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found in three forms, viz., the soluble, reverted and insolu- 

 ble forms. The first is soluble in water, the second is sol- 

 uble in neutral citrate of ammonia, and is termed by the 

 fertilizer law "citrate soluble," while the third form is in- 

 soluble both in water and citrate of ammonia, but is soluble 

 in acids, and is therefore designated "acid soluble phos- 

 phoric acid." 



In nearly all of our fertilizers the phosphoric acid is in 

 combination with lime, and in bones, phosphate rock and 

 the chief mineral phosphates, is in the form of what is 

 termed "bone phosphate" or insoluble phosphate of lime. 

 While this form of phosphoric acid is insoluble in pure 

 water, it dissolves slowly in water containing carbonic acid 

 (carbon dioxide) in solution, and upon many soils, especially 

 in the presence of organic matter, and where the particles of 

 the fertilizer are in a finely divided condition, has been 

 employed to good advantage. 



In order to reduce crude phosphates such as bones, phos- 

 phate rock, etc., to a state of fine division, and also to con- 

 vert the phosphoric acid into a form soluble in water, the 

 crude materials are treated with sulphuric acid and a pro- 

 duct is obtained which is known as superphosphate of lime 

 or acid phosphate, while gypsum (sulphate of lime) is also 

 produced and remains admixed with the phosphate. 



Pure superphosphate of lime is completely soluble in 

 water, and were the precise amount of sulphuric acid re- 

 quired to completely convert the insoluble phosphate into 

 the superphosphate employed, the whole of the phosphoric 

 acid of the acid phosphate would be in a soluble condition. 



In actual practice, however, the full theoretical amount 

 of sulphuric acid is seldom employed in the manufacture of 

 superphosphates, and a small amount of insoluble phos- 

 phoric acid is left in the product. 



This insoluble phosphate, in contact with the soluble 

 phosphate, leads to the formation of a compound inter- 

 mediate between the two in composition, and this substance 

 is the "reverted" or "reduced" phosphate. 



