150 MANURES. VII. 



(1) As tricalcium phosphate, Ca 3 (PO 4 ) 2 . 



(2) ,, dicalcium phosphate, Ca 2 H 2 (P0 4 ) 2 or CaHPO 4 . 



(3) ,, monocalcium hydrogen phosphate, CaH 4 (P0 4 ) 2 . 



(4) ,, free phosphoric acid, H 8 PO 4 . 



(5) ,, ferric or aluminium phosphate, FeP0 4 or A1P0 4 . 



(6) ,, tetracalcium phosphate, Ca 4 P 2 9 . 



Tricalcium Phosphate is a white, almost insoluble sub- 

 stance which dissolves easily in acids. It is the form in 

 which phosphoric acid occurs in bones, in most mineral phos- 

 phates, and to a large extent in guano. The solubility of the 

 salt in water free from carbon dioxide and air is, according to 

 Pollacci,* 0-0098 gramme per litre at 12-5, or, if the phosphate 

 be dried at 25 instead of moist, Q'0181 gramme ; if the water 

 be saturated with carbon dioxide it dissolves 0'1605 at 10'5. 

 The solubility and particularly the rate of solution depend 

 greatly upon the physical condition of the phosphate, being 

 favoured by fineness of subdivision, porosity, and an amorphous 

 state. 



The most important mineral phosphate is apatite, which has 

 a composition corresponding to the formula 3Ca 8 P 2 H .CaCl 2 , or 

 3Ca 3 P 2 O 8 .CaF 2 , the varieties being sometimes distinguished by 

 the names chlor-apatite and fluo-apatite ; this substance 

 occurs as hexagonal crystals, often of a green or yellow colour. 

 Many of the mineral phosphates consist of what is practically 

 amorphous apatite. 



Dicalcium Phosphate, CaHP0 4 , when prepared by precipi- 

 tation, is a white solid containing 2 molecules of water. It is 

 only slightly soluble in water, but its solubility is greatly 

 increased by the presence of many neutral salts, eg., ammo- 

 nium citrate. It probably is more readily dissolved by the 

 acid juices of plants' roots than is tricalcium phosphate. By 

 long boiling with water it is said to yield a mixture of 

 tricalcium and monocalcium phosphates. 



4CaHPO 4 - Ca 8 P 2 H + CaH 4 P a O H - 



Monocalcium Phosphate, CaH 4 P 2 O K .H 2 0, can be obtained 

 in thin rhombic plates. It is best prepared by dissolving dical- 



* Jour. Chem. Soc. 1897, abst. ii. 2<50. 



