io6 PHOSPHATIC MANURES [chap. 



the market as precipitated phosphate ; it is also known 

 as retrograde or reverted phosphate, because it arises 

 when the soluble compound next to be described, 

 "superphosphate," passes into the insoluble condition. 

 When tri-calcium phosphate is treated with such an 

 amount of sulphuric acid as is required to combine with 

 two out of the three lime radicles in the molecule, a 

 mixture is obtained of gypsum and of soluble mono- 

 calcium phosphate, Call^PoOg, which is known as 

 superphosphate. Some uncertainty may still be 

 supposed to exist as to the exact identity of this 

 compound, but in practice a readily soluble mixture of 

 phosphoric acid and lime in something like these pro- 

 portions does get formed, and the reactions of this 

 solution are explained accurately enough by the formula 

 CaH^PoOy. Soluble phosphoric acid itself, H3PO4, also 

 exists, and some is always supposed to be present in a 

 free state in superphosphate. 



It has already been mentioned that by the continued 

 treatment of ordinary insoluble phosphates with water 

 a more and more basic phosphate is formed, to which 

 VVarington gave the formula (Cag P205j)Xa(OH)., ; and a 

 definite phosphate of this type, Ca^PgO^ or 4CaO, PgOg, 

 has been isolated in crystals from basic slag and may be 

 supposed to mark the compound of lime and phosphoric 

 acid which is stable at high temperatures. It is this 

 tetra-calcium phosphate which is supposed to constitute 

 the greater part of the phosphoric acid compounds of 

 basic slag, but little is really known of its existence. 



Of the phosphatic manures, the earliest and for a 

 long time the only ones to be employed on a large 

 scale were those derived from bones. It would be 

 impossible to attribute the discovery of the fertilising 

 value of bones to any individual ; in common with all 

 other waste materials of animal origin, they were prob- 



