1840.] On an error in Dr. Thomson's Mineralogy. 311 



of magnesia, but Tliomson says that it contains only twenty-lwo per 

 cent. In this, however, it would appear, that Dr. Thomson has made 

 a mistake, because he says that caustic ammonia, and phosphate of soda, 

 also biphosphate ammonia, give the same precipitate; but this is impos- 

 Bible, because in using caustic ammonia and phosphate of soda, there is 

 only one atom of the acid, which combines with one atom of ammonia, 

 and one of magnesia, to form a subphosphate ; and, after ignition, the 

 ammonia having been volatilized, there will remain a neutral phosphate 

 of magnesia, which, according to Dr. Thomson's table of atomic weights, 

 will contain 35.7 per cent of magnesia, which is not much diiFerent 

 from the proportion given by Rose. But if biphosphate of ammonia 

 is used, in case the solution does not contain an excess of ammonia, 

 two atoms of the acid will be present, one of which will leave the 

 ammonia, and take up an atom of magnesia, and fovm a double neutral 

 salt, and not a sub-salt as before. On igniting this precipitate, an 

 atom of the amm^mia being volatilized, there will remain a biphos- 

 phate of magnesia, containing 21.7 per cent of magnesia. We may 

 ther<?f<>re conclude that Dr. Thomson has been in the habit of using 

 diphosphate of ammonia always as a precipitate, and that there are 

 Hwo insoluble double salts of phosphoric acid, ammonia, and magnesia* 

 Dr. Thomson after giving the composition of the ammonia-phosphate 

 of magnesia in his chemistry, remarks — RifFault analysed an ammonia- 

 ** phosphate of magnesia, which would seem to have contained only 

 ** half the phosphoric acid in the preceding salt." It contained one 

 atom of diphosphate of ammonia, and one atom of diphosphate of mag- 

 nesia, which must be the very salt to which Rose alludes. I am in- 

 formed that Brand e, in his chemistry, mentions there are two salts of 

 this kind, and one is composed of 



Ammonia...... 1 atom. 



Magnesia I atom. 



Phosphoric acid 1 atom. 



Water , ^. 5 atoms. 



but between this RifFault's salt, there is a discrepancy— for this, Dr. 

 Thomson says, is composed 



1 atom of. diphosphate magnesia. 



1 atom of diphosphate ammonia. 



5 atoms of. water. 



which is equivalent to 



2 atoms.... ...of magnesia. 



2 atoms. , . . e ..of., ; . ♦ . . . ammonia. 



