IRON 



973 



The proportion of nitrogen to oxygen, as an average deduced from these analyses, is 

 79'2 to 27- The product of the combustion of coal gives the same proportions as 

 those existing in atmospheric air, viz. 79'2 : 20-08. The excess of oxygen must 

 therefore depend upon the carbonic acid of the limestone, and tlie oxygen of the ore 

 given to carbon during the process of reduction. Now, as at a depth of 24 feet the 

 gas collected contained 27'6 and 26'5 oxygen to 79 '2 nitrogen, it is held that at this 

 depth the gas must already have accumulated all the oxygen' of the ore, and the car- 

 bonic acid of the limestone ; and the conclusion is drawn that, in hot-blast furnaces fed 

 with coal, the reduction of the iron and the expulsion of the carbonic acid from the 

 limestone takes place in the boshes of the furnace. The exact region of the furnace 

 in which the melting of the iron and the formation of slag are effected is not exactly 

 defined, but it is assumed that the point of fusion is at the top of the hearth. The 

 region of reduction in a furnace smelting with coal must be much lower than when the 

 fuel is coke or charcoal, because a large portion of the body of the furnace must be 

 taken up in the process of coking, and the temperature is thereby so depressed, that 

 it is sufficient neither for the reduction of the ore, nor for the expulsion of carbonic 

 acid from the limestone. 



The mean general results obtained by M. Ebelmen from a charcoal-furnace at 

 Clerval are given below. The methods of analysis adopted by this chemist were 

 altogether different from those employed by Messrs. Bunsen and Playfair. For details, 

 we refer to his memoir in the Annales des Mines, vol. xix. p. 89, 1851. 



I. Gas taken a short time after the introduction of the charge : II. the same taken 

 a quarter of an hour after charging : III. gas collected through a cast-iron tube four 

 inches in diameter ; it rushed out with a noise and gave a sheet of flame, carrying 

 with it particles of charcoal and dust : IV. gas collected by boring the masonry ; it 

 rushed out violently, burning with a blue-coloured flame: V. the same taken an 

 hour after : VI. gas collected by boring the masonry at the back of the furnace 

 about 3 feet above the tuyere ; it burnt with a white flame, giving off fumes of oxide 

 of zinc ; it was collected through porcelain tubes : VII. gas collected through gun- 

 barrels lined with porcelain ; it was evolved with sufficient force to project scoriae 

 and even cast iron. 



The furnace was working with cold blast nnder a pressure of '44 inch of mercury. 

 The charges had the following composition : Charcoal, 253 Ibs. ; minerals (various), 

 397 Ibs. ; limestone, 254 Ibs. Thirty-two charges were driven in twenty -four hours ; 

 the furnace was stopped after every twenty charges ; the produce being 3,970 Ibs. of 

 black cast iron ; the daily yield being about 6,175 Ibs. 



The experiments show that while the carbonic acid progressively diminishes down- 

 wards, the carbonic oxide progressively increases, the former altogether disappearing 

 at a depth of 27 feet. On examining the numbers representing the oxygen and 

 carbon referred to 100 nitrogen, it is seen that they diminish progressively to a depth 

 of 19 feet, the oxygen combined varying from 42'5 to 28*2. The proportion of carbon 

 in the same zone rises from 28'5 to 32'8 : a result brought about as much by the car- 

 bonic acid disengaged from the minerals as from the gaseous products of the distilla- 

 tion of the charcoal. It is seen that the reduction of the mineral is already considerably 

 advanced at the depth of 19^ feet; and this, so to speak, without any consumption of 

 charcoal, but through the conversion of carbonic acid into carbonic oxide. The hy- 

 drogen decreases as the carbonic oxide increases ; showing that this gas exercises no 

 influence in the reduction of the ore. 



