134 HOW CROPS GROW. 



trate this statement with the three phosphates of lime, 

 giving in one view their mode of derivation, their sym- 

 bols, and the names which we shall employ in this treatise. 



a. 3 H 2 O, P 2 O 5 and CaO give H 2 O and 2 H 2 O, CaO, 

 P 2 O 6 , the monocalcic* phosphate or acid-phosphate of 

 lime. 



b.3 H 2 O, P 2 O 5 and 2 CaO give 2 H 2 O and H 2 O, 2 Ca 

 O P 2 O 5 , the dicalcic * phosphate or neutral phosphate of 

 lime. 



c 3 H 2 O, P 2 O 5 and 3 CaO give 3 H 2 O and 3 CaO P a 



O B , the tricalcic * phosphate or basic-phosphate of lime. 



Phosphates Of Potash. Of these salts, the neutral and 

 subphosphates exist largely (to the extent of 40 to 50 per 

 cent) in the ash of the kernels of wheat, rye, maize, and 

 other bread grains. None of these phosphates occur in 

 commerce ; they closely resemble the corresponding soda- 

 salts in their external characters. 



Phosphates Of Soda. Of these the disodic, or neutral 

 phosphate, 2 Na 2 O, H 2 O, P 2 O & + 12 Aqf, alone needs no- 

 tice. 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, saline taste, and is 

 very soluble in water. 



Phosphates Of Lime. Both the neutral and subphos- 

 phate of lime probably occur in plants. The neutral or 

 dicalcic salt, (2 CaO H 2 O, P 2 O B + 2 Aq), is a white crys- 

 talline powder, nearly insoluble in water, but easily soluble 

 in acids. In nature it is found as a urinary concretion in 



* These names indicate the proportions of acid and base in the compounds, 

 and may be translated into common English, thus : One-lime phosphate, two-lime 

 phosphate, and three-lime pliosphate respectively. 



t The water which is found in crystallized salts and which usually may be ex- 

 pelled at a gentle heat, is termed water of crystallization, and is often designated 

 by Aq., (from the Latin Aqua), to distinguish it from basic water, which is more 

 intimately combined. 



