THE ASH OF PLANTS. 147 



III nature, sulphate of lime is usually combined with 

 two molecules of water, and thus constitutes Gypsum, 

 0aSO4 . 2 H2O, which is a rock of frequent and exten- 

 sive occurrence. In the cells of many plants, as for 

 instance the bean, gypsum may be discorered by the 

 microscope in the shape of minute crystals. It requires 

 400 times its weight of water to dissolve it, and being 

 almost universally distributed in the soil, is rarely absent 

 from the water of wells and springs. 



Land plaster is ground gypsum, that from N"ova 

 Scotia being" white, that from Onondaga and other local- 

 ities in New York State gray in color. 



The Phosphates which require special description 

 are those of Potassium, Sodium, and Calcium. 



Numerous phosphates of each of these bases exist, or 

 may be prepared artificially. But three classes of phos- 

 phates have any immediate interest to the agriculturist. 

 As has been stated (p 132), phosphoric acid, prepared by 

 boiling phosphorus pentoxide with water, is represented 

 by the symbol HsPOi. The phosphates may be regarded 

 as phosphoric acid in which one, two,, or all the atoms 

 of hydrogen are substituted by one or several metals. 



Potassium Phosphates or Phosphates of Potash. 

 — There are three of these phosphates formed by replac- 

 ing one, two, or three hydrogen atoms of phosphoric 

 acid by potassium, viz. : KH2PO4, primary or mono- 

 potassic phosphate ; K2HPO4, secondary or dipotassic 

 phosphate, and KsPOj, tertiary or tripotassic phos- 

 phate.* Of tlfese salts, the secondary and tertiary phos- 

 phates 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. The potassium phosphates do not occur 

 in commerce ; they closely resemble the corresponding 

 sodium-salts in their external characters. 



*Tlie primary phosphates are often designated add or m.per-pho»- 

 phates, the secondary neutral phosphates, and the tertiary basic phos- 

 phates. 



