66 Existence of Compound Radicals in Amphide Salts. 



existence of the different states of combination in which the phos- 

 phoric acid actually exists. It will consequently be necessary to 

 study separately the properties of the three compounds of phos- 

 phoric acid with water. 



" Monobasic phosphate of water. — A solution of this body re- 

 acts powerfully acid, it precipitates albumen (white of egg) in 

 white curds ; when neutralized by a base, it gives salts which 

 contain but one atom of base, their formula being P0 5 +RO; 

 and a soluble salt of it produces in solutions of silver, a white, 

 soft precipitate, PO 5 +AgO. This is the least stable of the phos- 

 phates of water, it gradually passes into the other forms, particu- 

 larly when its solution is boiled. 



" Bibasic phosphate of water. — This form of the acid may be 

 prepared by decomposing bibasic phosphate of lead by sulphuret- 

 ted hydrogen. It is characterized by combining always with two 

 equivalents of base, forming salts, whose formula is P0 5 +2RO; 

 its salts give, with nitrate of silver, a white precipitate PO s + 

 2AgO, which is not pasty like the monobasic phosphate. The 

 salts of this acid may contain only one equivalent of fixed base, 

 the other being water, and may hence at first sight appear to be 

 constituted like the monobasic salts ; the basic water is, however, 

 easily known to be present, by its not being expelled by a mod- 

 erate he^, with the water of crystallization, but requiring a tem- 

 perature approaching to ignition for its expulsion. 



" Tribasic phosphate of water. — This is the form of phospho- 

 ric acid which represents the class of salts most generally known ; 

 it is characterized by not precipitating albumen, and by combi- 

 ning with three equivalents of base when fully neutralized. In 

 the majority of cases of the three equivalents of base, one is water; 

 thus the common phosphate of soda is a tribasic phosphate, its 

 formula being (PO 5 -f 2NaO.HO)-f24Aq; when moderately heat- 

 ed, or even by long exposure to dry air, it loses the 24Aq, but it 

 requires to be melted at a red heat, in order to drive off the twen- 

 ty fifth atom of water; and if this be done, on redissolving the 

 fused mass in water, it crystallizes in a totally different form, and 

 is found to have been changed into bibasic phosphate of soda, 

 the formula of which is (PO 5 + 2NaO) + 10Aq. The difference 

 is remarkably shown by the action of these salts on a solution of 

 silver; common phosphate of soda precipitates nitrate of silver of 

 a canary yellow, and the solution becomes acid ; one equivalent 



