550 CAST IRON MALLEABLE IRON. 



sand, and thus cast into ingots or pigs, as they are called. Cast 

 iron is an exceedingly variable mixture of reduced substances, 

 of which the principal is iron combined with carbon. The 

 theoretical constitution to which that variety of it, most definite 

 in its composition, approaches, is the following : 



WHITE CAST IRON. 



Four atoms iron. .... 94.7 



One atom carbon. . 5.3 



100.0 



The difference in appearance and quality of the varieties of 

 cast iron is not well accounted for by their composition. The 

 grey or mottled cast iron, forming the qualities, Nos. 1 and 2, 

 presents a fracture composed of small crystals, is easily cut by 

 the file, and is preferred for castings. It is generally supposed 

 that a portion of uncombined carbon is diffused through the 

 iron of these qualities, in the form of graphite. No. 3, or white 

 cast iron is more homogeneous ; its fracture exhibits crystalline 

 plates, like that of antimony, and is nearly white; it is exceed- 

 ingly hard and brittle. 



Malleable iron. The great proportion of cast iron manufac- 

 tured is afterwards refined, or converted into bar or malleable 

 iron. Previous to refining, the cast iron is always fused, and 

 cooled suddenly by throwing water on the melted surface, by 

 which it becomes white cast iron, if not so before. In this 

 condition it is most easily deprived of its carbon, which is the 

 object of the refining. The principal operation, called the 

 puddling process, consists in heating masses of the iron in a 

 kind of reverberatory furnace, with a certain access of air. 

 The metal fuses, and by means of a sort of spatula is stirred 

 about, and every part of it exposed to the flame. The carbon 

 is thus gradually burnt out, partly by the direct action of oxy- 

 gen in the flame, and partly by cementation with oxide of iron ; 

 and the metal becomes less fusible, but thick and tenacious, 

 so that it sticks together, and is formed into a ball. In 

 this condition it is removed by tongs, compressed into a cylin- 

 drical form by a few blows of a loaded hammer, and quickly 

 converted into a bar, by pressing it between grooved rollers. 

 The tenacity of the metal is increased by welding several bars 

 together; a pile of bars is brought to a full red heat in an 



