite to that recommended 

 by Sir Humphry, it will be found more com- 

 pletely accordant with the above principles, as 

 well as with those of Mr. Kirwan : for, by leav- 

 ing dung openly spread on the surface, it is 

 evident, that the influence of the air, the heat of 

 the sun, and light, will be the least controlled 

 or obstructed j and, consequently, the decom- 

 position will be more rapid, regular, and con- 

 formable to the wants of the plants. Under such 

 circumstances, more carbonic acid may be ge- 

 nerated J but as this elastic fluid is heavier than 

 the atmospheric air, it will fall on and penetrate 

 the open surface of the soil, and thus accord in 



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