964 



IRON 



that have boon made during the last few ycnrs concur in proving. Hot-blast grey- 

 iron smelted with mineral coal, contains a much higher percentage of silicon than the 

 same variety of cast iron smelted from the same ores by cold blast ; in other resins 

 provided the process of reduction is complete, i.e. when little or no iron passes off with 

 the slag, there is very little chemical difference between the two varieties, as will be 

 seen in the following Table, which contains the results of a series of analyses of hot- 

 and cold-blast iron, which Dr. Noad had occasion to make, under circumstances 

 peculiarly favourable for instituting the comparison, the furnaces working with the 

 same ores, and making the same class of iron, viz., good No. 3 grey pig. 



Analyses of Cast Iron No. 3, smelted by Hot Blast. 



Analyses of Cast Iron No. 3, smelted by Cold Blast. 



The true reason of the frequent inferiority of hot-blast iron has been correctly given 

 by Mr. Blackwell. Furnaces blown with heated air exert greater reductive power 

 than those in which a cold blast is used. This has led, since the introduction of hot 

 blast, to the extensive use in iron smelting of refractory ores not formerly smelted, a 

 large part of which have been ores of a class calculated to produce inferior iron ; and 

 it is to the use of ores of this nature, far more than to any deterioration in quality, 

 arising from a heated blast, that this inferiority of Hot-blast iron is to be ascribed. 



Utilisation of the waste gases given off from the furnace-head. The agent in the 

 blast-furnace by which the oxide of iron is reduced is carbonic oxide, the presence of 

 which therefore in great excess is indispensable to the operation of the furnace. The 

 flames rising from the tunnel-head, which makes a blast-furnace at night such an im- 

 posing object, are occasioned principally by the combustion of this gas, on coming into 

 contact with the oxygen of the atmosphere. The attention of practical men was first 

 called to the enormous waste of heat which this useless flame entailed by Messrs. 

 Bunsen and Playfair, and the application of the gas to a useful purpose may be ranked 

 next to that of the heated blast, as the most important of the recent improvements in 

 the iron manufacture. The gases evolved from iron furnaces where coal is used as 

 the fuel contain the following constituents : vizf. nitrogen, bmmdnia, carbonic acid, car- 

 bonic oxide, light carburettcd hydrogen, olefiant gas, carburettcd hydrogen of unknown 

 composition, hydrogen, sulphuretted hydrogen, and aqueous vapour. The nature of the 

 combustible gas stands in a relation so intimate to the changes suffered by the 

 materials put into the furnace, that its different composition in the various regions of 

 the furnace indicates the changes suffered by the materials introduced as they descend 

 in their way to the entrance of the blast. Now as the examination of this column of 

 air in its various heights in the furnace must be the key to the questions upon which 

 the theory and practice of the manufacture of iron depend, it was of the first import- 

 ance to subject it to a rigid examination; this accordingly has boon done by the above- 

 named eminent chemists, and subsequently by M. Ebelmon. We shall return to a con- 

 sideration of the results they obtained presently, confining our attention at present to 

 the composition of the gases at the mouth of the furnace, and to the methods which 

 have been adopted to utilise them. 



In order to arrive at a knowledge of the composition of these gases, M. Bunsen 

 first studied minutely the phenomena which would ensue wore the furnace filled with 



