348 PHOSPHORUS. 



larly distinguished as the phosphates or the common phos- 

 phates. 



Deuto-hydrate of phosphoric acid, or bibasic phosphate of 

 water, 2HO + PO 5 . Dr. Clark first discovered that when the 

 phosphate of soda is heated to redness, it is completely changed, 

 and after being dissolved in water affords crystals of a new salt, 

 which he named the pyrophosphate of soda, an observation 

 which led to the most important results.* If a solution of this 

 salt, which it is not necessary to crystallize, be precipitated by 

 acetate of lead, the insoluble salt of lead washed and decom- 

 posed by sulphuretted hydrogen, as before, an acid liquor is 

 obtained which [contains the deuto-hydrate of phosphoric acid. 

 It must not be warmed ta expel the excess of sulphuretted 

 hydrogen, but be left in a shallow bason for 24 hours to permit 

 the escape of that gas. This acid, when neutralized with car- 

 bonate of soda, gives Dr. Clark's pyrophosphate of soda. It 

 also gives a white precipitate with nitrate of silver; all the 

 salts which it forms, have uniformly two atoms of base. Their 

 trivial name is the pyrophospkates, and since that term has come 

 into general use, it is not likely to be superseded by the syste- 

 matic, but rather cumbrous designation of bibasic phosphates. 

 A dilute solution of the deuto-hydrate of phosphoric acid may 

 be preserved for many months without change, but when the 

 solution is exposed for some time to a high temperature, it 

 passes entirely into the terhydrate. 



Protohydrate of phosphoric acid. If the biphosphate of soda 

 be heated to redness, a salt is formed, which treated in a similar 

 manner with the last, gives an acid liquor, containing the pro- 

 tohydrate of phosphoric acid. To prepare the biphosphate 

 itself, a solution of the terhydrate of phosphoric acid is added 

 to a solution of common phosphate of soda, till it is found that 

 a drop of the latter is no longer precipitated by chloride of 

 barium. The biphosphate of soda, which is now in solution, 

 can only be crystallized in cold weather. The glacial phos- 

 phoric acid also, is in general almost entirely the protohydrate. 

 This hydrate is characterized by producing a white precipitate 

 in solution of albumen, and in solutions of the salts of earths 

 and metallic oxides precipitates which are remarkable semifluid 



* Edinburgh Journal of Science, vol. VII p. 298, (1826) j or An. de Ch, et de Fh. 

 t. 41. p. 276. 



