84 THE PRINCIPLES OF AGRICULTURE. 



and natural fertilizer for the farmer, they are called 

 artificial fertilizers. 



The materials for them are obtained from a variety 

 of sources, some of the more common of which are 

 as follows : 



Sources of Nitrogen. 1. Nitrate of soda, or Chili salt- 

 peter, is extensively obtained in a natural state from 

 some parts of South America. 



2. Sulphate of ammonia is obtained for a fertilizer 

 from " gas liquor," or the water in which illuminating 

 gas has been washed. The ammonia which the liquid 

 contains is obtained by treating it with sulphuric acid, 

 with which the ammonia combines. 



3. Fish scrap, meat scrap, dried blood, and all forms 

 of animal refuse, are rich in nitrogen, and are generally 

 used for manufacturing fertilizers. 



Sources of Phosphoric Acid. 1. Bones are largely com- 

 posed of phosphate of lime, which is converted into an 

 available form, called superphosphate , by an application 

 of sulphuric acid. Crushed and ground bones are also 

 used directly as fertilizers, but yield their phosphoric 

 acid more slowly. Bone-ash, produced by burning bones 

 until they crumble easily, is less valuable than ground 

 bones, as the process of burning removes from the bones 

 what nitrogen they contain. 



2. Bone-black, or charred bone, is used at sugar refin- 

 eries for cleansing sugar. After it becomes useless for 

 this purpose it is treated with sulphuric acid to convert 

 its elements into a soluble form, and is then sold as a 

 fertilizer. 



3. -Mineral deposits are found at various points of the 

 earth containing a large percentage of phosphate of lime. 

 Their origin is supposed to have been an accumulation of 



