ORGANIC CHEMISTRY. 



376 



How is yellow 944 P^ssiATE OF POTASH. Cyanide 



prussiate of of iron is always incidentally formed from 



potash pre- . 



pared? Men- the iron of the vessel in the above process. 



tion its uses, j f Wftter ig added t() th 



cyanides dissolve ; although the latter, when alone, is 

 entirely insoluble. From the solution, the double cy- 

 anide of potassium and iron, mentioned in a preceding 

 paragraph, is obtained, by evaporation, in splendid yel- 

 low crystals. It is known in commerce as yellow 

 prussiate of potash, and is largely used in the arts for 

 the production of prussian blue and cyanide of potas- 

 sium. Prussian blue is obtained by adding its solution 

 to a salt of the peroxide of iron. As any solution of 

 iron is readily peroxydized by the addition of a little 

 nitric acid, the yellow prussiate may be employed as a 

 test for this metal. 



945. FERROCYANIDES. The yellow 



What is said 3 



of ferrocya.no- prussiate of potash, produced as above de- 

 scribed, is not properly a double cyanide 

 of iron and potassium. There is reason to believe 

 that the cyanogen is more intimately combined with 

 the iron than such a name would imply. It seems to 

 have lost its ordinary properties, in the compound. 

 Neither the alkalies, or sulphide of ammonium, which 

 usually precipitate iron from its solutions, have any 

 power to precipitate it from this salt. The three mole- 

 cules of cyanogen, which enter into its composition, 

 seem to have hidden and absorbed it. They have 

 formed with it, indeed, a new compound radical, called 

 ferrocyanogen. The double salt above mentioned is 

 therefore more properly a ferrocyanide of potassium. 



