The saving of fuel effected by the employment of the hot blast is immense, and is much greater than was at first anticipated : 2 tons of coal are now amply sufficient for the production of a ton of iron, from ore which would have required 8 tons when the cold blast was used. This saving is effected owing to the operation of several causes, one of which is, that raw coal may now be used in the furnace instead of coke: moreover, as a smaller quantity of fuel is required in the furnace to raise the injected air to the necessary temperature, so also a smaller quantity of air is needed to maintain the combustion: combustion takes place within a short time, so that the maximum heat of the furnace is obtained lower down in the crucible,' and the upper portions of the furnace do not become so intensely heated: the reduction of the ore consequently takes place nearer to the bottom, and the heat is thus concentrated and economized.

In every metallurgical process a particular temperature must be attained in order to secure the occurrence of the reaction, or of the fusion which is desired. All fuel consumed at temperatures below that point is ineffective, and is therefore burned to waste. It must be remembered that in every case of combustion where the same chemical compounds are produced, a definite weight of fuel always emits a definite amount of heat; consequently it will raise a definite weight of air, and of materials in the furnace, through a definite number of degrees of temperature : -Say that a certain weight of fuel will raise the temperature of a given charge in the furnace from 60° to 2500°. Now, the same weight of fuel (if we neglect the quantity of heat absorbed by alteration of the specific heat with rise of temperature) will also raise the same charge from 660° to 3100°. Suppose, now, that iron required a temperature of 2800° for its fusion, no amount of fuel burned so as to produce a temperature of 2500° would be of any avail in effecting the fusion of the metal, whilst a comparatively small quantity, starting from the initial temperature of 660°, would produce the desired result.

with great success in welding the joints of wrought-iron tubes. Messrs. Siemens prefer to distil the coal in furnaces through which a regulated supply of air is transmitted, thus furnishing a mixture of gaseous hydrocarbons with carbonic oxide and the nitrogen of the spent atmospheric air, and these combustible gases are conveyed by a flue and burned at the spot where the heat is required. The gases after having done their work are passed through the regenerator above described; and in the furnace where the combustion is effected a temperature can thus be obtained, limited at present only by the powers of the fire-brick to resist its fusing action. The gas furnace, for such it is, has already, in some cases, superseded the old coal furnace in glassmaking.

COMPOSITION AND PROPERTIES OF CAST IRON.

583

Even in a hot-blast furnace, however, the quantity of fuel which is wasted is enormous. Bunsen and Playfair, from their elaborate experiments at Alfreton, make the almost incredible estimate that somewhat more than 4ths of the total quantity of heat producible from the fuel consumed is lost, owing to the escape of unburned combustible matter in the form of gases, such as carbonic oxide, carburetted hydrogen, and hydrogen, which are still fit for use. Since the publication of these researches, Mr. Budd and other ironmasters have economized a portion of the heat contained in the escaping gases, in heating the blast and in generating steam.

The iron obtained by the use of the hot blast is inferior in tenacity to cold-blast iron; a circumstance which appears to be partially due to the fact that the proportion of silicon is greater in hot- than in cold-blast iron; it is also to be noticed, that in the employment of the hot blast uncoked coal is used, a fuel which contains more sulphur, and possibly also more phosphorus, than coke, which is required in working with the cold blast.

A furnace in full work requires an hourly supply of rather more than 1 ton of solid material, consisting of an average of 5 parts of coal, 5 of roasted ore, and 2 of limestone. The roasted clay-iron ore yields on an average 35 per cent. of iron, and each furnace when in full activity furnishes from 8 to 10 tons of metal in the 24 hours. Every morning and evening it requires to be tapped on these occasions the iron is run into shallow grooves in the sand, and forms the cast iron, or pig-iron of commerce. A good furnace, if well managed, may be made thus to work uninterruptedly without repair for many years.>

(747) Varieties of Cast Iron.-The iron as it runs from the furnace, however, is not a pure carbide or carburet, for in the intense heat, not only is the iron reduced, but portions also of silicon, aluminum, and calcium, and occasionally other bodies derived from the flux and from the fuel. These bodies enter in small quantity into combination with the iron, the properties of which they materially modify. Manganese generally accompanies

* The production of iron in Great Britain, in 1862, amounted to about 3,943,000 tons. It was estimated in 1855, by Mr. Blackwell, that the annual production of iron in different countries was then as follows:

In all, six millions of tons, of which Great Britain supplied one-half.

584

COMPOSITION AND PROPERTIES OF CAST IRON.

the ores of iron in greater or less quantity, and frequently combines with the reduced metal. Cast iron differs greatly in quality; the differences observed in it depend in part upon differences in the proportion of carbon and silicon which it contains. The composition of these carbides varies considerably within certain limits; but it does not appear that iron is capable of combining with more than about 5 per cent. of carbon. A compound of carbon having the composition of Fe,€, or the tetracarbide, would consist of 94'92 of iron, and 5'08 of carbon; and this is very nearly the composition of the hardest and most fusible kind of white cast iron, which, from the circumstance of its crystallizing in flat brilliant tables, is termed by the Germans spiegeleisen (or mirror iron) : according to Gurlt, the specific gravity of this carbide is 7·65. Spiegeleisen is, however, not a pure carbide of iron, but always appears to contain manganese in amount varying from 4 to 10 or 12 per cent. Faraday and Stodart found the most highly carburetted iron which they could produce to consist of-iron, 94'36; carbon, 564. Gurlt (Chem. Gaz., 1856, p. 231) has described another definite form of cast iron (Fe,E), the octocarbide, which when pure contains 2.63 per cent. of carbon. It has a sp. gr. 775, is of an iron-grey colour, and has a hardness much inferior to that of the tetracarbide, being slightly malleable. It crystallizes in confused octohedral groups, and according to Gurlt is the principal constituent of grey cast iron. The existence of this compound is probable, but cannot be regarded as absolutely proved. In many varieties of cast iron the carbon exists in two distinct forms,one portion being chemically combined with the metal, the other being mechanically diffused through it in the condition of graphite, the scales of which may be distinctly seen with a magnifying lens, when the surface of a freshly fractured bar is examined. These scales remain unacted upon when the metal is dissolved in diluted acids; the combined carbon under such circumstances unites with hydrogen, and forms an oily-looking liquid of ill odour.

In addition to carbon, cast iron also contains silicon, the proportion of which is equally liable to variation; the quantities of silicon which have been found in pig-iron range between 35 and 0.25 per cent.*

* Karsten found that when cast iron was melted with sulphur in a covered clay crucible, there was formed, on cooling, a layer of sulphide of iron upon the surface, then a layer of graphite, and beneath this a layer of carbide of iron in the maximum degree of carburation. These effects may be thus explained-Carbon is incapable of decomposing sulphide of iron, but sulphur can displace carbon from the carbide. On the addition of sulphur to the melted cast iron the carbon gradually becomes concentrated in that part of

COMPOSITION AND PROPERTIES OF CAST IRON.

585

The following table will serve to illustrate the general composition of some varieties of cast iron :—

Gurlt's specimens were all made in the same furnace, and with the same material; the grey at the highest temperature, the white at the lowest.

The fusing-point of cast iron varies with its composition; that of an average specimen was estimated by Daniell at 2786° F.

In commerce there are three principal varieties of cast iron, known respectively as Nos. 1, 2, and 3. No. is called grey cast iron; No. 2, mottled cast iron; and No. 3, white cast iron. The first two contain carbon disseminated in an uncombined form through the mass. Grey cast iron is soft; it may be filed, drilled, and turned in the lathe, and though somewhat less fusible than the white, is preferred for casting, since when melted its liquidity is more perfect. This variety is that which is generally produced from a furnace in good working order; if cooled suddenly, it is often converted into white cast iron.* The fracture of the mottled variety is in large coarse grains, among which points of

the iron not combined with the sulphur, until its point of saturation with carbon is reached, and then the graphite is separated. According to the same authority, both phosphorus and silicon act in a similar manner, phosphide and silicide of iron being formed, whilst the carbon becomes concentrated in the remainder until the excess of carbon is expelled and crystallizes in the form of graphite. When the proportion of phosphorus, of silicon, or of sulphur, is but small, the compounds which they form with the iron remain disseminated through the mass of cast iron, and exert an important influence upon its texture and tenacity.

* According to Le Guen (Ann. de Chimie, III. lxix. 282), if good grey pigiron be fused with 2 per cent. of powdered wolfram, the cast iron so produced is rendered much stronger and more elastic, the tenacity being increased from 3 to 4 if the quantity of wolfram be increased to 3 per cent. the metal becomes still harder, but not so tough.

586

VARIETIES OF CAST IRON-MALLEABLE CAST IRON.

graphite are distinctly visible; it is very tough, and is valued for casting ordnance. It may be obtained for this purpose by partially refining good grey iron. White cast iron contains about the same amount of carbon as the mottled iron, but the whole of the carbon appears to be chemically combined with the metal. The white variety passes through a pasty condition as a preliminary to liquefaction; it is more fusible than either of the others, is lighter in colour, very hard and brittle, has a lamellar crystalline fracture, and a specific gravity varying between 7.2 and 7·2 7.6. It usually contains less silicon, but more sulphur and phosphorus than grey iron. White cast iron seems in some cases to owe its colour to the presence of manganese. A much higher temperature in the furnace, and consequently a greater consumption of fuel is required for the production of grey than of white iron. This may probably arise from the fact, that if white iron be melted and exposed to a temperature considerably higher than its melting point, the tetracarbide of iron is decomposed, and if it be allowed to cool very gradually, a portion of the carbon crystallizes out as graphite, and grey cast iron is produced. In the process of casting heavy articles this carbon separates, and is thrown off in the form of brilliant scales, termed by the casters kish.

The peculiar value of iron for castings depends upon its property of expanding at the moment of solidification. It thus furnishes an admirable material for taking the most minute impressions, as is well exemplified in the beautiful castings obtained from Berlin.

Small articles made of cast iron, such as key-blocks, stirrupirons, &c., may be rendered malleable by packing them in powdered hæmatite, then heating them to redness for some hours, and allowing them to cool very slowly. In this case the oxygen of the oxide removes a portion both of the carbon and of the silicon, by