IRON. 551 



oblong furnace, and then extended between the grooved rollers 

 into a single bar. The texture of malleable iron is fibrous. 

 Although the purest commercial form of the metal, it still con- 

 tains about one-half per cent of carbon, with traces of silicon 

 and other metals. 



Steel. Only the best qualities of malleable iron, those pre- 

 pared from a pure ore, and reduced by means of charcoal, such 

 as the Swedish iron, are converted in steel. An iron box is filled 

 with flat bars of such iron and charcoal powder, in alternate 

 layers, and kept at a red heat for forty-eight hours, or longer. 

 The surface of the bars is found afterwards to be blistered, and 

 they have absorbed from 1.3 to 1.75 "per cent of carbon. This 

 is the process of cementation, to which allusion has already 

 been made (Note, page 545). It is known that iron can be 

 converted into steel without being in actual contact with char- 

 coal, provided the iron and charcoal are in a close vessel toge- 

 ther, and oxygen be present; the carbon reaching the 

 surface of the metal in the form of carbonic oxide gas. The 

 iron becomes harder by this change, and more fusible, but can 

 still be hammered into shape, and cut with a file. The pro- 

 perty in which steel differs most from soft iron, is the capacity 

 it has required of becoming excessively hard and elastic, when 

 heated to redness and suddenly cooled by plunging it in water 

 or oil. This hardness makes it invaluable for files, knives, and 

 all kinds of cutting instruments. But the steel, when hardened 

 in the manner described, is harder than is required for most of 

 its applications, and also very brittle. Any portion of its origi- 

 nal softness can be restored to the steel by heating it up to par- 

 ticular temperatures, which are judged of by the colour of the 

 film of oxide upon its surface, which passes from pale yellow at 

 about 430, through straw yellow, brown yellow, and red purple 

 into a deep blue at 580, and allowing the steel afterwards to 

 cool slowly. Articles of steel are tempered in this manner. 



Properties of iron. Iron is of a bluish white colour, and ad- 

 mits of a high polish. It is remarkably malleable, particularly 

 at a high temperature, and of great tenacity. Its mean density 

 is 7-7j which is increased by fusion to 7-8439. When kept for 

 a considerable time at a red heat, its particles often form large 

 cubic or octohedral crystals, and the metal becomes brittle. 

 Malleable iron softens before entering into fusion, and in this 



