548 SMELTING OF IRON. 



drawn off forms a solid slag, and the carburet of iron, or metal, 

 which is the heavier of the two. It may he drawn from 

 observations made by Dr. Clark, in 1833, on the working of the 

 Scotch blast furnaces, under the hot blast, that the relative pro- 

 portions of the materials, including air, and product of cast iron 

 are as follows :* 



Weight. 



Coal. 5 



Roasted iron stone. ..... 5 



Limestone. I 



Air. 11 



Average product of cast iron. ... 2 



The ultimate fixed products are the slag and carburet of iron, 

 but the formation of these is preceded by several interesting 

 changes, which the ore successively undergoes in the course of 

 its descent in the furnace. A portion of the oxide of iron is 

 certainly reduced to the metallic state, soon after its introduc- 

 tion, in the upper part of the furnace, by carbonic oxide and 

 volatile combustible matter ; but the reduced metal does not 

 then fuse. A large portion of the oxide of iron must combine 

 also, at the same time, with the silica and alumina present in 

 the ore, which act as acids, and a glass be formed, of which 

 the oxide of iron is scarcely reducible by carbon. But this 

 injurious effect of the acid earths is counteracted by the lime 

 of the flux, which being a more powerful base than oxide of 

 iron, liberates that oxide from the glass, and neutralises the 

 silica ; so that the slag eventually becomes a silicate of lime 

 and alumina, with scarcely a trace of oxide of iron, when the 

 proportions of the materials introduced into the furnace are 

 properly adjusted. The whole oxide of iron comes thus to be 

 exposed to the reducing action of the volatile combustible, and 

 consequently the whole iron is probably, at one time, in the con- 

 dition of pure or malleable iron. But when the metal descends 

 somewhat farther in the furnace, it attains the high tempera- 

 ture, at which it combines with the carbon of the coke in con- 

 tact with it, and it fuses for the first time, in the form of car- 

 buret of iron. It has not yet, however, attained its ultimate 

 condition. When it reaches, in its descent, the region of the 



* Edinburgh Phil. Trans, vol. 13, 



