318 SALTS. 



806. Ordinary phosphoric acid has the 



Why is ordi- ,, , . . 



naryphospho- property of combining with and neutrah- 



edtrTba ticT~ zing tnree e( l uivalents f Dase j instead of 

 one, as is the case with most other acids. 

 It is therefore called a tribasic acid. The hyd rated 

 acid contains, also, three equivalents of water, and may 

 be regarded as a salt in which the water acts the part 

 of base. Arsenic acid is similar in this respect, as well 

 as in the amount of oxygen which it contains, and in 

 the salts which it forms with bases. Two other kinds 

 of phosphoric acid may be prepared from that above 

 mentioned ; the first combines with one, and the second 

 with two equivalents of base. 



807. PREPARATION. The phosphates of 



How are the . f J 



phosphates the alkalies may be produced by the ac- 

 preparcd? tion of phosphoric acid on the proper car- 

 bonates. The remaining phosphates may be precipi- 

 tated by solution of phosphate of soda, from solutions 

 of the metals or their salts. As in other cases of pre- 

 cipitation, there is here a double decomposition, with 

 exchange of acids and bases. 



808. SUPERPHOSPHATE OF LIME. A 



Describe the . - 



preparation mixture bearing this name, formed by the 



action of dilute sulphuric acid on burned 

 bones, is extensively used as a fertilizer of 

 the soil. The sulphuric acid, when added, appropri- 

 ates part of the lime of the bones, forming with it 

 gypsum ; at the same time, it leaves the phosphoric 

 acid which it displaces, free to combine with another 

 portion of phosphate of lime and thereby to render it 

 soluble. The commercial article is a mixture of this 



