FURNACE. The common grate is the most familiar example of a furnace; and in the smaller chemical operations various con trivances are used, midway in character be tween a grate and a furnace; but the furnaces usually so called are the following :— The Wind Furnace gives a very high tem perature by the aid of a powerful draught. It is from 12 to 15 inches square, and is fur nished with moveable bars and a cover. The air is conveyed by pipes directly from without doors to the ash-pit, and the chimney is nar row and high. Such a furnace is much em ployed in the reduction of metals, and in the assaying of copper and various other ores. The fuel used is either coke or a mixture of coke and charcoal.
The Assay or Cupelling Furnace is a small furnace made of iron, lined with refractory clay, and containing a muffle; it is used prin cipally for the cupellation of gold and silver, which is placed upon a cupel in the muffle, previously heated to redness. The interior of the furnace contains merely the muffle resting upon two bars of iron ; it is put about two thirds into the furnace, and there is conse quently left a space between it and the back part of the furnace. Charcoal is used in this furnace.
For metallurgic operations on a large scale, as well as in making alkalies, red lead, &e., the Reverberatory Furnace is much used. There is a space furnished with a grate or bars, to contain the fuel, which is either coal, coke, or wood, according to circumstances. Beyond and behind this is a large low vacant space, so shaped that the flame may reverbe rate from the brick roof, and strike down upon the substance to be heated, which is placed upon a flat brick surface. There is a very high chimney that produces the draught, and which may be closed by a damper.
The Smelting Furnace, for iron works, is a large structure of brick, having a small square receptacle for the fuel beneath, a large inte rior space for the ore to be smelted, a wide mouthed chimney at the top, and air-holes at the bottom to admit either the hot or cold blast. Some of the South Wales furnaces have an internal cavity of 5000 cubic feet.
In scarcely any department of machinery or manufacturing apparatus are there more nu merous patented inventions than in the con struction of furnaces. Newton, Grist, Jukes,
Pollock, Burrows, Holcroft, Baker, Coed, Clarke, Bramwell, Homersham, Barker, Doo ley, Williams, Newcorne, Hall, Mackintosh, White, Howard, Wilson, Pidding, Robinson, Lee, Knowlys, Prideaux, Bessemer, Galloway, Johnson, Cliffe, Dalton—all these names, and many others, are connected with improvements in furnaces within the last two or three years. Some of these improvements relate to the furnaces of locomotives, some to those of ma rine engines, some to those of stationary en gines, and some to furnaces in general. In one case the patentee seeks to ecouomize space, in another fuel, in another to avoid the' smoke nuisance.' Some relate to metallic furnaces, some to fire-brick furnaces. In some the object is to raise steam in great quantity ; while in others it is to substitute rapidity for quantity.
An important principle has just been put in operation by the Ebbw Vale Iron Company of South Wales. It is no less than the applica tion of blast-furnace gases to heating pm-poses. After a furnace has performed the work for which it is intended, various gases escape with the smoke, at the upper orifice; and these gases carry with them a large amount of valu able heat. If the heat could be abstracted and usefully applied, without lessening the power of the furnace, an economical benefit would result. The above named company have ].1 blast furnaces, five engines to produce the blast, and twenty-five boilers to supply the engines with steam. The greater number of these boilers are wholly heated by the waste heat from the blast furnaces ; and various ovens and stoves in the Works are heated by similar means. It is said that 1000 tons of coal per week are saved by this admirable con trivance. The heated gases are arrested near the top of the furnace, carried out by a hori zontal tube, mixed with atmospheric air which enters in thin sheets or layers, and ignited by a small fire. It forms a true gaslight; and this gas-light heats a largo flue, which is sur rounded by a boiler containing water ; and thus is a supply of steam obtained.