of Phosphoric Acid. 331 



grms. of it, when heated to redness, gave 2*7900 grms. of pyro- 

 phosphate of soda. This phosphate of soda, therefore, still 

 contained 0*2735 grm., or 8*92 per cent, of water. 3*126 grms. 

 of the same salt were exposed for a considerable time to de- 

 finite temperatures ; it was found that by gradually heating the 

 salt, almost the whole of the water it contained could be ex- 

 pelled at a temperature of 464'° F. The following are the de- 

 tails of this experiment : — 



The above quantity of the salt weighed after exposure to a 

 heat of 



320° F. during 1 1 hours 3*054' grms. 



Had the salt been kept for a still longer time at a temperature 

 of 464° F., it would undoubtedly have lost the whole of the 

 water it contained. The quantity used would then have 

 weighed 2846 grms. 



But when the salt was examined, it was found that even at 

 the above temperature it had become almost completely con- 

 verted into pyrophosphate of soda. The solution gave a white 

 precipitate with nitrate of silver, which was mixed with as 

 much of a yellow one as would have been expected from the 

 quantity of water still existing in the heated salt. 



Solution of pyrophosphate of soda yields, with very many 

 neutral salts of metallic oxides, precipitates which are partly 

 soluble in excess of the pyrophosphate of soda. The peculia- 

 rity of the pyrophosphate of soda in readily formingdouble salts, 

 has already been very distinctly pointed out by Stromeyer. 

 Persoz has recently studied this point, without alluding to Stro- 

 meyer's memoir; he has however confirmed all the facts given 

 by him. Schwarzenberg has recently examined most of the 

 pyrophosphates quantitatively*, and Baer has made the inter- 

 esting discovery, that those insoluble precipitates which are 

 produced by a solution of pyrophosphate of soda, and are not 

 soluble in excess of it, are frequently insoluble double salts 

 of the soda salt with the pyrophosphates formed, in which 

 the soda replaced the other base, apparently without the 

 two existing in the double salt in a definite simple propor- 



* Chem. Gaz., vol. vi. pp. 181, 190. 



