IRON 975 



relation to the space in which the transformation of carbonic acid into carbonic oxide 

 takes place. 



3. That the ascending current, which consists of carbonic oxide and nitrogen, with a 

 little hydrogen, produces in ascending two distinct effects: it communicates one 

 part of its sensible heat to the materials of the descending column ; it becomes charged 

 with all the volatile products disengaged at different heights, and it reduces the oxide 

 of iron to the metallic state. Sometimes this transformation gives rise to an increase 

 in the quantity of carbonic oxide ; sometimes, on the contrary, it effects the conversion 

 of carbonic oxide into carbonic acid without change of volume, and without com- 

 bustion of fuel. Whenever the reduction of oxide of iron takes place with the produc- 

 tion of carbonic oxide, there is a consumption of fuel, and an absorption of latent heat. 

 It is essential, therefore, to the good working of the furnace, that the minerals should 

 arrive completely reduced to that part where the temperature is sufficiently elevated 

 for the conversion of carbonic acid into carbonic oxide by contact with carbon : this 

 condition is nearly always realised when the oxide of iron is in a. free state in the 

 mineral. The reduction of the oxide when in combination with silica requires, on the 

 other hand, a high temperature, and it can only take place in that zone of the furnace 

 where the carbonic acid has completely disappeared. 



4. That the zone where carbonic oxide exists alone is much more extended in coke- 

 than in charcoal-furnaces, and is nearer the mouth in the former than in the latter ; 

 it falls lower, however, in the cylinder with hot blast, the quantity of heat remaining 

 the same. 



6. That the volatile gaseous matters from the distillation of the charcoal pass into 

 the escape gases, and exert no influence on the reduction of the minerals. 



The mutual relation of the carbonic acid and carbonic oxide, which is observable 

 in the analyses of M. Ebelmen, is not found in those of Messrs. Bunsen and Playfair ; 

 this is attributed by M. Ebelmen to the circumstance that the latter chemists collected 

 their gases through narrow iron tubes, which, becoming intensely heated and partially 

 choked by the fragments of ore and fuel introduced by the rapid stream of gas, so 

 modified the composition of the gases, that the analysis, however carefully conducted, 

 could not represent accurately their real composition. M. Ebelmen collected his gases 

 through wide tubes, and from the lower parts of the furnace, by piercing the solid 

 masonry. It is obvious, however, that none but very general conclusions can be drawn 

 from the analysis of the furnace-gases, in whatever way they may be collected, for their 

 composition cannot be the same under all circumstances, the nature of the fuel, the 

 pressure of the blast, and (as Mr. Parry's experiments prove) the shape of the furnace 

 itself, must each exert an influence in modifying the circumstances which affect their 

 composition. Although, therefore, it is impossible to fix the precise region of the furnace 

 where the reduction of the oxide of iron begins to take place, that is, to define pre- 

 cisely the limits of the ' zone of reduction,' we may, in considering the theory of the 

 production of crude iron, divide the furnace into four zones : 1. The zone of reduc- 

 tion ; 2. The zone of carburation ; 3. The zone of fusion ; 5. The zone of oxidation. 

 The zone of reduction will vary in extent according as the furnace is working with 

 coal or with coke, with hot blast or with cold. The zone of carburation commences 

 just below the top of the bosses, the reduced metal in a soft and malleable state here 

 acquires carbon, its rapid sinking being retarded by the contraction which the sides 

 of the furnace begin to undergo from this point downwards. As the carburized 

 metal passes through the zone of fusion it melts, together with the earthy matters 

 which serve to protect it from the oxidising effects of the fourth zone, that of oxida- 

 tion, through which it passes^ in its passage to the crucible. If the temperature of 

 the zones of fusion and oxidation be not much higher than the melting point of spe- 

 cular iron, the metal in the crucible will be white, with little or no graphite ; and if 

 the iron remain sufficiently long^in the zone of carburation to take up the maximum 

 quantity of carbon, it will be bright iron. The reduction of silicon appears to take 

 place at about the melting temperature of specular iron : it exists therefore in small 

 quantity in white iron, and in the greatest abundance in the grey iron smelted from 

 refractory ores, which require a high temperature. 



The proportion of carbonic acid in the gases obtained from different heights in a 

 furnace, has been studied by MM. E. Montefiore Levi and Dr. Emil Schmidt (Zeit- 

 schrift des dster Ingenieurvereines, 1852). They found that the zone from which this 

 gas is entirely absent is of very limited extent, for although it is not met with at a 

 height of _8 feet from the tuyere, it exists at 9 feet to the extent of 478 per cent., 

 above which point it diminishes up to 15 feet, where it is 0. From this point it 

 again ^increases, amounting at a height of 30 feet to 3'5 per cent. It then gradually 

 diminishes, until, at a point from 37 to 39 feet above the tuyere, it amounts to only 

 1*69 or 1'91 per cent.; after which it goes on increasing with rapidity and regularity 

 up to the furnace mouth. The carbonic acid existing in the furnace-gases between 



