990 CYANOGEN COMPOUNDS. 



pounds, generally crystallizable, which are not decomposed or 

 modified by alkaline carbonates nor metallic chlorides. 



Protocyanide of iron, FeCy, with certain other cyanides, 

 comports itself in a peculiar manner with other metallic cya- 

 nides and also with hydrocyanic acid. The cyanide named in 

 combination with two equivalents of cyanide of potassium, 

 FeCy + 2KCy, forms a neutral salt, ferrocyanide of potassium, 

 (page 450), which is not poisonous, gives no hydrocyanic acid 

 with sulphuric acid at the ordinary temperature, while its iron 

 is not precipitated by an alkali or alkaline carbonate ; or in this 

 compound the properties of both a cyanide and salt of iron 

 entirely disappear. If the potash of this double salt is precipi- 

 tated by tartaric acid, a new double cyanide is formed, highly 

 acid and crystallizable, containing protocyanide of iron with 2 

 equivalents of hydrocyanic acid, FeCy+2HCy, known asferro- 

 cyanic add, or ferrocyanide of hydrogen. On the view of the 

 constitution of these salts proposed by M. Liebig, they contain 

 a bibasic salt-radical ferrocyanogen FeCy 3 , in combination, with 

 2K in the first, and with 2H in the second. Ferrocyanide of 

 lead, 2Pb,FeCy 3 , is a white precipitate with a shade of yellow, 

 formed on adding ferrocyanide of potassium to a salt of lead. 

 Ferrocyanide of copper, Cu 2 ,FeCy 3 , a reddish brown precipitate, 

 insoluble in dilute acids, formed on adding ferrocyanide of potas- 

 sium to a salt of black oxide of copper. Sesquiferrocyanide of iron 

 or Prussian blue, Fe 4 ,3FeCy 3 , is precipitated on adding the same 

 soluble ferrocyanide to a persalt of iron (page 564). With a 

 protosalt of iron, a greenish white precipitate is produced, which 

 according to Berzelius, is a double ferrocyanide of potassium 

 and iron, K.Fe 3 ,2FeCy 3 (page 557). 



Sesquicyanide of iron, Fe 2 Cy 3 , in combination with other 

 cyanides, forms another class of compounds, analogous to the 

 preceding. With 3 equivalents of cyanide of potassium, (Fe 2 Cy 3 

 H-SKCy,) it forms the redprussiate of potash, or ferricyanide of 

 potassium, K 3 ,Fe 2 Cy 6 (page 450). With 3 equivalents of hydro- 

 cyanic acid, FeCy 3 -f 3HCy, it forms ferricyanic acid or ferri- 

 cyanide of hydrogen, H 3 ,Fe 2 Cy 6 ; it is obtained by decomposing 

 the insoluble ferricyanide of lead by sulphuretted hydrogen. 

 The ferricyanide of potassium produces with a protosalt of iron, 

 ferricyanide of iron, Fe 3 ,Fe 2 Cy 6 , a beautiful variety of Prussian 



