an utensil or vessel proper to contain fire, or to raise and maintain a vehement fire in, whether of coal or wood.—Of these there are a great variety, according to the different uses to which they are applied.

In all furnaces the principal things to be attended to are, 1. To confine the heat as much as possible to the matter to be operated upon; 2. To prevent its being dissipated; 3. To produce as much heat with as little fuel as possible; and, 4. To have it in our power to regulate the degree of heat according to our pleasure.

To answer the first intention, the fire is usually confined in a chamber or cavity built on purpose for it, and furnished with a door for putting in the fuel; a grate for supporting it, and allowing air to pass through, as well as the ashes to drop down into a cavity provided on purpose, and called the ash-pit. Thus the heat produced by the inflamed fuel is confined by the sides of the furnace, and obliged to spend great part of its force upon the subject inclosed.

The second intention, viz. to prevent the dissipation of the heat, is obtained by shutting the door of the furnace; taking care that the chimney be not too wide, and that the matter to be acted upon be placed in such a manner that the fire may have its full effect upon it as it goes up the chimney.

The third intention, which is the most important, is at the same time the most difficult to answer, and depends entirely upon the proportion between the spaces betwixt the furnace bars and the wideness and height of the chimney. This will appear from a consideration of the principles on which the degrees of inflammation are produced. These depend entirely on the current of air which passes through the inflamed fuel. As soon as the fuel is set on fire, a certain degree of heat is produced; but unless a constant influx of air is admitted through the burning fuel, the fire is instantly extinguished; nor is it possible by any means to renew the inflammation until we admit a stream of fresh air among the fuel. When this is done, a rarefaction commences in the air of the fire-place of the furnace; so that it is no longer a counterpoise to the external air, and is therefore driven up the chimney by that which enters at the ash-pit. This again passing through the fuel, is rarefied in its turn; and giving place to fresh quantities, there is a constant flow of air up the chimney. In proportion to the rarefaction of the air in the fire-place, the greater is the heat. But by a certain construction of the furnace, the under part of the chimney will become almost as strongly heated as the fire-place; by which means, though a very strong current of air is forced through the fuel, yet as great part of the heat is spent on the chimney, where it can be of no use, the fuel is wasted in a very considerable degree. To avoid this, we have no other method than to contract the throat of the chimney occasionally by a sliding plate; which when put quite in, shuts up the whole vent; and by being drawn out more or less, leaves a larger or smaller vent at pleasure. This plate ought to be quite drawn out till the fuel is thoroughly kindled, and the furnace well heated, so that a current of air may flow strongly through the fuel. After this the plate is to be put in a certain length, so as just to prevent the smoke from coming out at the door of the furnace. The rarefaction of the air in the fire-place will solicit a very considerable draught of air, which will keep the fuel inflamed to a great degree; at the same time that the heat, being reflected from every part of the furnace excepting that narrow passage where the smoke goes up, becomes very intense. A large quantity of fuel may be put in at once, which will consume slowly, and thus require but little attention in comparison with those furnaces where no such precaution is used. The sliding-plate may be made of cast-iron in those furnaces where no great heat is excited; but in others fire-clay will be more convenient. The contrivance, however, is scarce applicable to those furnaces where great quantities of metal are to be melted; and accordingly the waste of fuel there is immense. It is computed, that the iron works of Carron in Stirlingshire consume annually as many coals as would be sufficient for a city containing 700,000 inhabitants.

The fourth intention, viz. that of regulating the heat, is accomplished by allowing only a certain quantity of air to pass through the fuel. For this purpose, according to Dr Black, it is necessary to have the command of the furnace below; the parts above being frequently filled with small quantities of foot. The best method of managing this is to shut up the door of the ash-hole perfectly close, and to have a set of round holes bearing a certain proportion to one another; and their areas being as 1, 2, 4, 8, 16, &c. Seven or eight of these ought to be made in the door of the ash-pit, which will give a sufficient command over the fire. When the fire is to be increased to the utmost, all the passages both above and below are to be thrown open, and the height of the vent augmented; which, by increasing the height of the column of rarefied air, increases also the motion of that through the fuel, and of consequence also the heat of the furnace. Macquer recommends another tube applied to the ash-pit, widest at the end farthest from the furnace, and tapering gradually towards it. The intention of this is to augment the current and velocity of the air by its being made to pass from a wider into a narrower vent; but though this is no doubt true, the air will not ultimately move with greater velocity than if the tube were not there. It can only be useful therefore in cases where the furnace is placed in a small room, and the tube itself has a communication with the external air.

An Effluy or Cupelling Furnace is made in the following manner. 1. Make with iron plates a hollow quadrangular prism, eleven inches broad and nine inches high (aa, bb), ending at top in a hollow quadrangular pyramid (bb, cc) seven inches high, terminating in an aperture at top seven inches square. This prism must be closed at bottom with another iron plate, which serves as a basis or bottom to it (aa). 2. Near the bottom make a door (e), three inches high, and five inches broad, that leads to the ash-hole. 3. Above this door, and at the height of six inches from the basis, make another door (f), of the figure of a segment of a circle, four inches broad at its basis, and three inches and a half high in the middle. 4. Then fasten three iron plates on the forespart of this furnace. Let the first of them (gg), eleven inches long and half an inch high, be fastened, so that its lower edge shall rest against the bottom of the furnace, with three or four rivets; and in such a manner, that there may be between the upper edge of the said plate and the side of the furnace a groove so wide, as that the sliders of the lower door (kk) may be put into it, and freely move backwards and forwards therein: these must be made of a thicker iron-plate. The second iron-plate (bb), eleven inches long, three inches high, and perfectly parallel to the foregoing plate, must be fastened in the space between the two doors, in such manner that both the upper and the lower edges of it may form a hollow groove with the side of the furnace. One of these grooves, which is turned downwards, serves to receive the upper edge of the sliders that shut the lower door (No 2). The other, that turns upwards, is to receive the inferior edges of the sliders of the small door above (No 3). The third plate (ii), which is like the first, must be riveted close above the upper door, in such manner, that it may form a groove turning downwards, and contiguous to the upper edge of the upper door (No 3.) 5. In order to shut both doors (No 2, 3.), you must adapt to each of them two sliders made of iron-plates, that may move within the above mentioned grooves (kk, ii). But the two sliders belonging to the upper door (No 3.) must have each a hole near the top; that is, one a small hole one fifth part of an inch broad, and one inch and a half long (m); and the other a semicircular aperture, one inch high and two inches broad (n). Let, besides, each slider have a handle, that they may be laid hold of when they are to be moved. 6. Moreover, let five round holes, one inch broad, be bored in the furnace; two of which must be made in the forespart of the furnace (oo), two others in the back part; all at the height of five inches from the bottom, but three inches and a half distant from each side of the furnace; and, finally, a fifth hole (p), at the height of one inch above the upper edge of the upper door (f). 7. In short, let the inside of the furnace be armed with iron-hooks, jetting out half an inch, and about three inches distant from each other, to fasten the lute with which the furnace is to be covered over within. 8. Let then an iron, moveable, hollow, quadrangular pyramid (g), three inches high, be adapted to the upper aperture (d) of the furnace, at the basis seven inches broad, ending upwards in a hollow tube (r), three inches in diameter, two inches high, almost cylindrical, though somewhat convergent at top. This prominent tube serves to support a funnel or base, which is almost cylindrical, hollow, made of iron plates, and two feet high; and which, when a very strong fire is required, is put perpendicularly upon the shorter tube, in such a manner, that it enters close into it, one inch and a half or two inches deep, and may again be taken off at pleasure, when there is no need of too strong a fire. But this pyramidal cover (g) must besides have two handles (ss) adapted to it, that it may be laid hold of, and thus be taken away or put on again: and that this, being put on the aperture (d) of the furnace, may not be easily thrown down, let an iron plate be rivetted to the right and left upper edge of the furnace (cc), and be turned down towards the inside, so as to make a furrow open before and behind, into which the latter edges of the cover may enter and be fastened, and at pleasure be moved backwards and forwards, whenever it must be put on, or moved.

9. Let a square ledge, made of a thick iron-plate, be fastened at top of the upper edge of the lower door (e): this is designed to support the grate and the lute: but it must be made of two pieces, that it may be easily introduced into the cavity of the furnace. Thus you will have an affay-oven, which must afterwards be covered over on the inside with lute. This you are to do as follows:

That the fire may be better confined, and that the iron may not be destroyed by growing red-hot, the whole inside of the furnace must be covered over with lute, one finger or one finger and a half thick. The lute fit for this is described under the article Chemistry, No 604, 605. But before you cover the inside of your furnace with this lute, you must first put within the furnace small iron bars, equal in length to the diameter of the oven, quadrangular, prismatical, half an inch thick, having their extremities supported by a square iron ledge, and three fourths of an inch distant from each other; and you must fasten them so, that their flat sides may be oblique with regard to the transverse section of the furnace, and that the two opposite angles may look one upwards and the other downwards: the bars must not be laid flat, but edgewise; by which situation you hinder the ashes of the fuel of the fire from being detained too long between the interstices of the said iron-bars, and from making an obstruction that would oppose the free draught of the air. The furnace being then covered over with lute, and dried up by a gentle heat, is at last fit for doctrinal operations, and especially for such as must be performed in the affay-oven.

If then an operation is to be made in the furnace hitherto described, you must let through the four lower holes above described of the furnace (oo) placed before and behind, and directly opposite to each other, two iron-bars one inch thick, and long enough that their extremities on every side may jut out of the holes a small matter. These serve to support the muffle and its bottom. You then introduce the muffle through the upper aperture of the furnace (d), and place it upon the above described iron-bars, in such a manner, that the open fore-side of it be contiguous to the inward... Furnace. ward border of the upper door (f). The fuel of the fire is introduced through the top of the furnace (d); the cover of which (g), on this account, must be moveable, and not very heavy. The belt fuel for the fire is charcoal made of the hardest wood, especially of beech, broken into small pieces of the bigness of an inch, wherewith the muffle must be covered over some inches high. We then reject larger bits of coal, because they cannot fall through the narrow interstices, between the sides of the muffle and those of the furnace, and cannot of course sufficiently surround the circumference of the muffle. Whence it happens, that there are on every side places void of fuel, and the fire is either not strong enough or unequal. But if, on the contrary, you use coals too small, then a great part fall immediately through the interstices of the grate into the ash-hole; and the tenderest particles of them turn too soon into ashes, and, by increasing the heap of ashes, obstruct the free draught of the air, which is here greatly requisite.

A perfect management of the fire is most commonly necessary in the performing of operations in this furnace; therefore the chemical reader must give attention to what follows. If the door of the ash hole (e) is quite open; and the sliders of the upper door (f) drawn towards each other, so as to touch one another in the middle of the door; and if, besides, the cover (q), and the funnel adapted to its tube (r), is upon the top (g) of the furnace; the fire will be then in the highest degree possible; though, in the mean time, it is hardly ever necessary to put the funnel on, except in a very cold season: but if, after having disposed the furnace in the manner just described, you put red burning coals into the open upper door (f) of it, the fire is still more increased thereby: however, this artifice is never, or very seldom, necessary. When you shut the upper door with only that slider that has a narrow oblong hole in it (m), then the heat becomes a little less; but it diminishes still more when you shut the door with the other slider that has in it the semicircular hole (n), which is larger than that of the first slider: nay, the heat again is less when you take away the funnel put at the top of the cover: finally, the door of the ash-hole being either in part or totally shut, the heat is still diminished; because the draught of air so necessary to excite the fire, is thereby hindered: but if, besides all these, you likewise open the upper door quite, then the cold air, rushing into the muffle, cools the bodies put under it, that are to be changed, to a degree never required in any operation, and such as will entirely hinder the boiling of lead. If, during the operation, the fire begins to decay, or to grow unequal, it is a sign that there are places void of coals between the sides of the furnace and those of the muffle: therefore, in this case, you must stir your coals on every side with an iron-rod, which is to be introduced through the upper hole (p) of the furnace, that they may fall together, and thus act in a proper manner and equality.

However, you are to observe concerning the regimen of the fire just described, that though the apparatus is made with all the exactness mentioned, nevertheless the effect does not always answer it. The cause of which difference has most commonly its origin in the various dispositions of the air: for as every fire is more excited by coals in proportion as the air, more condensed, and more quickly agitated, strikes them more violently (which the effect of the bellows plainly shows); it thence appears, that in warm and wet weather, when the atmosphere is light, the fire must be less efficacious in furnaces; that likewise, when several furnaces, situated near each other, are burning at the same time, the fire is in part suffocated, because the ambient air is thereby rendered more rare and lighter. The same effect is produced by the sun, especially in summer-time, when it shines upon the place where the furnace is situated. The atmosphere, on the contrary, being heavier in cold dry weather, excites a very great fire.

The heat of the fire acts the stronger upon the bodies to be changed, as the muffle put in the furnace is less; as the said muffle has more and larger segments cut out of it; as the sides of this muffle are thinner; in short, as there are more vessels placed in the hinder part of the muffle; and on the contrary. In this case, when many of the conditions requisite for the exciting of fire are wanting, then indeed the artificer, with all his skill, will hardly be able to excite the fire to a sufficient degree, in order to perform operations well, in common assay-ovens, even though he uses bellows, and puts coals into the upper door of the furnace. For this reason, the grate ought to be put almost three inches below the muffle, lest the air, rushing through the ash-hole, should cool the bottom of the muffle, which happens in common assay-ovens; and again, that the smaller coals, almost already consumed, and the ashes, may more easily fall through the interstices of the grate, and the larger coals still fit to keep up the fire be retained. Lastly, the above-mentioned funnel is added, that the blowing of the fire being, by means of it, increased as much as possible, this might at last be carried to the requisite degree; for the fire may always be diminished, but not always be increased at pleasure, without the assistance of a proper apparatus.

Fig. 2. Represents a longitudinal section of a Refluxing Furnace used in the smelting of ores. 1. The masonry. 2. The ash-hole. 3. A channel for the evaporation of the moisture. 4. The grate. 5. The fire-place. 6. The inner part of the furnace. 7. A basin formed of sand. 8. The cavity where the melted metal is. 9. A hole through which the scoria is to be removed. 10. The passage of the flame and smoke, or the lower part of the chimney; which is to be carried up to a height of about 30 feet. 11. A hole in the roof, through which the ore is thrown into the furnace. This furnace is 18 feet long, 12 feet broad, and 9½ high.

Fig. 3. Represents a longitudinal section of the Refining Furnace. 1. The masonry of the pillars and walls surrounding the furnace. 2. The channels for carrying off the moisture. 3. Other small channels which join in the middle of the basin. 4. The basin made of bricks. 5. A bed of ashes. 6. The hollow or basin in which the metal is melted and refined. 7. The great flame-hole. 8. The two openings for the entry of the tuyeres of the bellows. 9. The vault or dome of the furnace. 10. The fire-place. 11. The grate. 12. The draught-hole. 13. A hole in the vault, which, being opened, serves to cool the furnace.

Portable Furnace. See Chemistry, No. 600, &c.