350 PHOSPHORUS. 



protohydrate of phosphoric acid combines with no more than 

 one, and the deutohydrate with no more than two proportions 

 of soda, although three or a larger number of proportions of 

 alkali be added to it. The excess of alkali remains free. Again, 

 supposing an equivalent quantity of the terhydrate of phos- 

 phoric acid in solution, and one equivalent of soda added to it, 

 one equivalent only of water is displaced, and two retained, and 

 a phosphate formed, containing one of soda and two of water 

 as bases, which is the salt already adverted to under its old 

 name of biphosphate of soda. Let a second equivalent of soda 

 be added to this salt, and a second basic atom of water is dis- 

 placed, and a tribasic salt produced, containing two of soda and 

 one of water as bases, which is the common phosphate of soda 

 of pharmacy. A third equivalent of soda added to the last salt 

 displaces the remaining atom of basic water, and a tribasic 

 phosphate is formed, of which the whole three atoms of base 

 are soda, and which has had the name of subphosphate of soda. 

 But this last salt can unite with no more soda. The 

 same three salts may be formed by means of the tribasic 

 phosphate of water, in another manner. That acid hydrate 

 decomposes chloride of sodium, but only to a certain extent, 

 expelling hydrochloric acid, so as to acquire one of soda, and 

 becoming 2HO, NaO + PO 5 , or the biphosphate of soda (ap- 

 plying the old trivial terms) ; the same acid hydrate applied to 

 the carbonate or the acetate of soda, can assume two propor- 

 tions of soda, displacing twice as much of the weaker carbonic 

 and acetic acids, as of the hydrochloric acid, and so becomes 

 HO, 2NaO -f PO 5 , or the common phosphate of soda ; and the 

 same acid hydrate applied to the hydrate of soda (caustic soda), 

 assumes three of soda, and becomes 3 NaO + PO 5 , or the sub- 

 phosphate of soda. 



From soluble tribasic phosphates, such as those mentioned, 

 insoluble salts may be precipitated, which are likewise tribasic, 

 by adding solutions of most metallic salts. Thus 1 equivalent 

 of the common phosphate of soda, added to the nitrate of silver 

 in excess, decomposes 3 equivalents of it, and produces the 

 yellow tribasic phosphate of silver, as explained in the following 

 diagram, in which the name of a substance is understood to 

 express one equivalent of it, and the figures, numbers of equi- 

 valents : 



