ARTIFICIAL MANURE MANUFACTURE 65 



is commercially called "soluble phosphate' ' i.e., 

 the amount of phosphoric acid found, on analysis, 

 to be soluble, is calculated back to Ca3(PO 4 )2. 

 An ordinary superphosphate will contain 24 to 

 30 per cent of soluble phosphate, 40 to 45 per cent 

 calcium sulphate, and 2 to 3 per cent of insoluble 

 phosphate. Attempts have been made to pro- 

 duce superphosphate containing more soluble phos- 

 phoric acid than the quantity mentioned above, 

 in order to save carriage; thus "double super- 

 phosphate " is made by extracting an ordinary 

 superphosphate with water, removing the calcium 

 sulphate by means of a filter press, and evap- 

 orating the liquor with phosphate that has already 

 been treated with a quality easily attacked by 

 phosphoric acid. Such preparations may contain 

 80 to 90 per cent of soluble phosphate. 



When superphosphate is kept, a portion of the 

 soluble phosphate becomes insoluble in water 

 owing, it is supposed, to the interaction of calcium 

 superphosphate CaH 4 (PO 4 ) 2 , and normal calcium 

 phosphate, CasCPO^, thus: 



CaH 4 (P0 4 ) 2 +Ca 3 (P0 4 ) 2 =4CaHPO 4 . 



A further reaction, which has been already men- 

 tioned, occurs between the ferric oxide and alumina 

 contained in crude phosphate and a portion of 

 the phosphoric acid, insoluble ferric phosphate 

 (FePO 4 ) and aluminium phosphate (A1PO 4 ) being 

 formed. Although such "reverted phosphate" is 

 insoluble in water, it is more soluble in saline solu- 



