IRON. 293 



indicated generally by their color (red, reddish-brown, or yellowish- 

 red), as iron is the most common of all natural, inorganic coloring 

 agents. It is found also, though in small quantities, in plants, and 

 in somewhat larger proportions in the animal system, chiefly in the 

 blood. In the metallic state iron is scarcely ever found, except in 

 the meteorites or metallic masses which fall occasionally upon our 

 earth out of space. 



The chief compounds of iron found in nature are : 



Hematite, ferric oxide, Fe 2 O 3 . 



Magnetic iron ore, ferrous-ferric oxide, FeO.Fe 2 O 9 . 



Spathic iron ore, ferrous carbonate, FeCO s . 



Iron pyrites, bisulphide of iron, FeS 2 . 



The carbonate and sulphate are found sometimes in spring waters, 

 which, when containing considerable quantities of iron, are called 

 chalybeate waters. Finally, iron is a constituent of some organic 

 substances which are of importance in the animal system. 



Manufacture of iron. There is no other metal manufactured in 

 such immense quantities as iron, the use of which in thousands of 

 different tools, machines, and appliances is highly characteristic of 

 our present age. Iron is manufactured from the above-named oxides 

 or the carbonate by heating them with coke and limestone in large 

 blast furnaces, which have a somewhat cylindrical shape, and are 

 constantly fed from above with a mixture of the substances named, 

 while hot air is forced into the furnace through suitable apertures 

 near its hearth. The chemical change which takes place in the upper 

 and less heated part of the furnace is a deoxidation of the iron oxide 



by the carbon : 



Fe 2 O 3 + 30 == SCO -f 2Fe 



The heat necessary for this decomposition and fusion of the re- 

 duced iron is produced by the combustion of the fuel, maintained by 

 the oxygen of the air blown into the furnace. At the same time the 

 lime and other bases combine with the silica contained in the ore, 

 forming a fusible glass, called cinder or slag. The iron and slag 

 collect at the bottom of the furnace, where they separate by gravity, 

 and are run off every few hours. 



Iron thus obtained is known as cast-iron, or pig-iron, and is not 

 pure, but always contains, besides silicon (also sulphur, phosphorus, 

 and various metals), a quantity of carbon varying from 2 to 5 per 

 cent. It is the quantity of this carbon and its condition which im- 

 parts to the different kinds of iron different properties. Steel contains 



