116 ELEMENTARY AGRICULTURAL CHEMISTRY 



phate, does not correctly describe the state of existence of the 

 phosphorus pentoxide in many manures ; while even " potash," 

 which really means the substance Kfi, is not correctly applied 

 as the name of a constituent of such a manure as '' muriate of 

 potash " — i.e., KCl. The trade method of expressing the com- 

 position of a sample of superphosphate may be given. 



Per cent. 



Monocalcium phosphate, or monobasic phosphate of lime 15 

 {=tricalcium phosphate rendered soluble, or 

 " soluble phosphates," 23*5 per cent.) 

 Insoluble phosphates ....... 3 



Hydrated calcium sulphate 

 Alkaline salts 

 Water .... 

 Silica 



54 

 2 



22 

 4 



100 



By " monobasic phosphate of lime " in such an analysis is 

 meant, not true monocalcium tetrahydrogen phosphate, 

 CaH^PgOg, but that substance less two molecules of water, 

 CaPgOfi (though this is really calcium metaphosphate, which 

 does not occur in manures). The relation between this sub- 

 stance and the tricalcium phosphate is easily seen from their 

 formulae : 



CaP.O, Ca3P,03 



40 + 62-1-96 = 198 120-1-62 4-128 = 310 



i.e., the amount of " monocalcium phosphate "x310-rl98 = the 

 amount of tricalcium phosphate, or " tribasic phosphate of 

 lime," as it is sometimes called. 



In many superphosphates a portion of the phosphorus pent- 

 oxide is in the form of calcium hydrogen phosphate, CaHPO^. 

 This substance, though insoluble in water, is soluble in many 

 salt solutions — e.^., ammonium citrate. It is sometimes referred 

 to as " reverted," " retrograde," or " reduced " phosphate ; it 

 would be better to call it " citrate soluble." In fact, since 

 phosphorus pentoxide is the really valuable ingredient, it would, 



