Chemical Science. 179 



Sulphate of Magnesia. 



Magnesia 13.249 



Sulphuric acid 33.75 1 



Water 53 



100 

 Hydrate of Magnesia. Sub Hjdrate of Magnesia. 



Magnesia 52.997 79.218 



Water 47.003 20.782 



100 100 



Magnesia. 

 Magnesium 49.174 



Oxygen 50.826 



100 

 Ann. de Chim. p. xii. 255. See Mr. Phillips on the subject, 

 p. 313, Vol. VI., and p. 392, Vol. VII., of this Journal. 



On the Nature of Prussian Blue. — In addition to the number 

 of chemists who have turned their attention to this subject, M. 

 Robiquet has also engaged himself upon it, and given some in- 

 teresting details to the public. He has observed, as has been 

 done by Mr. Porrett, that the white precipitate, caused by add- 

 ing prussiate of potash to proto-sulphate of iron, contained 

 potash, however perfectly it be washed ; and he therefore ob- 

 serves that it may be considered as a compound intermediate 

 between the triple prussiate of potash and Prussian blue. 

 Whilst acid is present it is insoluble, but when that is removed 

 by repeated washing, the salt becomes somewhat soluble in 

 water. 



When sulphuric acid is added to Prussian blue, or rather 

 the pure prussiate of iron, it makes it perfectly white, without 

 liberating any odour of prussic acid ; if water be added to it, the 

 blue colour immediately returns. The sulphuric acid appears 

 to separate nothing but water in this case, for if it be withdrawn 

 from off the white precipitate, it is found to afford no traces of 

 prussic acid or of iron. The experiment also succeeds per- 

 feclly ill rucm, so that the air has nothing to do in it. M. Ro- 

 N 2 



