PHOSPHORIC ACID. 347 



acid remains, in a fused state, which is known as glacial phos- 

 phoric acid, from its resemblance to ice. 



To exhibit many of its properties phosphoric acid must be 

 first dissolved in water, when the compound will be found to be 

 marked by an inconstancy and variableness in its characters, 

 most unusual in a strong acid. This arises from the circum- 

 stance that it is not actual phosphoric acid which dissolves in 

 water, any more than it is true sulphuric acid, which dissolves 

 in water, when oil of vitriol is added to that fluid. It is a hydrate 

 of both acids, which is soluble ; the phosphate of water in the 

 one case and the sulphate of water in the other. But the phos- 

 phoric acid differs from the sulphuric, in a singular and almost 

 peculiar capacity to form three different salts of water, instead 

 of one only ; and these three phosphates of water are all soluble 

 without change, and exhibit properties so different that they 

 might be supposed to contain three different acids. When the 

 dry acid from the combustion of phosphorus is thrown into 

 water, it produces a mixture, in variable proportions, of the 

 three hydrates ; but each of them may be had separately and 

 in a state of purity by a particular process. 



Terhydrate, or tribasic phosphate of water , 3HO + PO 5 . The 

 common phosphate of soda of pharmacy may be had recourse to for 

 all the hydrates of phosphoric acid ; but it should be first dissolved 

 and crystallized anew to purify it. To a warm solution of the 

 pure phosphate of soda in a bason, add a solution of acetate of 

 lead in distilled water, so long as it occasions a precipitate ; the 

 phosphate of soda requires rather more than an equal weight of 

 acetate of lead. The dense insoluble phosphate of lead, which 

 precipitates, is washed, and being afterwards suspended in cold 

 water, is decomposed by a stream of sulphuretted hydrogen gas 

 sent through it. The liquor may then be warmed, to expel the 

 excess of sulphuretted hydrogen, and filtered from the black 

 sulphuret of lead : it is very sour, and contains the terhydrate 

 of phosphoric acid. The characters of this acid solution are, to 

 give a yellow precipitate with nitrate of silver, to yield the 

 common phosphate of soda when neutralized with carbonate of 

 soda, to form salts which have invariably 3 atoms of base to 1 

 of phosphoric acid, and to be unalterable by boiling its solution 

 or keeping it for any length of time. The class of salts which 

 this hydrate forms are the old phosphates, which have been 

 long known, and it is convenient to allow them to be particu- 



