148 HOW CROPS GROW. 



Sodium Phosphates, or Phosphates of Soda. — 



Of these the disodic phosphate, Na2HP04, alone needs 

 notice. It is found in the drug-stores in the form of 

 glassy crystals, which contain X% molecules (56 per cent) 

 of water. The crystals become opaque if exposed to the 

 air, from the loss of water. This salt has a cooling, sa- 

 line taste, and is very soluble in water. 



Calcium Phosphates, or Phosphates of Lime. 

 — Since one atom of calcium replaces two of hydrogen, 

 the formulae of the calcium phosphates are written as 

 follows : monocalcic or primary phosphate CaHiPaOj ; 

 diealeic or secondary phosphate, OaHP04 ; tricalcic or 

 tertiary phosphate, Ca8P208.* Both the secondary and 

 tertiary phosphates probably occur in plants. The sec- 

 ondary is a white crysballine powder, nearly insoluble 

 in water, but easily soluble in acids. In nature it is 

 found as a urinary concretion in the sturgeon of the Cas- 

 pian Sea. It is also an ingredient of guanos, and proba- 

 bly of animal excrements in general. 



The tricalcic phosphate, or, as it is sometimes termed, 

 tone-phosphate, is a chief ingredient of the bones of ani- 

 mals, and constitutes 90 to 95 per cent of the ash or 

 earth of bones. It may be formed by adding a solution 

 of lime to one of sodium phosphate, and appears as a 

 white precipitate. It is insoluble in pure water, but dis- 

 solves in acids and in solutions of many salts. In the 

 mineral kingdom tricalcic phosphate is the chief ingre- 

 dient of apatite and phosphorite. These minerals are 

 employed in the preparation of the commercial super- 

 phosphates now consumed to an enormous extent as a 

 fertilizer. Plain superphosphate is essentially a mixture 

 of sulphate of lime with the three phosphates above no- 

 ticed and with free phosphoric acid. 



The Phosphates of Magnesium, Iron, Alumin- 

 ium and Manganese, are bodies insoluble in water, 



* These formulae coiTcspond to 2 molcoules of phosphoric acid, 

 =HeP20s, with 2 and 4 H-atoms replaced by Ca. 



