PHOSPHATIC MANURES. 151 



ciurn phosphate in phosphoric acid and washing the crystals 

 obtained with alcohol and ether." It is not hygroscopic if 

 free from excess of phosphoric acid. When treated with a 

 small quantity of water a portion of the salt is decomposed, 

 with the formation of dicalcium phosphate as a precipitate and 

 free phosphoric acid ; but with larger quantities of water or in 

 the presence of free phosphoric acid this does not occur. 



Free Phosphoric Acid, H 8 P0 4 , is a thick, semi-solid mass, 

 of sp. gr. 1'88, formed by decomposing, say, calcium phosphate 

 with sulphuric acid 



Ca 8 P a 8 + 3H. 2 S0 4 - 3CaS0 4 + 2H 8 P0 4 . 

 It is soluble to practically any extent in water. 



Ferric Phosphate, FeP0 4 , and Aluminium Phosphate 



are practically insoluble in water and, unlike tricalcium phos- 

 phate and most other phosphates, are not dissolved to any 

 appreciable extent by weak vegetable acids, e.g., acetic acid. 

 Consequently they are not easily available to plants and 

 possess little value as manurial ingredients. 



Tetracalcium Phosphate, Ca 4 P. 2 O y , is found in the slag 

 produced in the dephosphorisation of cast-iron by the Basic 

 Bessemer or Basic Siemens process. It is practically in- 

 soluble in water, but dissolves in many saline solutions. It is 

 therefore available as a plant food. 



The chief varieties of phosphatic manures yet to be described 

 are 



Coprolites. 



Phosphorites, of which there are many varieties. 



Redonda phosphate. 



Mineral superphosphates. 



Bone superphosphate. 



Basic slag. 



Coprolites are concretionary nodules found in the chalk or 

 other deposits in the South of England and in France ; they 



* Stocklasa, Jour Chem. Soc. 1890, abst. 695. 



