IRON. 549 



furnace where the heat is most intense, its carbon reacts on the 

 silica, alumina, lime and other alkaline oxides contained in the 

 fluid slag, with which it is accompanied, reducing portions of 

 silicon, aluminum, calcium and other alkaline metals, which 

 combine with the iron. The proportion of carbon replaced by 

 silicon and metallic bases, is generally found to be greater in 

 iron prepared by the hot -than by the cold blast, owing, it is 

 presumed, to the higher temperature of the furnace with the 

 hot blast. 



The introduction of air already heated to support the com- 

 bustion of the blast furnace, for which a patent was obtained 

 by Mr. J. B. Neilson, has greatly reduced the proportion 

 of coal required to smelt a given weight of ore, enabling the 

 iron master indeed, to eifect a saving of more than three fourths 

 of the coal where that is of a bituminous quality. The air is 

 heated betwee^the blowing apparatus and the furnace, by 

 being made to circulate through a set of arched tubes of mode- 

 rate diameter, heated by a fire beneath them. The air can be 

 heated in this manner to low redness, or to near 1000, but 

 there is found to be no proportional advantage in raising its 

 temperature much above "the melting point of lead (612), which 

 is already higher than the point at which charcoal inflames. 

 Considering the great weight of air that enters the furnace, 

 the temperature of that material must greatly affect the whole 

 temperature of the furnace, particularly of the lower part, 

 where the air is admitted, and which part it is desirable should 

 be hottest. Now a certain elevated temperature is [required 

 for the proper smelting of the ore, and unless attained in the 

 furnace, the fuel is consumed to no purpose. The removal of 

 the negative influence of the low temperature of the air, appears 

 to permit the heat to rise to the proper point, which otherwise 

 is attained with difficulty and by a wasteful consumption of 

 fuel. Professor Reich of Freiburg, has observed that heating 

 the air likewise alters the relative temperatures of different 

 parts of the furnace, depressing in particular, and bringing 

 nearer the twyeres, the zone of highest temperature. The ad- 

 mixture of steam with the air has, he finds, precisely the oppo- 

 site effect, elevating the zone of highest temperature in the 

 furnace ; so that the effect of the hot blast, may be exactly neu- 

 tralised by mixing steam with the hot air. 



Cast iron. The fused metal is run into channels formed in 



