The cast iron formed in blast furnaces is not always of the same quality. When slowly cooled it is soft, has a grey colour, and is not

causes the whole contents of the furnace to subside, thus forming an empty space at the top, which is again filled up with the afore mentioned mixture. During its downward course this mixture is subjected to increasing heat. This rise of temperature first drives off the moisture of the ore mixture, and then leads to the formation of

the products of the dry distillation of coal or charcoal. Little by little the subsiding mass attains a temperature at which the heated carbon reacts with the carbonic anhydride, passing upwards through the furnace, and transforms it into carbonic oxide. This is the reason why carbonic anhydride is not evolved from the furnace, but only carbonic oxide. As regards the ore itself, on being heated to about 600° to 800° it is reduced at the expense of the carbonic oxide ascending the furnace, and formed by the contact of the carbonic anhydride with the incandescent charcoal, so that the reduction process in the blast furnace is without doubt effected by the formation and assistance of carbonic oxide and not by carbon itself-thus, Fe2O3+3CO=Fe2+3CO2. The reduced iron on

proportion to the amount of manganese, and it is then known under the name of 'spiegeleisen' (and 'ferro-manganese ').o

Cast iron is a material which is either suitable for direct application for casting in moulds or else for working up into wrought iron and steel. The latter principally differ from cast iron in their containing less carbon--thus, steel contains from 1 p.c. to 0.5 p.c. of carbon and far less silicon and manganese than cast iron; wrought iron does not generally contain more than 0.25 p.c. of carbon and not more than 0.25 p.c. of the other impurities. Thus the essence of the working up of cast iron into steel and wrought iron consists in the removal of the greater part of the carbon. This is effected by means of oxidation, because the oxygen of the atmosphere, oxidising the iron at a high temperature, forms solid oxides with it; and the latter, coming into contact with the carbon contained in the cast iron, are deoxidised, forming wrought iron and carbonic oxide, which is evolved from the mass in a gaseous form. It is evident that the oxidation must be carried on with a molten mass in a state of agitation, so that the oxygen of the air, passing over the surface of the iron, may be brought into contact with the whole mass of carbon contained in the cast iron,

9 The specific gravity of white cast iron is about 75. Grey cast iron has a much lower specific gravity, namely, 70. Grey cast iron generally contains less manganese and more silica than white; but both contain from 2 to 3 p.c. of carbon. The difference between the varieties of cast iron depends on the condition of the carbon which enters into the composition of the iron. In white cast iron the carbon is in combination with the iron-in all probability as the compound CFe, but perhaps in the state of an indefinite chemical compound resembling a solution. In any case the compound of the iron and carbon in white cast iron is chemically very unstable, because when slightly cooled it decomposes, with separation of graphite, just as a solution when slowly cooled yields a portion of the substance dissolved. However, the separation of carbon in the form of graphite on the transition of white cast iron into grey is never complete; part of the carbon remains in combination with the iron in the same state as it is combined in white cast iron. Indeed, when grey cast iron is treated with acids, the whole of the carbon does not remain in the form of graphite, but a part of it is separated as hydrocarbons, which proves the existence of chemically-combined carbon in grey cast iron. It is sufficient to re-melt grey cast iron and to cool it quickly to transform it into white cast iron. However, it is not carbon alone that influences the properties of cast iron; when it contains a considerable amount of sulphur, cast iron remains white even after having been slowly cooled. The same is observed in cast iron very rich in manganese (5 to 7 p.c.), and in this latter case the fracture is very distinctly crystalline and brilliant. When cast iron contains a large amount of manganese, the quantity of carbon may also be increased. Crystalline varieties of cast iron rich in manganese are in practice called ferro-manganese, and are prepared for the Bessemer process. Grey cast iron not having an uniform structure is much more liable to various changes than dense and thoroughly uniform white cast iron, therefore the latter oxidises much more slowly in air than the former. The cast iron destined for the manufacture of the best kinds of wrought iron and steel ought to contain very little sulphur and phosphorus (not more than 0.05 p.c.). The silicon, manganese, and part of the sulphur oxidise during the processes, and hardly enter into the steel.

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in the barcoal Geni the tre does Di matain any are the quality of the inn or steel-tre instance, sulphur or Ure caly wood thars al may be need with may, from which process can only be carried i where the manufacture of son can be 5's fael. Coal aLi ecke contain the above-mented impanties and would defcere produce iron of a brittle nature, and thus it would be necessary to have recourse to pedong, where the fael is barat on a special hearth, separate from the Case #reby the impurities of the fuel do n't come into contact with it. The martains care of steel from cast-iron may also be conducted in tires; but, in addition to this is so now prepared by many other methods One of the long-known processes is called the cation, by which steel is prepared from wrought iron but not from cast itou. Fyfus process strips of iron are heated red-hot for a considerable time whilst mmcised n now Jezed charcoal; during this operation the iron at the surface combines with the chase, but the interior contains but little of it; after this the iron strips are orged, drawn out again, and cemented anew, repeating the process until a steel of the ered quality is forced-that is, containing the requisite proportion of carbon. The

impurities. Chemically pure iron may be obtained by precipitating iron. from a solution (a mixture of ferrous sulphate with magnesium sulphate Bessemer process occupies the front rank among the newer methods (since 1856); it is so called from the name of its inventor. This process consists in running melted cast iron into retorts (converters, holding about 6 tons of cast iron)-that is, eggshaped receivers, fig. 94, capable of revolving on trunnions (in order to charge in the

FIG. 94.-Bessemer converter, constructed of iron plate and lined with ganister. The air is carried by the tubes L, O, D to the bottom, M, from which it passes by a number of holes into the converter. The converter is rotated on the trunnion d by means of the rack and pinion H, when it is required either to receive molten cast iron from the melting furnaces or to pour out the steel. cast iron and discharge the steel), and forcing a stream of air through small apertures at a considerable pressure. Combustion of the iron and carbon at an elevated temperature then takes place, resulting from the bubbles of oxygen thus penetrating the mass of the cast iron. The carbon, however, burns to a greater extent than the iron, and therefore a mass is obtained which is much poorer than cast iron in carbon. As the combustion proceeds very rapidly in the mass of metal, the temperature rises to such an extent that even the wrought iron which may be formed remains in a molten condition, whilst the steel, being more fusible than the wrought iron, remains very liquid. In half an hour the mass is ready. The purest possible cast iron is used in the Bessemer process, because sulphur and phosphorus do not burn out like carbon, silicon, and manganese.

The presence of manganese enables the sulphur to be removed with the slags, and the presence of lime or magnesia, which are introduced into the lining of the converter, facilitate the removal of the phosphorus. This basic Bessemer process or ThomasGilchrist process, introduced about 1880, enables ores containing a considerable amount of phosphorus, which had hitherto only been used for cast iron, to be used for making wrought iron and steel. Naturally, the greatest uniformity will be obtained by re-melting the metal. Steel is re-melted in small wind furnaces, in masses not exceeding 30 kilos; a liquid metal is formed, which may be cast in moulds. A mixture of wrought iron and steel is often used for making cast steel. Large steel castings are made by simul