148 HOW CROPS GROW. 



Sodium Phosphates, or Phosphates of 

 Of these the disodic phosphate, Na 2 HP0 4 , alone needs 

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

 glassy crystals, which contain 12 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 Lirrie. 

 Since one atom of calcium replaces two of hydrogen, 

 the formulae of the calcium phosphates are written as 

 follows : monocalcic or primary phosphate CaH 4 P 2 8 ; 

 dicalcic or secondary phosphate, CaHP0 4 ; tricalcic or 

 tertiary phosphate, Ca 3 P 2 8 .* Both the secondary and 

 tertiary phosphates probably occur in plants. The sec- 

 ondary is a white crystalline 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, 

 bone-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 ard with free phosphoric acid. 



The Phosphates of Magnesium, Iron, Alumin- 

 ium and Manganese, are bodies insoluble in water, 



These formulae correspond to 2 molecules of phosphoric acid, 

 with 2 and 4 H-atonis replaced l>y fa. 



