476 SODIUM. 



fusion at 170. This salt continues tribasic after being exposed 

 to a red heat. 



Biphosphate of soda ; 2HO, NaO, PO 5 -f2HO ; 1733.1 or 

 138.88. Obtained by adding tribasic phosphate of water to 

 phosphate of soda, till the latter ceases to produce a precipitate 

 with chloride of barium. The solution affords crystals, in cold 

 weather, of which the ordinary form is a right rhombic prism, 

 having its larger angle of 93 54'. But this salt is dimorphous, 

 occurring in another right rhombic prism, of which the smaller 

 angle is 78 30', terminated by pyramidal planes, isomorphous 

 with binarseniate of soda. The biphosphate of soda is very so- 

 luble, and has a distinct acid reaction. Like all the other solu- 

 ble tribasic phosphates, it gives a yellow precipitate with nitrate 

 of silver, which is tribasic phosphate of silver. 



Phosphate of soda and ammonia, Microcosmic salt ; HO, 

 NH 4 O, NaO, PO 5 + 8HO. This salt is obtained by heating 

 together 6 or 7 parts of crystallized phosphate of soda, and 2 

 parts of water, till the whole is liquid, and then adding 1 part of 

 powdered sal ammoniac. Chloride of sodium separates, and the 

 solution, filtered and concentrated, affords the phosphate in 

 prismatic crystals. It is purified by a second crystallization. 

 This salt occurs in large quantity in urine. It is much employed 

 as a flux in blow-pipe experiments. By a slight heat it loses 

 8HO, by a stronger heat it is deprived of its remaining water 

 and ammonia, and converted into metaphosphate of soda, 

 which is a very fusible salt. It will be observed that the three 

 atoms of base in this phosphate are all different, namely water, 

 oxide of ammonium, and soda ; of which the two last belong to 

 the same natural family. This salt, I believe, proved the key 

 to the constitution of the bibasic and tribasic organic acids, by 

 supplying the canon, founded upon it by myself, that bases of the 

 same family may exist together in the salts of such acids, but not 

 in ordinary double salts ; which was happily applied to elucidate 

 the salts of the acids in question by MM. Liebig and Dumas. 

 No phosphate exists, corresponding with microcosmic salt, but 

 containing potash instead of oxide of ammonium; the phosphate 

 of soda, with 14HO, has been mistaken for such a salt. 



Pyrophosphate of soda ; 2NaO, PO 5 + 10HO; lb'74.1 + 1125, 

 or 134.15-1-90. Procured by heating the phosphate of soda to 

 redness, when it loses its basic water as well as its water of crys- 

 tallization. The residual mass dissolved in water affords a salt, 



