ened on the outside by heavy boiler plate iron. The top of the furnace is so constructed that it is practically airtight, and any gases formed within must pass off through a pipe near the top shown in Fig. 70. However, as the process is a continuous one, additions of

material are made repeatedly through a hopper which opens mechanically when a load is dumped upon it and then closes again when the material has fallen into the furnace. Near the bottom is an opening for drawing off the molten iron, which is kept closed by means of clay until needed. At some distance above is an opening for the exit of the slag. As the quantity is usually large, this is open most of the time with a steady outflow of the molten material. Above this, entering from the sides, are the blast pipes, often called tuyeres, through which the air is forced into the furnace. This is needed to give the intense heat required. The furnace is tapped usually every twelve hours and the molten iron run off into trenches and side trenches in the ground. These short trenches are called "pigs" and the iron formed in them "pig iron." Sometimes the molten iron is run into steel molds bolted upon an endless chain, which moves slowly forward as each is filled from a large cauldron. (See Fig. 71 for illustration of method of molding pig iron.)

5. The Charge. The materials used in the "charge" put into the furnace are usually iron ore, coke and limestone. Most ores when brought from the mine contain more or less silica. In the furnace the limestone with the silica forms a species of glass, called slag. It floats above the molten metal and protects it from the oxidizing action of the blasts of air. When the ore contains limestone as a gangue, instead of silica, some silicious material is added to serve as the flux.

6. The Chemical Action.-Owing to the large amount of air introduced through the tuyeres, at the bottom of the furnace carbon dioxide is produced. (See p. 169.) As this flows upward it meets red-hot carbon and is changed to carbon monoxide, thus,

CO2+C→2CO.

This being an unsaturated gas removes the oxygen from the iron ore and the free iron thus produced flows to the bottom of the furnace. The gas, which escapes at the top, contains much carbon monoxide as well as other combustible gases. This is used to heat the air forced

Fig. 71.-A blast furnace, showing the molds for the "pigs" in the sand.

in and to run the fans for the air pressure. The iron thus obtained is known as cast iron. It contains a high percentage of carbon, sometimes as much as 3 or 4 per cent as well as silica and other impurities. It is coarse grained, brittle, hard, and has a melting point relatively low, about 1250° C.

7. Wrought Iron.-Wrought iron is made by heating the cast iron in a reverberatory furnace with ferric oxide. The purpose of the oxide is to furnish oxygen for union with the carbon in the iron, by which carbon monoxide is formed. These are sometimes called puddling furnaces, because the molten pig iron is "puddled" or stirred so as to facilitate the action. When the mass has become stiff and very difficult to stir, it is known that the carbon is burned out. Pure iron melts with much more difficulty than impure, and the temperature of these furnaces is only sufficient to keep the impure form in a molten condition. It is the same principle, seen often before, in which a solution will remain liquid while the solvent will solidify. At this stage the iron, called "bloom," is removed from the furnace, and beaten with trip hammers, which forces out the silica and any slag remaining. Such is now wrought iron. It is malleable and may be welded; it is fine grained, somewhat more dense than cast iron, melts at about 1,500° C. and contains only about 0.1 to 0.2 per cent of carbon. It should not contain much phosphorus or sulphur. Wrought iron is made into sheets, and is used in chains, wire, and in many other ways where a cheap, malleable metal is desired.

8. Steel. The oldest method of making steel was by the cementation process. Bars of wrought iron were imbedded in powdered charcoal and heated strongly for a number of days. The iron slowly absorbed small portions.

of the carbon and was changed thereby into steel. However, it was far from satisfactory, for the reason that the carbon was not taken up uniformly. Some portions would contain so much as to be more or less brittle, while others would not contain enough. Cast steel was first made by melting cementation steel and molding it into a bar so as to give uniform composition. This greatly improved the produce previously obtained. The process is entirely too. slow for modern demands; for years, until recently, most of the steel used has been made by the Bessemer process

(Fig. 72). This process uses a movable furnace, called a converter, made of boiler-plate iron and lined with a silicious earth called ganister, which is infusible. The compressed air passes up through a supporting post, through the horizontal trunnion, and down the pipe into the tuyere box or air chamber, then into the body of the converter. For use it is filled half or two-thirds with molten cast iron and the air is turned on. No heat is applied; the interaction of the oxygen of the air with the carbon and other impurities in the iron is sufficient not only to keep the metal molten, but even raise the temperature.