furnace, enclosed space for the burning of fuel. There are many kinds of furnaces, the type depending upon the fuel and the use to which the heat produced within it is put. Most familiar are the furnaces used in the heating of buildings. In the hot-air furnace, fuel is burned within an inner wall and air, led into a space between the inner and the outer wall, is heated and is led away to the various rooms of the building. Hot-water furnaces, by which water is heated to be led through pipes to radiators, and furnaces that turn water to steam for heating purposes are common. The kiln is a kind of furnace. In metallurgy, the separation of many metals from their ores is accomplished by the use of various kinds of furnaces, e.g., the blast furnace and the reverberatory furnace. The structure of these furnaces makes possible a good control of temperature. In the production of steel, however, the open-hearth furnace and the Bessemer converter are used in the treatment of cast iron. The electric furnace is extensively employed in the production of high-grade steels for use in making steel alloys and for the manufacture of high-speed tools. Heat may be generated in such a furnace by using an electric arc or by sending an electric current through resistive elements in the furnace. If the material to be processed is electrically conductive, heat may also be generated by creating an electric current in the material by induction or by inserting into it electrodes to which a voltage is applied. In the preparation of phosphorus from calcium phosphate, this compound of phosphorus is mixed with sand and coke and treated in an electric furnace. An electric current is sent from one electrode to another through the mass to create the extremely high temperature needed to bring about the chemical action that results in the production of free phosphorus. Graphite is produced from coal or coke in an electric furnace, and the extremely hard substance carborundum is made there by the combination of carbon and silicon (from sand). Nitrogen is obtained from the air (in the Birkeland-Eyde process) by passing a stream of air through an arc. The nitrogen and oxygen of the air combine to form nitric oxide.
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