542 FURNACE purposes they are designed to serve. The iron manufacturer, smelting the ores upon a vast scale, builds an immense structure with a capa- city of hundreds of tons, and furnishes it with heavy machinery for supplying the great vol- ume of air blown in almost without ceasing, as the operation is continued during a single blast of two years or more. All furnaces em- ployed in melting refractory materials those for assaying, as well as those operating upon a large scale require a free supply of air, pro- portional in quantity to the amount of fuel they consume. The generation of heat depends upon the rapid chemical combination of carbon with oxygen, and a sufficient supply of the latter ele- ment is as essential as is that of the former. Every pound of good bituminous coal, accord- ing to Dr. Thomson, requires 150 cubic feet of air, or allowing one third more for waste, there should be supplied at least 200 cubic feet. So immense is the quantity of this invisible ele- ment consumed and wasted in the large fur- naces for smelting iron ores, that its weight even is greater than that of all the other mate- rials, ores, coal, and flux, introduced; and the power required to force this volume of air through the dense column of heated matters far exceeds that expended in charging the fur- nace with its solid contents, even adding to this the power involved in the removal of the products of the operation. To provide for this large supply is then a matter of the first conse- quence to furnaces ; and according to the mode in which this is effected they are separated into two classes. The kind called air or wind or reverberatory furnaces receive their supply by means of the current produced by a tall chimney, the heated column rushing upward through the flue. To fill the space in the lower part of the flue, air presses in from without through every aperture; and none being al- lowed except those leading through the recep- tacle for the fuel, the supply of air is thus se- cured, heat is generated for the purposes re- quired, and a portion is expended in furnishing the mechanical power involved in the move- ment of the current of air. Fireplaces, stoves, and grates are examples of air furnaces ; and by means of the blower, which causes the air admitted into the chimney to pass first through the fire, the flue is prevented from becoming chilled by the entrance of cold air, the column ascends more rapidly, an increased supply of air is furnished to every portion of the body of fuel, and the chemical process goes on with augmented intensity and generation of heat. The other classes of furnaces are supplied with air through bellows or other blowing appara- tus. (See BLOWING MACHINES.) They are called for this reason blast furnaces, and are used when the resistance opposed to the passage of the current of air by the density of the con- tents of the furnace is so great, that sufficient quantity cannot penetrate to keep up thorough combustion throughout the mass ; or when the operations do not admit of the large openings beneath the fire, which the free admission of such bodies of air would require ;' or again, when the nature of the operation demands an intensity of heat concentrated in one spot. The blast in this case acts like the jet of the blow- pipe, and its effect is in many cases greatly in- creased by its being conveyed through iron pipes which are highly heated by exposure in suitable ovens to the waste heat of the smoke and gases which escape from the chimney. It thus restores to the interior of the furnace, in the form of highly heated air, a portion of the caloric that would otherwise be lost. Furnaces of both classes are often used in the chemical laboratory; but the blast furnace is rather preferable because it can always be more perfectly controlled. The one commonly em- ployed for general purposes is a wind furnace, built of fire brick, and strongly secured with iron rods and straps. It has a flat top, with two or more openings, and on these are placed pans of cast iron for holding sand in which vessels are placed for exposure to moderate heat. The furnace has under the flue that leads into the chimney an oven for drying. With a good draught this furnace produces sufficient heat for many crucible operations. These are, however, better conducted in small- er furnaces, either wind or blast, constructed specially for this use. The construction and manner of using the various kinds of reverbe- ratory, blast, and assay furnaces will be found described under the heads ASSAYING, BLOOM- AEY, CASTING, COPPEE SMELTING, IRON MANU- FACTURE, and others which treat of processes involving the use of these furnaces. Gas fur- naces employ gas instead of solid fuel, and are constructed in a variety of forms, but always upon the principle of the Bunsen's burner. (See FLAME.) Griffin's blast gas furnace, for metallurgic operations requiring high heat, is shown in section in fig. 1. Two fine clay cylin- ders, a, a, form the body of the furnace. They rest upon a perforated fire-clay plate, 5, into which the gas burner, c, is introduced. A plumbago crucible, <Z, sets upon a perfora- ted plumbago cylin- der, e, and is cover- ed to a considerable depth with quartz pebbles from half an inch to an inch in di- ameter ; //are plugs which may be re- moved to admit of in- spection. The burn- er is represented in fig. 2, and consists _ _ of two chambers of Fm.l.-Griffin's Gas Furnace, cylindrical cast iron, one for the reception of air and the other for gas. Tubes, varying in number from 6 to 20 or more, pass from the air chamber through the gas chamber, and
