142 PHOSPHATIC MANUKES 



nitrate and potassium chloride, which when mixed in solution and 

 evaporated yield, first, sodium chloride crystals, and then, on cooling, 

 potassium nitrate. 



The properties of saltpetre are well known and need not be de- 

 scribed here. 



II. PHOSPHATIC MANURES. Several of the manures already 

 described are mainly valued for the phosphates they contain ; this is the 

 case with the non-nitrogenous guanos, steamed and burnt bones, etc. 

 There are other sources of phosphoric acid of greater importance, which 

 must now be described. Before mentioning the manures themselves, 

 it may be advisable to briefly describe the various forms in which 

 phosphoric acid occurs in fertilisers. These are 



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



(2) dicalcium phosphate, Ca 2 H 2 (P0 4 ) 2 or CaHP0 4 . 



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



(4) ,, free phosphoric acid, H 3 PO 4 . 



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



(6) ,, tetracalcium phosphate, Ca 4 P 2 O y . 



Tricalcium phos%)hate is a white, almost insoluble substance, which 

 dissolves easily in acids. It is the form in which phosphoric acid 

 occurs in bones, in most mineral phosphates, and, to a large extent, in 

 guano. The solubility of the salt in water, free from carbon dioxide 

 and air, is, according to Pollacci, 1 O0098 gramme per litre at 12'5, 

 or, if the phosphate be dried at 25 instead of moist, 0-0181 gramme ; 

 if the water be saturated with carbon dioxide it dissolves (M605 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 3 P. 2 O 8 .CaCl 2 , or 

 3Ca 8 P 3 O 8 .CaF 21 the varieties being sometimes distinguished by the 

 names chlor-apatite and fluo-apatite ; this substance occurs as hexa- 

 gonal 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 precipitation, is 

 a white solid containing 2 molecules of water. It is only slightly 

 SDluble in water, but its solubility is greatly increased by the presence 

 of many neutral salts, e.g., ammonium 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. 



4CaHP0 4 = Ca 3 P 2 8 + CaH 4 P 2 8 . 



Monocalcium phosphate, CaH 4 P.,O 8 .H 0, can be obtained in thin 

 rhombic plates. It is best prepared by dissolving dicalcium phosphate 

 in phosphoric acid and washing the crystals obtained with alcohol and 

 ether.' 2 It is not hygroscopic if free from excess of phosphoric acid. 



1 Jour. Chern. Soc., 1897, Abstracts, ii, 260. 

 2 Stoklasa, Jour. Chem. Soc., 1890, Abstracts, 695. 



