A general name for those contrivances used in the arts and in chemistry, by which an extraordinary degree of heat is generated, much more intense than what obtains in our ordinary fire-places. These are of very extensive use, particularly where metals and minerals are the subjects of manufacture or of examination, such substances requiring a very powerful heat for their reduction, calcination, or fusion. Hence the various kinds of furnaces which occur in the manufacture of iron, steel, brass, copper, zinc, &c., in smelting, casting, and forging the metal, and in numerous other processes connected with the formation of it. In other cases, such as glass-works, potteries, and various others, it is in general not so much a very intense heat that is wanted, as a steady temperature, and the fire of such magnitude as will keep a large body of material in fusion, and at a very equable state for working. Another large and extensive class of furnaces, and one which is becoming every day of more and more importance, is that applied to the generation of steam in the boilers of steam-engines; and here also it is not so much an intense heat which is necessary, as a very large quantity for a rapid production of the elastic fluid. The construction of furnaces therefore forms an interesting and very important branch of practical mechanics. We shall endeavour at present to explain the general principles of their construction, and describe a few of those most generally in use; and further information will be found in treating of the different arts, manufactures, and processes where the furnaces are chiefly employed.

In regard to the general principles on which the operation of furnaces depends, the exact nature of the process of combustion is, we believe, hardly yet settled among chemical philosophers; and experiments are wanting to determine many points which might be practically useful. It was thought, for example, by Sir Humphry Davy, that by a vivid and rapid combustion of our fuel, a much greater quantity of heat would be generated than by a slow and languid action; and this theory, were it consistent with fact, would often be of great importance where economy of fuel is the object. Be this, however, as it may, it is certain that, in our ordinary furnaces, whatever be the fuel, the grand supporter of combustion is the atmospheric air, or rather the oxygen, which forms one of its constituent parts; and it is the combination of this aerial element with the solid mass of the combustible material in the furnace, which somehow or other, not well understood, produces all the heat that we obtain. To provide, therefore, a supply of this vital principle is the first object in the construction of every furnace. It appears, from some experiments on the products of combustion made by our celebrated chemist Dr Thomson of Glasgow, that for every pound weight of good caking coal consumed in a furnace, there will be required at least 150 cubic feet of common air; and adding fifty more for what cannot be rendered effective, we may allow at least 200 cubic feet to each pound of coal; and this may give us some idea of the vast quantity of air which must be required for many of the large furnaces and fires which are frequently used. In many of the furnaces, for instance, for the boilers of our steam-vessels, the consumpt of coal is more than twenty cwt. each hour; and by the above calculation this will require 7400 cubic feet of air per minute. It may seem at first no easy matter to supply so large a draught stream, and to make it pass regularly through the fire; yet by chimney, on the principle of the ascensional power of heated air; and few examples occur in practical mechanics of so simple, beautiful, and highly useful an application of a general principle. Air, like every other substance, is expanded by heat, and this in a remarkable degree, owing to the feeble aggregation of its elementary parts. With every degree of Fahrenheit, from the temperature of freezing to that of boiling mercury, or 680°, it expands the 488th part of its bulk. This is according to the accurate experiments of Messrs Petit and Dulong; and these philosophers also found the important law to obtain exactly within the above range of the thermometer, that equal increments of heat produced equal increments of expansion. Hence with a heat of 488°, which is far surpassed in our fires, the volume of the air is doubled; but with every such expansion the air becomes in proportion specifically lighter, and ascends by its buoyancy through the colder and of course denser medium around it. The fire therefore heating the air above it, this ascends, and the chimney forming a tube or perpendicular shaft, either directly above the fire-place, or in communication with it, the heated air fills it, and forms a rarefied column standing in the midst of the ordinary medium. The external columns, therefore, pressing on all sides into the partial vacuity, must rush in below where there is a free communication through the fire; thus a constant draught of air sets in towards the fire and up the chimney; and by this means the combustion in the furnace is kept up. It is exactly what takes place on a great scale over the globe itself, by the sun heating excessively the regions under the line; and this causing everywhere a current of heated air upwards, draws continually a current of cold air from the regions on each side of the tropics; and it is this which produces the phenomenon of the trade-winds, the opposite currents coming into collision under the equator, and destroying each other's effect as to north or south, while the rotation of the earth gives the whole an apparent motion nearly due east. The chimney then is the most essential part of every furnace, and its use is not only to create a draught of heated air, but to receive and discharge the smoke and other aerial products which arise from the combustion. In ordinary fire-places, where only a very moderate degree of heat is wanted to heat our apartments comfortably, the chimney is left open below, immediately above the fire-place, for the convenience of getting fuel or cooking utensils put on. In that case only a very small portion of the air which ascends up the chimney passes through the fire, namely, that which ascends from under the grating; and perhaps a little in the front. The greater proportion ascends above the fire-place, and this mixing with the heated air, cools it, and moderates the draught, which would otherwise become too great. In furnaces, however, where all the heat is required which can be produced, a different arrangement becomes necessary. The fire-place in these must be closed on all sides, and every access to the air excluded, excepting below, where the fuel rests on an open grating, consisting of iron bars laid parallel to each other, and at such distances apart as will retain the fuel, while the air is admitted through them, and the ashes and other refuse drop down into the ash-pit below. Hence arises the same general construction in every fur- nace; and, however they may vary in particular forms and dimensions, each contains a fire-place, grating, ash-pit, and chimney. It is very essential, particularly where an in- tense temperature is wanted, that the fire-place should be surrounded with substances of a very slow conducting power for heat, otherwise this would be soon dissipated in the sur- rounding atmosphere, and no high degree of temperature attained. Clay is found very useful for this purpose in small furnaces intended for experiments; and the effect is most strikingly exhibited if we try the action of a fire-place of sheet iron: the heat is extremely feeble, and is observed dissipating itself on all sides; but if we line it with an inch thickness of clay or pounded charcoal, a very intense heat may then be generated. In large furnaces there is no ma- terial so useful for this purpose as brick, which is not only a slow conductor, but can be made to withstand very in- tense heats. The magnitude of the fire must evidently be determined by the size of the grating; for, however large the fire-place be made, or great the quantity of fuel in it, unless the grating is such as will admit a suf- ficiency of air, the combustion will not proceed. In large fires, therefore, intended to generate a great quantity of heat, such as those for steam-engine boilers, the grates must be made large in proportion. In fires, again, where it is not so much quantity as an intense temperature, the grate may be quite small, as the temperature will depend chiefly on the power of the draught. Most of the opera- tions of the brass-founder, and of gold and silver-smiths, are performed with furnaces of which the grate does not exceed two feet in area. It may be reckoned generally, that for every ten pounds of coal consumed in an hour, there should be a square foot of area in the grating. The spaces between the bars should be about three eighths or half an inch. The bars themselves should be loose, and made to rest on two cross bars, one at each end, so that they can be taken out and renewed at pleasure. In that case they are made about an inch thick, and with a swell in the middle for greater strength. They are seldom made more than two feet or two and a half feet in length, one or more of such lengths being used if required.

But the grating itself, though it will admit it, will not determine any air to enter and pass through the fire. This is done by the draught in the chimney; and according as this is more or less powerful, the fire will burn with great- er or less force. It is of the first importance, therefore, to determine what are the circumstances in the form and di- mensions of the chimney on which the effect of this draught depends, and by which it may be regulated to our purposes. It is evident, in the first place, that the warmer the air is in the chimney, the greater will be its buoyant force, and the more powerful, therefore, the draught upwards. Every means, therefore, should be taken to preserve the heat in the chimney; and hence the great advantage in dwelling- houses of conducting the vents as much as possible through the interior walls, which are not exposed to the cooling ef- fects of the atmosphere, or of wind and rain, and also car- rying them up in a mass together, by which each pro- tects the other from the cold; secondly, the interior of the chimney should be as straight, smooth, and regular as possible, that the current of air may traverse it with the least possible degree of impediment. There should be no projections, nor any sudden inequalities in width, which are sure to create eddies, and obstruct the progress of the air. Hence even the cylindrical form of a chimney will be found to improve the draught, because it presents a larger area than a square in proportion to its circumference, and therefore presents less obstruction. A decided im- provement has lately been introduced in the construction of domestic chimneys, by the use of cylindrical cans of fire- brick, about a foot long, and built in, one above the other, to the top of the chimney. These are glazed inside, and present on the whole a very smooth and equal bore for the heated air. The cylindrical form of a chimney also pre- sents externally less extent of cooling surface than the square. Still there are facilities of construction in the square form which give them an advantage. But whether circular or square, as the air becomes necessarily rather cooler at the top of the chimney than the bottom, and therefore denser, hence the chimney should have a slight degree of taper in proportion to this.

But of all the circumstances affecting the draught of fur- naces, that which has the most powerful influence is the height of the chimney. The reason of this will be evident, if we consider that the mechanical force producing the draught is nothing else than the superior weight of the external air over that within the chimney. The latter forms a column expanded by heat, and having thus lost part of the air which it would have contained had it been at the same temperature with the external air. The ex- ternal column, therefore, presses on the internal with a force exactly equal to this difference, and which is evidently proportional to the height of the chimney. For sup- pose we had a chimney fifteen feet high, and the air within it heated to 488°, then one half of the air originally in the chimney will be expelled. The column of fifteen feet would now, if the tube were extended, occupy thirty feet. This fifteen feet, therefore, of the expanded air, being taken out of the scale of the internal column, it is evident that the external air will now act with a preponderating force exactly equal, or with such a pressure as will be due to a column of the expanded air fifteen feet high, which is just equal to a column of the ordinary air seven and a half feet high. But suppose the chimney is thirty feet high under the same circumstances, then the column of expanded air expelled by the heat will be thirty feet high, and the preponderating column of ordinary air fifteen feet, just double of the former, in proportion to the double altitude, and so of every other height; and hence we de- duce an important rule, that the height of the preponde- rating part of the column of air, by which the draught is produced in the chimney, is in every case proportional to the height of the chimney itself; and it is easy, from what has been stated, to form a calculation of this height in every case. Let H be the height of the chimney, n the number of degrees by which the internal air is heated be- yond the external, taking of course the average heat of the top and bottom of the chimney, or any other rule for giv- ing a true average from top to bottom, then \( \frac{nH}{488} \) will be the height of the internal column when expanded by the heat, and \( \frac{nH}{488} \) the height of the preponderating part of the external column, supposing it to have the den- sity of the internal. But suppose it to be of the ordinary density, it will be \( \frac{nH}{488 + n} \).

These principles then being established, it is easy now to calculate the velocity with which the external air will rush into the chimney; and this is the most important point to arrive at, as it is this which must determine the rate of sup- ply, and whether it will be more or less than what is cal- culated on. This velocity, then, or V, according to the well-known formula, will be each second eight times the square root of the height of the preponderating part of the column; and as this height is proportional to that of the chimney itself, hence the rule that the velocity is proportional to the square root of the height of the chimney. Taking then the above expression for the height of the preponderating part of the column, we have the velocity of the external air rushing into the chimney \( V = \frac{nH}{480 + n} \).

But in passing along the sides of the chimney, the air, particularly with great velocities, encounters considerable obstruction; and for this, and for what may arise from inequalities or other causes, we may allow 6 instead of 8 for the co-efficient in the above formula; this gives \( V = \frac{nH}{480 + n} \). The air is also greatly obstructed at the grating, and in passing through the interstices of the fuel; but this is supposed to be allowed for by giving the grating an enlarged capacity on this account, so that it may have the same facility in passing through the fire as it has in the chimney. This source of obstruction therefore need not be taken into account.

The above simple formula will be found to agree very nearly with facts; and we have been thus particular in deducing it from well-known and established principles, as rules and calculations have been given on this subject by different authors, the results of which differ considerably from each other. In cases of this nature, it is really superfluous to enter into all the refinements of calculation; there are in actual practice so many anomalies and deranging circumstances, that the only rules of any utility are those which embody simply, but with accuracy, the leading principles involved in the action; more complicated formulas, even though rigidly exact, instead of guiding, serve often rather to deter the practical engineer from any calculation at all. In an ingenious paper on this subject, by an eminent mathematician, in the Quarterly Journal of Science, rigorous formulæ are given for the above calculations; but they are more complex, and the accuracy of some parts may be doubted. The rarefaction of the air by heat, for example, is calculated by a rule which makes it, for 1500°, to be expanded upwards of twenty times its natural volume; whereas, by Petit and Du Long's experiments, it would not exceed 4½ times. Let us now take an example. Suppose we have a chimney thirty feet in height, and the average temperature of the air in it 300° above the external atmosphere. What will be the velocity with which the external air will enter the chimney? Here \( H = 30 \) \( n = 300 \), therefore \( V = \frac{9000}{788} \), or about 20 feet per second. Hence a chimney at this height, and one foot square, ought to supply 1200 feet per minute; and, from what we have seen, this would be sufficient to consume six pounds of coal per hour; and by enlarging the area of the chimney it would be adapted to a larger fire. But suppose it is raised to 100 feet, the velocity being in every case as the square root of the height, this would give the ratio of 5½ to 10, or 37 feet per second, and a supply, with a one-foot chimney, of 2220 feet a minute. This shows how slowly the effect of raising the chimney proceeds. It requires the height to be raised four times to double the effect, and nine times to triple it; and this explains the great effect, comparatively, which is often produced by chimneys of moderate height judiciously constructed.

In this manner it is easy to calculate the effect of different heights and areas of chimneys, and proportion our dimensions to the nature of the fire to be used. In the above calculations it must be kept in view that the velocity referred to is always the velocity of the external air entering under the fire. The velocity in the chimney itself may be often very different; and by not attending to this distinction, serious mistakes may be committed. The formula for this velocity is simpler, being \( V = \frac{nH}{480} \), and in the above case it is twenty-five in place of twenty, and the supply 1500 feet in place of 1200.

In the smelting of iron, and other uses, where a more intense temperature is required than can be produced by the natural draught of a chimney, it is necessary to resort to artificial means for increasing the effect; and this gives rise to the blast-furnace, where the draught is produced by the action of bellows or blowing cylinders throwing by mechanical force a violent current of air into the fire. The same object has recently been attained in a very convenient manner by means of fans, which was originally proposed, we believe, by Desaguliers in the Philosophical Transactions, under the name of the centrifugal wheel, for the purpose of ventilating hospitals, ships, &c., and was a few years ago revived and successfully adapted to the blast-furnace by Messrs Carmichael, engineers, Dundee, and is now employed, not only for smelting, but instead of the bellows in the smith's forge, eight or ten forges being in some manufactories all supplied from one fan-wheel.

We shall now describe a few of the furnaces most generally in use. The first is the common air-furnace used for melting the metals. In this, the metal being placed in a crucible, is set in the heart of the fire, and the heat is communicated directly from the materials in combustion, of which charcoal or coke is the most powerful. The draft is produced by a chimney of proper capacity and height, and the heat generated is retained by a building of brick. The most perfect form of air-furnace of this description would be to have the chimney ascend air-firing right above the fire-grate, and to be nearly of the same capacity. In that case, the draft would operate with the most powerful effect. But in practice it is found more convenient to have the use of the space immediately above the fire-place for getting in and out the crucibles and the metal. The chimney is therefore set to a side, and the draft directed into it by a sloping vent, or even by a level flue, the height of the chimney making up for any deficiency in the direct course of the heated air. Plate CCLI. fig. 1, shows a furnace of this description, being a section and side elevation. A shows the fire-place or body of the furnace, with a melting-pot or crucible on its stand. The stand is often omitted, but is useful in raising the crucible above the grate, so as to allow the bottom of it to receive the full heat of the fire. B is the sloping flue, terminating in the chimney. C, and D is the cover or door for closing the opening when the crucible is set. The angle E at the top of the inclined vent serves for setting a crucible to heat when necessary. a is the grate, and F the ash-pit opening through the outer wall, or into a cellar below, which serves to prevent the cold air rushing in from injuring the workmen. d is the damper. Fig. 2 is a plan of the same furnace, showing the furnace-bars, &c.

Fig. 3 is a section of another air-furnace, with a horizontal flue above the fire-place. Fig. 4 is a section of a brass-founder's melting-furnace, which is found to answer the purpose extremely well. The body of the furnace is circular, composed of three courses of bricks formed to the arch, as shown in the plan, fig. 5. At the top and bottom, and at the two intermediate joints of the bricks, the whole is bound with hoops like a barrel, forming a very strong and durable construction. The flue is horizontal, and of small capacity compared with the fireplace. Above all there is an iron plate, with a flanch in front, holding the brick-work together, and an iron cover, a, shutting the opening. A is the ash-pit; B the pavement on the floor of the workshop. The next sort of air-furnace is what is termed the reverberatory furnace. In this the metal is acted on not by the heat generated among the combustible materials in the heart of the fire, but by the action of the flame and the heated air striking against it as it ascends up the chimney. The fuel best suited for this furnace, therefore, is coal, not coke; and for the purpose of directing the current, the space above the fire-place is arched over, and a horizontal or slightly descending flue is extended from this space to about three or four times the length of the fire-place, until it terminates in the bottom of the chimney. In this flue the metal is placed, and the flame and heated air striking against the top of the arch above the fire-place, is from thence reflected with full force against the metal, and from this sort of reverberation which takes place the furnace takes its name. From this action, and from the flame heating the interior of the chimney, the effect of this furnace is very powerful; and it is most extensively used in the iron manufacture.

At Plate CCLI. figs. 6, 7, and 8, are represented a plan and sections of one of these reverberatory furnaces. A is the fire-place with the arch above; B the descending flue; C the chimney; d the damper; E the ash-pit; F is an opening for introducing the metal or other substance to be acted on, which is laid on the inclined hearth at B, where the flame, reverberating from the top of the arch, strikes with full force. If the substance is intended to be melted, it runs down, and can be taken out at the opening G.

At fig. 9 is a view of another reverberatory furnace, contrived by Dr Black. The hearth or bottom of the flue B here is horizontal. Immediately above the fire-place there is an arched cover, H, of iron, from the top of which an arch of brick-work is extended all the way to the chimney, in which the flame reverberates down upon the hearth. There is only one opening, F, for the introduction of the metal. This furnace may be used with advantage for roasting various substances, such as the ores of metals, for the purpose of expelling their volatile matter. It may also be employed for the cupellation of metals, the door, F, being a little opened for admitting atmospheric air.

A furnace of great power was constructed and employed by Mr Mushet, in his numerous and valuable experiments on iron and steel, and was found very convenient for such operations. It is represented at Plate CCLII. figs. 1, 2, 3. Fig. 1 is the section of an assay or melting and annealing furnace, and also a small reverberatory furnace for fusing in very high heats with the flame of pit-coal.

The assay-furnace is cased in cast iron, with a flanch projecting inward the breadth of a brick, and about half an inch more, which serves instead of bearers for the bars (see fig. 2 at D). Upon this flanch the brick-work is reared. It ought to be of good fire-bricks on the bed. The furnace is nine inches square; total height twenty-seven inches. From the top of the flanch to the bottom of the flue the interval is eighteen inches; the flue is four inches high; the height above is five inches; flue seven inches long, and keeps opening into the chimney, as may be seen at fig. 3 at E. If the chimney is under twenty-five feet in height, a larger flue is requisite; and if beyond thirty-five feet, a smaller flue will throw the heat more regularly through the furnace. In general, however, more harm ensues from too small than too large a flue. G is the floor-line, and also represents the edge of a grate which covers the ash-pit, which is better seen on the ground-plan, fig. 3, and in fig. 2 at H. This grate lies nine inches from the bars, having an open space for the admission of air. It projects twenty-four inches outwards, and serves the operator to stand upon.

I I, in figs. 1 and 2, is the ash-pit. In fig. 3, which is a ground-plan of the chimney and furnaces, C is the annealing or cementing furnace, in which the crucibles are annealed or baked to a bright red heat, and from thence introduced, along with the matter to be operated upon, into the assay-furnace. It also serves instead of a cementing furnace, being easily made to produce a heat of 100° of Wedgwood. It may be made of any size, from nine to fourteen inches square; a nine-inch chimney being sufficiently wide to the extent of an eighteen-inch furnace.

The chimney to each furnace is carried up five feet perpendicular; they then gradually incline to the centre opening, which they enter about twelve feet above the flues. L, L, L are dampers. From the grates of this assay-furnace to the top of the chimney the interval is thirty-three feet.

This furnace has melted 400 grains of malleable iron in ten minutes, and half a pound from lumps in forty minutes. If the materials to be operated upon are prepared with judgment, any experiment to the extent of half a pound of matter may be performed in half an hour, and less quantities in much less time. When approaching to its highest heat, a Stourbridge clay crucible (which drops in 165° of Wedgwood) will disappear in fifteen minutes from the time that it is put in. The first five bring it to 140° of Wedgwood, at which cast iron boils. Steel boils in it at 160°, and malleable iron boils in it at 170° to 172° of Wedgwood. It is probable, however, that the advantages of this furnace do not result from the height of the chimney (which is not so great), or from the size of its opening. More, it is likely, depends upon the size, the opening of the grate bars, the size of the fuel, and particularly the feeding of the fire.

In many cases it is desirable to heat the articles of manufacture without exposing them to the smoke or dust arising from the fire or flame. This gives rise to a variety of furnaces variously adapted to the purpose; a kitchen oven is a furnace of this description; also the brass-founder's hot-plate or lacquering furnace; the furnaces used by enamellers; and all the different kinds known by the name of muffle-furnaces, in which a close vessel or cavity is formed above the fire-place, or in the draft of heated air from it, with a door opening externally, and by this the articles to be heated are introduced. Plate CCLII. fig. 4, represents a very convenient lacquering furnace or hot-plate for brass-founders. It stands at some distance from the wall of the apartment. The fire-place is immediately under the plate, and the flue carried horizontally into the chimney. Above the plate there is a square cover or box of tin plate, open below, which can be let down over the plate, and encloses the articles as in an oven.

Figs. 5 and 6 represent a convenient furnace for enamellers. It is merely a common air-furnace, similar to those already described, with a close vessel of earthenware placed over the fire-place, the latter being swelled out to allow the heated air to ascend on each side of the vessel. A is the fire-place; B the ash-pit; a the grate; C the vessel with its door; D the chimney; and d the damper; the other parts to be understood. This furnace may also be used for assaying metals by cupellation.

Figs. 7, 8, and 9, represent a muffle-furnace for producing very intense degrees of heat, and which was employed by Pott, and afterwards by D'Arcey, in their experiments on earths and stones exposed to a continued and violent heat. The fire-place or body of the furnace, B B, is in the form of an oblong coffin, swelling out in the middle. A shows the hole for the muffle; and the dome or upper part of the furnace is shown in D D, with a large door C, for introducing the fuel; g is the grate; i the ash-pit; and the other parts will be understood.

For an account of the glass-house furnaces, and these In regard to furnaces for steam-boilers, the object of these, as already mentioned, is to produce rapidly a large quantity of heat for the generation of the steam. For this purpose the fire-place and grate are of great extent, and the flame and heated air, after striking the bottom of the boiler, is conveyed in flues along the bottom, and then round and round the sides, so as to deliver the whole of the heat, or as much of it as possible, to the water, before it ascends into the chimney. Figs. 8, 9, 10, Plate CCLIII. represent a furnace and boiler of this description. A is the fire-place; B, B, the grate bars; C, C, the boiler; D, D, D, the flues running under and around the boiler, and terminating in the chimney, E. For a more particular account, we must refer to the article Steam-Engine.

In all these furnaces, and particularly those for steam-engines, a very important object has often been attempted, namely, a consumption of the immense volumes of smoke occasioned by the large fires which must be kept up for their use. For an account of these, see Smoke and Steam-Engine.

Figs. 11, 12, 13, and 14, represent the forms of fire-tongs found useful in the operations connected with furnaces.

Furnaces are of very extensive use in the numerous processes of chemistry, and are variously constructed, according to the notion of the chemist, the uses required, or the means within his reach. They may be divided into three kinds; crucible-furnaces, wind-furnaces, and blast-furnaces. The first are used with charcoal, or, in the larger ones, with charcoal mixed with coke. The draft for the supply of air is obtained from the mere height of the crucible, or by the addition of a funnel pipe. In the wind-furnaces, the draft is obtained by connecting them with the chimney of the laboratory, and therefore more powerful, and better adapted for various uses. In the blast-furnace, again, the air is supplied by bellows; and furnaces of this kind are capable of producing the most intense degree of heat required. The crucible-furnaces are thus described by Professor Faraday, in his well-known work on Chemical Manipulation.

An exceedingly useful furnace, either in a large or small laboratory, may be made out of a black-lead or earthen crucible. The proper crucibles for this purpose are known by the name of blue pots, and may be had of almost every size less than the height of twenty-two inches, and of twelve or fourteen inches diameter in the top. They are made of clay and plumbago mixed, and are easily cut by a saw, rasp, or file. The price of one which will make a very good furnace for small operations is about six shillings.

One of these vessels, of the height of twelve inches, and seven inches in width at the top within (Plate CCLIII. fig 1), will make a very useful furnace for the igniting of a small crucible, heating a tube, or distilling with large glass retorts at moderate temperatures, or with smaller glass or earthen retorts at higher temperatures. It is first, in the course of preparation, necessary to have certain round holes pierced in it. These are easily made; a gimlet, bradawl, or other small instrument, is to be used to penetrate the sides; and the small apertures thus produced are to be enlarged with a rat's-tail and round rasp, and ultimately finished with a half-round rasp, which will make them of the size required. Four of these holes are to be placed at equal distances from each other, and about two inches from the bottom of the pot. They may be one and a fourth or one and a half inch in diameter. A second ring of holes, five or six in number, is to be made halfway between the top and bottom of the pot, and a third row of five or six within two inches of the top. These holes should be rather smaller than the four lower ones.

The pot should now be bound round with iron or copper wire, to hold it together when it cracks, and a handle made to it of iron wire.

A small round grate of cast iron is only wanting to render this furnace complete for many operations; and several of these grates of different sizes should be ready at hand to drop in, and fit at different heights, as may be required. The part below the grate then forms the ash-pit, and the part above forms the body of the furnace. For the purpose of regulating the fire, if this requires to be diminished, the air-holes can be closed with stoppers made of soft brick or old blue pots; and, to increase the temperature, the body of the furnace itself may be enlarged by setting on the top of it a portion of an old pot, as in fig. 2, cut off so as to form a ring, several of which may be added as occasion requires; and they increase considerably the draft as well as the capacity of the furnace.

Another simple method of increasing the power of these furnaces is to set on the top a piece of straight funnel pipe, as at fig. 3, two feet long, four inches diameter, opening out below like a funnel, till it is about eight inches diameter, with a wooden handle for the convenience of taking off or putting on. These simple furnaces are very powerful, and are capable, without difficulty, of raising a crucible two and a half inches in diameter to a white heat. In fact, all the ignitions and heatings which belong to the analysis of siliceous and other minerals have long been made in furnaces of this kind at the Royal Institution.

If the funnel-pipe in one of these crucible furnaces be connected with any chimney or flue in the laboratory, which is easily done by kneeling the pipe at the top, and having two or three short pieces to fit in for adjusting it to different lengths, as at fig. 4, its power is greatly increased, and in fact it becomes a wind furnace. The funnel termination at the bottom should be lined with fire-clay, and have an opening for the introduction of fuel, to be closed by a stopper when the fire is in order; or a still better arrangement is to continue the furnace upwards by a deep ring with the feeding apertures in it, as at fig. 5.

On the same principle with the crucible-furnace is a small portable furnace contrived by Mr Knight, and represented at Plate CCLIV. fig. 1, and consisting of a cylinder of sheet-iron lined with an earthy composition. A B is the fire-place; B C the ash-pit, closed on all sides excepting at the register door, D, where air is admitted; E is an opening for fuel; F a recess for the neck of a retort. Fig. 2 is a funnel top for the furnace, with a pipe above extending by other lengths into the chimney. Fig. 3 is a sand-bath, which can be used in place of the dome, with a pipe rising up from it.

On the same principle is the well-known portable furnace of Dr Black, represented at fig. 4, Plate CCLIV., portable which is the most complete of the kind which has yet been contrived. It is made of sheet iron, formed into an elliptic shape for the purpose of getting a chimney separate from the body of the fire-place, and is carefully lined with clay well tempered. A B is the fire-place; B C the ash-pit; E a sliding door for the admission of fuel; another door for the same purpose, and also for the introduction of a muffle. Fig. 5 is a cover, and fig. 6 a sand-bath. F is the chimney, which is lengthened by pipes, and connected with the chimney of the house.

On the same general principle of the wind-furnace is the general laboratory or table-furnace, which, being of considerable magnitude, is fixed in the laboratory, and forms one of the most important and useful pieces of apparatus which the chemist employs. From the extreme facility which, if conveniently arranged, it gives to every operation, its use is partly domestic, partly chemical; for it has to warm and air the place, occasionally to heat water, as well as to supply the means of raising a crucible to ignition, or of affording a high temperature to flasks through the agency of a sand-bath.

These objects are best obtained by those furnaces which are built with a table top. The fire-place itself is constructed of brick-work with iron front and fittings; and the flue being carried horizontally for three or four feet, is afterwards carried off to and connected with the main flue existing in the wall. The fire-place and horizontal flue are covered with a large plate of cast iron, of from two to three feet in width. This is formed in the middle, over the heated part, into sand-baths; a round moveable one over the fire itself, and a long fixed one over the flue. The sand-baths supply every gradation of heat, from dull redness, if required, down to a temperature of 100° or lower; whilst on each side of them exists a level surface, which answers every purpose of an ordinary table, and supplies extraordinary facilities to experiments going on in the sand-bath or furnace. Nor are these advantages gained by any serious sacrifice of heating power in the furnace itself; for it is easy so to construct it as to make its ordinary combustion not more rapid than that of a common fire, and yet, by closing the fire-door and opening the ash-pit, to obtain a heat that will readily melt gold, silver, or cast iron.

A furnace like this is best placed in the middle, or towards one end of the laboratory, independent of the wall; for then it most effectually warms the air of the place, and there is working room all round it. The flue may then either descend and be carried off for a short distance under ground, or it may be connected by a funnel pipe with the upright draught chimney. But if more convenient, either as occupying less of the room of a small laboratory, or for other reasons, it may be placed with advantage against one side; and where the laboratory is made out of a room previously built, the best situation is generally against the fire-place, and the flue of the furnace is then easily connected with the chimney previously existing. When a furnace of this kind stands against the wall, it is frequently advantageous to construct a wooden hood over the sand-bath, to receive the fumes evolved during the digestions and solutions made upon it, and conduct them away to the chimney.

Being in constant requisition as a table, a furnace of this kind should be about thirty-four or thirty-five inches in height; its other dimensions, and even its form, must depend upon the space that can be allotted for it; and the following is a more particular description of the one in the laboratory of the Royal Institution, constructed under the direction of Mr Brande. It is represented at fig. 7, Plate CCLIV., and in section at fig. 8 through the line AB. It has the brick-work fifty-two inches in length and thirty-eight in width; the iron plate, including sand-bath, being fifty-seven inches by forty-two. Others have been constructed, the plates of which are only forty inches by twenty-seven. The principal part of this furnace is necessarily of brick-work, only the top plate, with the backs and the front, being of iron. The front is a curved iron plate, having two apertures closed by iron doors, one belonging to the fire-place, and the other to the ash-pit. It is thirty-four inches high and fourteen inches wide. The ash-hole door moves over the flooring beneath; the bottom of the fire-place door is twenty-two inches from the ground, and the door itself is eight and a half inches by seven. This front is guarded within, at the part which encloses the fire, by a strong cast-iron plate, having an opening through it, corresponding to the door of the fireplace. It has clamps attached to it, which, when the furnace is built up, are enclosed in the brick-work. In the setting or building of the furnace, two lateral brick walls are raised on each side the front plate, and a back wall at such a distance from it as to leave space for the ash-hole and fire-place. These walls are lined with Welsh lumps where they form the fire-chamber. Two iron bars are inserted in the course of the work, to support the loose grate bars in the usual manner, the grate being raised nineteen inches from the ground. The side walls are continued until of the height of the front, and are carried backward from the front until in two parallel lines, so as to afford support for the iron plate which is to cover the whole. The back wall of the fire-place is not raised so high as the side walls by six inches and a half, the interval which is left between it and the bottom of the sand-bath being the commencement of the flue or throat of the furnace. In this way the fire-place, which is fourteen inches from back to front, and nine inches wide, is formed, and also the two sides of the portion of the horizontal flue which belongs to the furnace, and is intended to heat the larger sand-bath. The bottom of this part of the flue may be made of brick-work, resting upon bearers laid on the two side walls; or it may be a plate of cast iron, resting upon a ledge of the brick-work on each side, and on the top of the wall which forms the back of the fire-place. When such an arrangement is adopted, the plate must not be built into the brick-work, but suffered to lie on the ledges, which are to be made flat and true for the purpose; for, if attached to the walls, it will by alternate expansion and contraction disturb and throw them down. The ends of the side walls, forming as it were the back of the furnace, may be finished either by being carried to the wall against which the furnace is built, or enclosed by a piece of connecting brick-work, to make the whole square and complete; or a warm air cupboard may be built in the cavity beneath the flue, and the door made to occupy the opening between the walls. Occasionally the flue may be required to descend there, and pass some distance under ground. These joints should be arranged and prepared before the plate constituting the top of the furnace is put on to the brick-work, so that when the plate with its sand-baths are in their places, they may complete the portion of the horizontal flue by forming its upper side.

The size of this plate is the first thing to be considered, and having been determined upon from a consideration of the situation to be occupied by the furnace, and the places of the sand-bath also having been arranged, the brick-work must then be carried up, so as to correspond with these determinations, and with the plate itself, which in the mean time is to be cast. The sand-bath and the plate are to be formed in separate pieces. The bath over the fire is best of a circular form, and of such diameter, that when lifted out of its place, it may leave an aperture in the plate equal in width to the upper part of the fire-place beneath; so that a still or cast-iron pot, or a set of rings, may be put into its place over the fire. The other sand-bath must be of such a form as to correspond with the shape and size of the flue beneath. These vessels are to be of cast iron, about three tenths of an inch thick; their depth is to be two and a half inches or three inches, and they are to be cast with flanges, so as to act in the corresponding depressions of the plate, that the level of the junctions may be uniform. This will be understood from the accompanying sections of the furnace, see fig. 8, given through the line A B of the view. It is essential that these sand-baths be of such dimensions as to fit very loosely into the apertures in the plate, a space of the eighth of an inch or more being left all round them when cold, as shown in the section, otherwise, when heated, they will expand so much as entirely to fill the apertures, and even break the plate. The plate itself should be half an inch thick.

When the plate and its sand-baths are prepared, and the brick-work is ready, the furnace is finished by laying the plate on the brick-work, with a bed of mortar inter- vening. If the walls are thin, or any peculiarity in their arrangement occasions weakness, they should be bound together, within by cranks built into the work, and without by iron bands. The alternate changes of temperature from high to low and low to high, to which the furnace is constantly subject, renders it liable to mechanical injury, in a degree much surpassing that which would occur to a similar piece of brick-work always retained nearly at one temperature.

The sand-baths which have been described are liable to an accident that has induced some chemists to substitute others made of wrought iron. When first heated, they frequently, indeed generally, crack from the unequal expansion in different parts; and the plate itself is subject to the same accident. If constructed of wrought iron, this effect is not produced; but then, after being used some time, they warp into very irregular and inconvenient forms, especially if made of thin metal; whilst, on the contrary, those of cast iron, when cracked, are rarely injured for the uses to which they are to be applied, and seldom suffer further change.

These baths should have washed sand put into them. It is heavy, and occasions no dust when moved; whilst, on the contrary, unwashed and bad sand contains much dirt, and occasions great injury in experimenting. A piece of straightened iron hoop, about twelve inches in length, should lie on the furnace, as an accompaniment to the baths, being a sort of coarse spatula with which to move away the sand.

The circular sand-bath is frequently replaced by a set of concentric rings, or a cast-iron pot, see fig. 9. The rings are convenient for leaving an aperture over the fire of larger dimension, according as a larger or smaller number are used at once; and being levelled at the edges, fit accurately into each other, without any risk of becoming fixed by expansion. The external one, like the sand-baths, should be made smaller than the depression in the furnace plate in which it rests. The iron pots are of various sizes, and adapted to the furnace by means of the rings. A red heat is easily obtained in them for sublimation.

In cases where a greater heat is required than can be obtained by the table furnace, or any of the portable furnaces already described, other wind furnaces may be constructed by proportioning the size of the chimney to that of the furnace, much surpassing these in the intensity of heat produced; but in these and other cases it is better to have recourse at once to the blast-furnace, the operations being more manageable and expedient, the heat greater, and the consumption of fuel smaller. By a little contrivance, one of the crucible furnaces before described is easily converted into a blast-furnace, and a very high temperature for small vessels obtained. This is done by closing the holes of the ash-pit with the stoppers, except one, and applying to that the nozzle of a pair of double-hand bellows, from which a draft is to be urged, and the furnace aided at the same time by the piece of upright funnel pipe; the fuel is to be charcoal.

A very powerful blast-furnace on a small scale has been contrived by Mr Aikin; it is represented at fig. 10, and is all made out of broken pots. The lower piece, A, is the bottom of one of these pots, cut off so low as only to leave a cavity of about an inch deep, and ground smooth above and below. The outside diameter over the top is five and a half inches. The middle piece or fire-place, B, is a larger portion of a similar pot, with a cavity about six inches deep, and measuring seven and a half inches over the top, outside diameter, and perforated with six blast-holes at the bottom. These two pots are all that are essentially necessary to the furnace for most operations; but when it is wished to heap up fuel above the top of a Furnace crucible contained, and especially to protect the eyes from the intolerable glare of the fire when in full height, an upper pot, C, is added, of the same dimensions as the middle one, and with a large opening in the side, cut to allow the exit of the smoke and flame. It has also an iron stem with a wooden handle (an old chisel answers the purpose very well), for removing it occasionally. The bellows, which are double, D, are firmly fixed by a little contrivance, which will take off and on, to a heavy stool, as represented in the plate; and their handle should be lengthened so as to make them work easier to the hand. To increase their force on particular occasions, a plate of lead may be firmly tied on the wood of the upper flap. The nozzle is received into a hole in the pot, A, which conducts the blast into its cavity. Hence the air passes into the fire-place, through six holes of the size of a large gimlet, drilled at equal distances through the bottom of the pot, and all converging in an inward direction, so that if prolonged, they would meet about the centre of the upper part of the fire. No luting is necessary in using this furnace, so that it may be set up and taken down immediately. Coke, or common cinders taken from the fire when the coal ceases to blaze, sifted from the dust, and broken into very small pieces, form the best fuel for higher heats. The fire may be kindled at first by a few lighted cinders and a small quantity of wood charcoal. The heat which this little furnace will afford is so intense, that its power was at first discovered accidentally by the fusion of a thick piece of cast iron. The utmost heat procured by it was 167° of Wedgwood's pyrometer, when a Hessian crucible was actually sinking down in a state of porcelainous fusion. A steady heat of 155° or 160° may be depended on if the fire be properly managed and the bellows worked with vigour.

The following is a description of a most excellent blast-furnace, which has been in use for some years in the laboratory of the Royal Institution, and is represented at fig. 11. It is sufficiently powerful to melt pure iron in a crucible in twelve or fifteen minutes, the fire having been previously lighted. It will effect the fusion of rhodium; and even pieces of pure platinum have sunk together into one button in a crucible subjected to its heat. All kinds of crucibles, including the Cornish and the Hessian, soften, fuse, and become frothy in it; and it is the want of vessels which has hitherto put a limit to its applications. The exterior consists of a blue pot, eighteen inches in height and thirteen inches in external diameter at the top. A small blue pot of seven and a half inches internal diameter at the top had the lower part cut off so as to leave an aperture of five inches. This, when put into the larger pot, rested upon its lower external edge, the tops of the two being level. The interval between them, which gradually increased from the lower to the upper part, was filled with pulverized glass-blowers' pots, to which enough of water had been added to moisten the powder, which was pressed down by sticks, so as to make the whole a compact mass. A round grate was then dropped into the furnace, of such a size that it rested about an inch above the lower edge of the inner pot: the space beneath it, therefore, constituted the air chamber, and the part above the body of the furnace. The former was seven and a half inches from the grate to the bottom, and the latter seven and a half inches from the grate to the top. Finally, a horizontal hole, conical in form, and one and a half inch in diameter on the exterior, was cut through the outer pot, forming an opening into the air-chamber at the lower part, its use being to receive the nozzle of the bellows by which the blast was to be thrown in. The furnace being thus completed, the next object was to dry it gradually, that Furneaux when used it might not be blown to pieces by confined aqueous vapour; a charcoal fire was therefore made in it, and left to burn some hours, being supplied with air only by the draught through the hole into the chamber beneath. When vapours ceased to be formed, the furnace was considered to be ready for use.

This furnace has always been used with a pair of large double bellows mounted in an iron frame, the furnace being raised upon a stool, so as to bring the aperture of the air-chamber to a level with the nozzle of the bellows. The latter has generally been inserted in the aperture; for this and similar furnaces are of such depth compared to their width, that when charged with a crucible and fuel, there is so much resistance to the passage of the air, when urged by a blast competent to create and sustain a vivid combustion, that a part returns by the side of the nozzle, if the aperture be left open. The bellows spoken of is far larger than necessary for the furnace described, and is rarely worked to one third of its power; for otherwise the heat rises so high as to destroy the crucible, and the results are lost. It is, however, at all times advisable to have an abundant command of air.

The heat produced in this furnace is such as at every violent operation to cause the production of some slag from the melting of the inner surface of the furnace itself, where the combustion has been most vivid. The slag running down the interior, collects round the edge of the grate, and should be removed with a chisel and hammer, or with an iron rod, after each operation, that the grate may be clear and free of obstruction for the next process. When in the course of time the interior of the furnace is so far injured as to become thin and weak, it must be displaced, and the furnace restored to its original state by the introduction of a new inside, as before.

The fuel to be used in this furnace is coke. Its consumption is very small, considering the heat that is obtained in consequence of the short period of each operation. The superiority of the blast-furnace over the wind-furnace, in many operations for which high temperatures are required, depends upon the rapidity of its action. It is requisite to employ this furnace in the open air, or under a well-arranged vent; for an immense number of sparks, much flame, and a current of hot air, are projected during its operation, which might occasion serious mischief in a room, unless the ceiling were at a considerable height, or guarded by a metal screen.

The fuel to be used in furnaces is of three kinds, coal, coke, and charcoal. Coal is the ordinary fuel for the laboratory table furnace, or that intended to be in use every day, and to serve for fusions, roastings, and other operations, for which its temperature may be sufficient. It is very desirable that this coal should be good, and not of the kind which contains much sulphur, or an abundance of earthy matter; for the first interferes with various fusions and ignitions, and the latter renders the fire dirty and dusty, and, when the temperature is raised to a high point, causes an abundance of clinkers. On certain occasions to be hereafter distinguished, especially if the coal be sulphureous and bad, it may be necessary at times to use both coke and charcoal in the table-furnace. Coal should never be used in the blast-furnace; for, in consequence of its softening and swelling by heat, it aggregates, closes the small channels by which the air finds a passage through the fuel, and impedes the combustion.

Coke is in constant requisition. It varies in quality with the coal from which it is obtained. Such as is intended for the service of the blast-furnace should be free from sulphureous, earthy, and metallic matter. Of this kind is the Staffordshire coke, which may be obtained at various wharfs on the canals near London. It is frequently so little altered in appearance as to resemble the original coal. It burns completely away in a blast-furnace, leaving scarcely a trace of slag, so that after several successive portions have been introduced, no material quantity of refuse is produced upon the grate, nor any thing that will act seriously on the crucible as a flux.

On the contrary, if common gas-coke be used in this furnace, the oxide of iron and earthy matter which it contains is so abundant, that slag is soon produced, which flowing over the crucible, corrodes and destroys it.

The charcoal intended for laboratory use may be of the ordinary kind, and must not be either too large or too small. If large, the pieces should be broken down, or they will be unfit for use in the crucible furnaces, for which it is principally intended. Charcoal is a quick fuel; but burning with facility, a small quantity of it can be easily retained in a state of regular combustion; and hence, in cases where but little space intervenes between the substance to be heated and the side of the furnace, or when a small temporary fire is required in the air, it is very convenient. Where Staffordshire coke will burn, and, by means of a blast or a draught of air, will give sufficient intensity of heat, it is very superior to charcoal in duration. Occasionally a mixture of coke and charcoal is convenient, since it affords a combination possessing the qualities of permanency and freedom of combustion.

A charcoal box is almost as essential in a laboratory as one for coal, and should have its appointed place.

It must be remembered, that all operations with furnaces should be carried on in safe situations; care being taken that no danger be incurred by the ascent of sparks, flame, or hot air, by lateral vicinity to combustible bodies, or by standing on an unprotected wooden surface.

When small furnaces are placed upon tables, stools, or trays, a brick or a piece of sheet iron should be interposed, according to the mode by which the heat is likely to be communicated.

For further information on the subject of furnaces, see the articles already referred to, and others connected with particular arts where they are used. See also Lewis's Philosophical Commerce of Arts; Faraday's Chemical Manipulation; Parke's Chemical Essays; Aikin's Dictionary of Chemistry; Tredgold on Warming and Ventilating Buildings; Tredgold's Account of the Steam-Engine; Davies Gilbert, Quarterly Journal of Science, vol. xiii.; Watt's Patent Furnaces, Repertory of Arts, vol. iv. p. 298; Raley's Patent Furnaces, Repertory of Arts, vol. x. p. 153; Miche on Reverberatory Furnaces, in Rozier's Journal, vol. xxxii. p. 385; Howard's Improved Air Furnace, in Tilloch's Philosophical Magazine, vol. v. p. 190; Percival's Chamber Lamp Furnace, Repertory of Arts, vol. iii. p. 28, and in the Transactions of the Royal Irish Academy, vol. iv. p. 91; Accum's Improved Universal Furnace of Dr. Black, in his System of Practical Chemistry, vol. ii. p. 357, and in Nicholson's Journal, vol. vi. p. 273, 8vo; Curaud's new Evaporating Furnace is described in the Annales de Chimie, No. 149, an. xii.; and in Nicholson's Journal, vol. ix. p. 204, 8vo.