MANURES. 167 



ed it, is a strong retainer. It will hold fast the gases 

 and the salts of the animal part ; whereas, if we put 

 uncomposted manure into a light soil, the gases which 

 it generates are liable to be blown away, and the salts 

 to be washed away. 



314. I will here state that nitrogen is considered to 

 be among the most important ingredients of animal 

 manures. Some have gone so far as to lay it down, 

 that animal manures are valuable just about in pro- 

 portion to the nitrogen they contain. When manures 

 ferment, the nitrogen combines with hydrogen, one 

 atom of the former to three of the latter forming am- 

 monia (NH^). This immediately combines with car- 

 bonic acid (CO'*), forming carbonate of ammonia 

 (NH', CO'), which is exceedingly volatile, and passes 

 off into the air, where it is dissolved in watery vapor, 

 and again returned to the earth in falling rain. 



815. These facts show the benefit of sprinkling 

 plaster on stable floors, which should always be done, 

 and on fermenting manure heaps. The explanation is 

 thus : plaster is sulphuric acid (SO'), and lime (CaO), 

 or sulphate of lime (CaO, SO'). Now, when ammonia 

 is escaping from manure in the form of a carbonate, 

 if plaster is present, the ammonia and the lime ex- 

 change acids with each other, by what is called a dou- 

 ble decomposition. The lime takes the carbonic acid 

 from the ammonia, becoming carbonate of lime, and 

 gives its sulphuric acid to the ammonia, making that 

 a sulphate of ammonia, which last is a fixed, and not 

 a volatile alkali, and therefore remains in the manure 



