10 CHEMICAL MANUEES. 



phorous or phosphorous acids, or by the action of water in excess 

 on pentachloride of phosphorus. In the concentrated state or- 

 dinary phosphoric acid forms a thick, inodorous, very acid, non- 

 poisonous syrup, of density 1-88. It crystalhzes with three mole- 

 cules of water when evaporated under a bell jar over sulphuric 

 acid. It is very soluble in water, and highly deliquescent. Under 

 the action of heat, it is converted at about 213° C. into pyrophos- 

 phoric acid, and towards a red heat into metaphosphoric acid. It 

 is reduced by red-hot charcoal, yielding phosphorus. It gives no 

 precipitate with barium chloride, nor with perchloride of iron, nor 

 with a solution of silver, nor with albumen. But if ammonia be 

 cautiously added, barium chloride gives a brownish precipitate, and a 

 solution of silver a yellow one ; both precipitates are soluble in acetic 

 acid. A characteristic test for phosphoric acid is the production of 

 the double phosphate of magnesium and ammonium, by " magnesia 

 mixture," consisting of ammonia, magnesium chloride, and water. 

 This precipitate is white and crystalline ; it suffices to see it once 

 not to confuse it with any other precipitate. A reagent which 

 enables traces of phosphoric acid to be detected, consists of a nitric 

 acid solution of ammonium molybdate. If the mixture of the two 

 solutions be heated the liquid assumes a bright yellow coloration.^ 

 Ordinary phosphoric acid is a very stable compound. It displaces 

 sulphuric and nitric acids from their compounds. 



Phosphates of Lime. — It has just been se:;n that phosphoric 

 acid is tribasic, i.e. it contains three atoms of hydrogen, replaceab'e 

 by metals. 



I OH i OM ( OM C OM 



PO { OH PO { OH PO OM PO { OM 



I OH I OH I OH I OM 



Orthophosphoric Monometallic Dimetallic Trimetallic 



Acid Orthophosphate Orthophosphate Orthophosphate 



When the hydrogen of phosphoric acid is replaced by calcium, a 

 divalent element, three types of phosphates are obtained to which 

 the commercial varieties of phosphatic manures correspond in a 

 very characteristic fashion. 



I ^H (^-^r., r^->ra 



PO OH PO [ 0^'^'^ PO { 0^^^ 



■^\p '^C^^^ ^O-^Cr 



0>Ca Q>Ca 0>^'' 



PO OH PO OH PO - O^p,^ 



Uh Uh ^ 0>^'' 



Monocalcic phos- Diealeic phosphate Tricalcic phosphate, 



phate or acid or monacid Neutral phosphate of lime, 



phosphate of lime, phosphate of Tribasic phosphate of lime, 



CaH,(PO,).. lime Ca3(P0,)._j 



^ But ammonium molybdate in nitric acid solution gives a yellow coloration 

 even in the absence of phosphoric acid. Unless there is a decided yellow pre- 



