MANGANESE, IRON, COBALT, NICKEL, CHROMIUM. 249 



than half of this salt is sulphur, heat will drive off a large 

 portion of it. It is therefore usually heated in clay retorts, 

 and the sulphur which passes off in vapor is collected. 

 The residue is taken out and thrown into heaps, and is 

 simply left exposed to the air. By the absorption of oxy- 

 gen from the air this sulphide gradually becomes a ferrous 

 sulphate, the oxygen converting the sulphur into sulphuric 

 acid, and the iron into oxide of iron, which unite to form 

 the sulphate. 



347. Other Salts of Iron. Metallic iron dissolves readily 

 in nitric and hydrochloric acid, forming nitrate and chloride 

 of iron. Two of each can be obtained, one in the ferrous 

 and the other in the ferric state. Ferric chloride is much 

 used in medicine and in the arts ; it is a valuable disinfect- 

 ant. Ferric solutions are usually yellowish red in color, 

 and ferrous solutions pale green. 



348. Cobalt. This is a brittle metal of a reddish-white 

 color. It exists in nature in combination with arsenic and 

 sulphur. There are two oxides, one of which, the monox- 

 ide, gives a beautiful blue color to glass. It is this colored 

 glass ground to a fine powder that constitutes the smalt 

 which is used to give. to writing-paper and linen a delicate 

 shade of blue. The blue colors on porcelain are also pro- 

 duced by cobalt, and the zaffer used to give a blue color 

 to common earthenware is an impure oxide of this metal. 

 The fly-poison, so commonly called cobalt by apothecaries, 

 is arsenic, and has not a particle of cobalt in it. The name 

 which this metal bears was given to it in a singular way. 

 When the superstitious miners of the Middle Ages found 

 the ores of cobalt, they expected, from their brilliancy, that 

 they should obtain something very valuable from them; 

 but they were disappointed in finding them crumble in 

 their smelting-furnaces into gray ashes, emitting at the 

 same time a disagreeable odor of garlic. They imagined, 



L2 



