978 IRON 



mised. How very great the change has been may be scon in the preceding table 

 giving the dimensions and dates of tho erection of new furnaces at the different 

 works since iron-smelting was commenced in Cleveland in 1851, in furnaces of 

 the Staffordshire type. 



Mr. Lowthian Bell, to whom we are indebted for an elaborate work on the chemistry 

 of the blast-furnace, considers that it is not certain whether any greater saving can be 

 effected by the use of furnaces of 30,000 feet cubic capacity than by those of 10,000 

 or 12,000 feet, as in either case for similar charges he finds the composition of tho 

 waste gases and their temperature will be about the same. 



Mr. Bell has pointed out that when peroxide of iron is exposed to carbonic oxide at 

 temperatures between 750 and 850 Fahr., that simultaneously with the reduction of 

 the ore to metallic iron, with the production of carbonic acid, a converse action takes 

 place to a small extent, the carbonic oxide being split up into carbon and carbonic 

 acid, the former being absorbed by the partially-reduced ore. This action is supposed 

 to play a considerable part in modifying the ordinarily-accepted theory of blast- 

 furnace action, as it is supposed that the carbon so absorbed is carried down with the 

 ore into the highly -heated region of the hearth and boshes, where the last traces of 

 oxygen are removed by its combustion, and the iron becomes fully carburised. 



This absorption of carbon is attended with the disintegration of the ore a fact 

 which accounts for the disappearance of the lumps of ore in tho interior of blast-furnaces 

 when examined through holes in tho brickwork of the stack made by accident or 

 design. In such cases the flux and coke retain their original form ; but the ore is 

 changed in many cases to a black crumbled mass, in which the separate lumps can no 

 longer be distinguished. The reduction by carbonic oxide of peroxide of iron to the 

 metallic state commences at the temperature of about 300 to 400 Fahr., but at the 

 higher point, of about 800, the spongy metal is reoxidised when brought into contact 

 with carbonic acid. The reducing action, therefore, of carbonic oxide in blast-furnace 

 gas must at the higher temperature become balanced by the oxidising action of car- 

 bonic acid in proportion as the two gases approach to ' an equality. This is a point of 

 great importance as affording a true measure of the working of a furnace, which may 

 be considered to have consumed its fuel properly, when the gases at the throat are 

 cooled to such a point that the particular dilution of carbonic oxide by carbonic acid, 

 to which they correspond, is no longer capable of reducing the ore. This is attained 

 in the Cleveland furnaces when the volume of carbonic oxide to that of carbonic 

 acid is as 2 to 1, and the temperature from 600 to 700. Beyond this point the 

 gases have only a calorific value, namely, their sensible heat and the amount that can 

 be developed by their combustion in stoves and boilers instead of coal. 



The increase of heat in the blast has in general been attended with a marked economy 

 of fuel up to temperatures of about 1000 Fahr. With more highly heated blast, 

 such as is obtained in the Siemens and Whitwell stoves, it is less certain whether any 

 greater saving can be effected. At Consett a distinct saving is claimed for blast at 

 1300 over the same furnace when blown at 1000. At Barrow, on tho other hand, 

 no saving was effected by the use of blast at 1500 instead of 1100, the consump- 

 tion of coke being the same, in either case about 20'18 cwts. to the ton of pig-iron 

 produced. 



Varieties and Chemical Constitution of Cast Iron. In commerce there are four 

 principal varieties of cast iron, known respectively as Nos. 1, 2, 3, and 4, or dark grey, 

 bright grey, mottled, and white; these terms, although convenient, do not, however, 

 indicate the intrinsic value of the iron thus denominated, as tho variable qualities of 

 ore, fuel, and limestone may exercise such an influence on the resulting crudo iron, as 

 to render a low denomination of one manufacturer of greater commercial value than 

 a higher denomination of other makers. The general characters of the four varieties 

 are these : No. 1. Colour, dark grey, in large rounded grains, obtained commonly 

 near the commencement of tho casting when tho furnace is in irood working order, 

 and when an excess of carbon is present ; in flowing it appears pasty, and throws out 

 blue scintillations. It exhibits a surface where crystalline vegetations develop them- 

 selves rapidly in very fine branches; it congeals or fixes very slowly; its surface. 

 when cold, is smooth, concave, and often charged with plumbago; it has but. a 

 moderate tenacity, is tender under tho file, and susceptible of a dull polish. Win-it 

 melted over again, it passes into No. 2, and forms tho best castings. No. 2, colour 

 bright grey, of small-grained structure, and interspersed only with small graphite; 

 laminae; possesses great tenacity, is easily filed, turned, and bored; may even 1m 

 hammered to a certain extent; does not readily crack from change of temperature. 

 No. 3 is a mixture of white and grey iron. On stroiir/ly rncittlrd iron, little stars and 

 spots of grey iron are found, interspersed in bright or flowery iron ; weakly mot tlJ iron 

 exhibits white specks on a grey ground. In streaked iron, grey iron is found alm-o 

 and below, and bright iron in the middle, with strong demarcations. No. 4. While 



