336 Prof. H. Rose on the Isomeric Modifications 



racteristic property of pyrophosplioric acid, that it does not 

 precipitate albumen, and by this very property it differs most 

 essentially from metaphosphoric acid, all the submodifications 

 of which possess the property of precipitating albumen in a 

 remarkable degree. 



I have already remarked above, that the aqueous solution 

 of pyrophosphoric acid obtained from the insoluble pyrophos- 

 phate of copper by sulphuretted hydrogen gas, exhibits the 

 same reactions as the acid obtained from the lead salt. 



Pyrophosphoric acid differs from metaphosphoric acid, in 

 addition to its characteristic reaction with albumen, also in 

 that with a solution of chloride of barium, although, as I have 

 remarked above, all the modifications of metaphosphoric acid 

 are not alike in this respect ; also in the difllerent properties 

 of the precipitate produced by a solution of silver, regarding 

 which, it must be observed, that one of the modifications of 

 metaphosphoric acid (the submodification described at p. 326) 

 forms a soluble salt with oxide of silver. The difference be- 

 tween the reaction of metaphosphoric acid and pyrophospho- 

 ric acid with albumen therefore forms the most important 

 distinction between these two acids. 



c-Phosphoric Acid [ordinart/ Phosphoric Acid). 



This modification of phosphoric acid is the one most com- 

 monly occurring in analytical investigations, because the other 

 modifications are converted into it by the action of acids. 



Its salts have been so frequently examined, that most of 

 their properties are known. One property however, which 

 especially characterizes the c-phosphates, appears to have been 

 hitherto overlooked. This consists in the solubility of a large 

 number of the insoluble phosphates in excess of the saline so- 

 lution from which they have been precipitated by means of 

 the phosphate of soda. The solution generally possesses 

 the property of producing a copious precipitate when heat 

 is applied, which disappears however on cooling. Hence 

 double salts are formed, which are decomposed by a high 

 temperature. Many of the precipitates thrown down by 

 pyrophosphate of soda, frequently dissolve, as has already 

 been mentioned, even in excess of the solution of the salt ; 

 this solution is also troubled by heat, but the precipitate is 

 not dissolved on cooling. As in chemical investigations the 

 c-phosphate of soda is so frequently used to precipitate oxides 

 fron\ the solutions of their salts, and as it appeared to me im- 

 portant to be accurately acquainted with the properties of the 

 precipitates, I shall offer no excuse for stating here the re- 



