296 METALS AND THEIR COMBINATIONS. 



In the preparation of fuming sulphuric acid (which see) by heating ferrous 

 sulphate there is left a residue of ferric oxide, known as rouge, which is used 

 as a red pigment and as a polishing powder. 



4FeS0 4 + H 2 = 2Fe,O, + H 2 S0 4 .SO 3 4- 2SO 2 . 



A specially fine variety of rouge for polishing is manufactured by heating 

 ferrous oxalate, FeC 2 O 4 , in contact with the air. 



Ferric hydroxide, Ferri hydroxidum, Fe(OH) 3 = 106.14, Is 

 obtained by precipitation of ferric sulphate or ferric chloride by am- 

 monium or sodium hydroxide (see Plate I., 3) : 



Fe 2 (S0 4 ) 3 + 6NH 4 OH = 3[(NH 4 ) 2 SOJ -f 2Fe(OH) 3 . 



Precipitation is complete, no iron remaining in solution as in the 

 case of ferrous salts. 



Ferric hydroxide is a reddish-brown powder, sometimes used as 

 an antidote in arsenic poisoning ; for this purpose it is not used in 

 the dry state, but after having been freshly precipitated and washed, 

 it is mixed with water, and this mixture used. 



Ferric hydroxide with magnesium oxide, U. S. P., is a mixture freshly made, 

 when called for, by adding magnesia to a solution of ferric sulphate, when 

 magnesium sulphate and ferric hydroxide are formed; the two substances are 

 not separated from each other, the mixture being intended for immediate 

 administration as an antidote in cases of arsenic poisoning. 



Ferrous-ferric oxide, FeO.Fe 2 O 3 (Magnetic oxide). This com- 

 pound, which shows strong magnetic properties, has been mentioned 

 above as one of the iron ores, and is known as loadstone. It has a 

 metallic lustre and iron-black color, and is produced artificially by 

 the combustion of iron in oxygen, or in the hydrated state by the 

 addition of ammonium hydroxide to a mixture of solutions of ferrous 

 and ferric salts. 



Iron trioxide, FeO 3 . Not known in a separate state, but in com- 

 bination with alkalies. In these compounds, called ferrates, FeO 3 

 acts as an acid oxide, analogous to chromium trioxide, CrO 3 , in chro- 

 mates. The composition of potassium ferrate is K 2 FeO 4 . 



Ferrous Chloride, FeCl 2 (Protochloride of iron), is obtained as a 

 pale-green solution by dissolving iron in hydrochloric acid : 

 Fe + 2HC1 = FeCl 2 + 2H. 



The anhydrous salt cannot be obtained by evaporation of the solu- 

 tion, as it decomposes ; but it may be made by heating iron in a cur- 



