556 IRON. 



p blue precipitate may be obtained by adding to a protosalt of 

 iron, a mixture of yellow prussiate of potash, chloride of soda, 

 and hydrochloric acid. The tint of this blue is lighter and 

 more delicate than that of prussian blue. It is occasionally 

 used by the calico-printer, who mixes it with permuriate of tin, 

 and prints the mixture, which is in a great measure soluble, 

 upon Turkey red cloth, raising the blue colour afterwards by 

 passing the cloth through a solution of chloride of lime, con- 

 taining an excess of lime. The chief object of that operation 

 is indeed different, namely, to discharge the red and produce 

 white patterns, where tartaric acid is printed upon the cloth, 

 but it has also the effect incidentally of precipitating the blue 

 pigment and peroxide of tin together on the cloth, by neutra- 

 lising the acid of the permuriate of tin. This blue is believed 

 to resist the action of alkalies longer than ordinary prussian 

 blue. Mr. R. C. Campbell observed that the ferricyanide of 

 iron may be distinguished from prussian blue by the circum- 

 stance, that when boiled in a solution of yellow prussiate of 

 potash, it affords red prussiate of potash, which dissolves, and a 

 grey insoluble residue of ferrocyanide of iron and ferrocyanide 

 of potassium (Liebig). 



Carbonate of iron is obtained on adding carbonate of 

 soda to the protosulphate of iron, as a white or greenish white 

 precipitate, which may be washed and preserved in a humid 

 condition in a close vessel, but cannot be dried without losing 

 carbonic acid and becoming peroxide of iron. It is soluble, 

 like the carbonate of lime, in carbonic acid water, and exists 

 under that form in most natural chalybeates. Carbonate of 

 iron occurs also crystallized in the rhomboidal form of calc 

 spar, forming the mineral spathic iron, which generally contains 

 portions of carbonates of lime, magnesia, and manganese. It is 

 generally of a cream colour or black, and its density rarely ex- 

 ceeds 3.8. This anhydrous carbonate does not absorb oxygen 

 from the air. Carbonate of iron is also the basis of clay-iron- 

 stone. There is no carbonate of the peroxide. 



Sulphate of iron,, Ferrous sulphate, Green vitriol, Copperas ; 

 Fe O, SO 3 , HO + 6HO; 940.4 + 787.5 or 75.3 + 63. This salt 

 may be formed by dissolving iron in sulphuric acid diluted with 

 4 or 5 times its bulk of water, filtering the solution while hot, 

 and setting it aside to crystallize. But the large quantities of 

 sulphate of iron, consumed in the arts, are prepared simulta- 



