MANURE. 135 



matter ; but that, under the influence of porous ma- 

 terials, aided by alkalies or lime, the elements of air 

 Cbmbine and form nitric acid and nitrates. This ac- 

 tion is greatly assisted by ammonia, which acts by 

 catalysis. The great use of the animal matter is to 

 produce this alkali or ammonia. If no alkaline base 

 is present, it becomes the source of the formation of 

 nitrate of ammonia. This salt being decomposed 

 by the living plant, its nitric acid acts on the sili- 

 cates, and saltpetre or nitrate of potash is produced. 

 The agency of this as a manure, has already been 

 considered (167, 168). The action, also, of other 

 salts in dung, will be easily understood by reference 

 to the fourth chapter. 



195. There is still a powerful effect due to the 

 geine, or to the hay in its conversion to that state. 

 During this process an immense quantity of carbonic 

 acid is liberated. The decomposing action of this 

 upon silicates of the soil, and the consequent libera- 

 tion of their alkali, has also been explained (133). 

 All these actions are to be remembered, in account- 

 ing for the action of cow dung. The geine, salts, 

 nitrogen, each acts — the geine has an action, the 

 salts an action, the nitrogen an action. They all 

 contribute to one end. Three substances, but one 

 result, viz : vegetation. 



