Metallurgy of Iron. 19 



furnaces placed around the great prismatic tube, and conducts it 

 into a narrow space between this and an outer wall ; through this, 

 by openings, a regulated supply of air is introduced for the com- 

 bustion of the gas, by which the ore contained in the tube is raised 

 to a red heat. The next step is to provide the reducing material 

 which shall remove the oxygen from the ignited ore, and for this 

 purpose we have already seen, that even in the ordinary smelt- 

 ing process carbonic oxyd is always the agent ; but instead of the 

 impure gas obtained from his furnaces, and diluted with the ni- 

 trogen of the air, M. Chenot prefers to prepare a pure gas, which 

 he obtains as follows. A small quantity of carbonic acid gas, 

 evolved from the decomposition of carbonate of lime, is passed 

 over ignited charcoal, and thus converted into double its volume 

 of carbonic oxyd gas ; this is then brought in contact with ignitfd 

 oxyd of iron, which is reduced to the metallic state, while the gas 

 is changed into carbonic acid, ready to be converted into carbonic 

 oxyd by charcoal as before. In this w ! ay the volume goes on 

 doubling each time the two-fold operation is repented. By intro- 

 ducing the carbonic oxyd thus obtained into the furnace charged 

 with ignited iron ore, and withdrawing a portion of the gas at a 

 higher level, for the purpose of passing it again over ignited char- 

 coal in a smaller tube apart, the process may be carried on inde- 

 finitely, the carbonic acid serving as it were to carry the reducing 

 combustible from the one tube, to the ore in the other. 



A modification of this process consists in mingling the ore with 

 an equal volume of small fragments of charcoal, and admitting a 

 limited supply of air into the body of the apparatus, by openings at 

 mid-height, the heat being as before applied from without. In 

 this case the action is analogous to that which takes place in the 

 ordinary blast furnace ; carbonic oxyd and carbonic acid are al- 

 ternately formed by the reactions between the oxygen of the air, 

 the ore and the charcoal ; but the supply of air being limited, 

 and the temperature low, neither carburation nor fusion of the 

 metal can take place, and five-sixths of the charcoal employed, 

 remain unchanged and serve for another operation. This simpler 

 way has the disadvantage that one half of the furnace is occupied 

 with charcoal, so that the product of metal is less than when the 

 reducing gas is prepared in a separate generator. In either case 

 the product is the same, and the iron remains as a soft porous 

 substance, retaining the form and size of the original masses of ore. 

 This metallic sponge is readily oxydized by moisture, and if pre- 

 pared at a very low temperature, takes fire from a lighted taper? 



