of Phosphoric Acid. 325 



Solution o^ chloride of calcium produces an extremely small 

 precipitate, which is perfectly soluble in excess of the acid. 

 Ammonia produces a dense voluminous precipitate in this 

 solution. Lime-water does not cause any precipitate until 

 added in excess. Both acids exhibit the same reactions with 

 sulphate of magnesia, nitrate of silver and albumen. 



These two acids therefore differ principally in their reac- 

 tion with chloride of barium. But as solutions of Graham's 

 metaphosphate of soda, as stated above, when decomposed 

 by solutions of neutral salts, yield liquids which possess an 

 acid reaction, the acid separated from the precipitates must 

 be different from that in the soda salt used. 



As the composition of the oily and resinous precipitates 

 produced by Graham's salt was entirely unknown, the salt of 

 silver was analysed by M. Weber. It was prepared in the 

 same manner as the salt precipitated for the purpose of ob- 

 taining the free acid. After having been dried for a long 

 time at 212^ F., 0*930 grm. lost at a red heat 0-019 grm., 

 or 2"04' per cent, of water. It fused into a mass of a yellowish 

 colour, which, when dissolved in nitric acid, yielded on the 

 addition of hydrochloric acid 0'815 grm. of chloride of silver, 

 corresponding to 70*09 oxide of silver. The phosphoric acid 

 in the filtered liquid was precipitated in the form of the phos- 

 phate of ammonia and magnesia; 0*257 grm. of pyrophos- 

 phate of magnesia was obtained after heating to redness, 

 corresponding to 27*87 per cent, of phosphoric acid. 



These results correspond to the formula 3AgO, 2P^ O^ 

 + HO. Berzelius examined this salt long ago, but he ob- 

 tained it in a different manner*. He precipitated a solu- 

 tion of phosphoric acid which had been recently heated to 

 redness, with nitrate of silver; he then obtained a precipitate, 

 the composition of which but little resembled that of AgO 

 P^ O^, the amount of oxide of silver differing nearly 3 per cent, 

 from that calculated. This salt was placed in boiling water; 

 in a few minutes it fused into a viscid mass resembling tur 

 pentine, and had the same composition as the compound men- 

 tioned above. 



The composition found above explains the acid reaction of 

 the liquid, when the neutral solution of Graham's salt is pre- 

 cipitated with solution of nitrate of silver. It also renders 

 evident why the acid separated from the silver salt may possess 

 different properties from metaphosphoric acid as obtained by 

 the combustion of phosphorus. The question, whether the 

 other oleaginous salts, which a solution of Graham's salt forms 

 with other neutral salts, possess an analogous composition to 

 * PoggendorfTs Annalen, xix. p. 333. 



