In reverberating furnaces, which are heated by the flame of pit coal, the maximum is much higher than in the melting furnace, which is heated with conk. This arises front the flame in the former heating the interior of the chimney. The melting furnace should have its chim ney not less than SO feet. The reverberatory furnace may with good effect be made much higher.
The chimney of a furnace should he perfectly distinct front every other, nor can it have any opening in its side without sustaining an injury proportionate to its size ; but its effect will be less, as its height above the fire is greater.
Sound philosophy and experience will ever discard the practice of attempting to make one furnace perform two operations at the sante time.
Having given some account of the principles of fur naces, we shall now describe several at present used in the arts, and in the laboratory. Plate CCLXII. Fig. 1, is a section and side elevation of a melting furnace. B is the body of the furnace, containing a crucible upon its stand. The use of the stand is to raise the crucible above the grate g, so as to allow its bottom to receive the greatest heat of the fire. A is the ash-pit, opening through the outer wall, or into a cellar below. This serves to prevent the cold air from annoying the operator. D is the damper, which when shut covers the under side of the grate. K is the cover, which is either funned by putting two fire bricks in an iron ring, or by moulding fire clay into proper form, and burning it afterwards. C is the chimney. Fig. 2. is a plan of the same furnace.
Figures 3. and 4. are a section and plan of a furnace for heating a sand-bath. A is the ash-pit, B the body of the furnace, d the door for fuel, g the grate, \V a wall, or one large fire-brick placed between the grate and the chimney, leaving no way for the smoke to pass but through the neck n. It then descends on the other side, and passes through the flue F into the chimney C. p is a plate of cast metal, formed of two plates ribbeting into each other, by which it is less liable to break by the heat than if the plate was in one. This plate covers the whole of the top of the fur nace so as not to allow the escape of smoke. I I is a frame of cast-iron lying loosely upon the plate, with as little touching surface as possible. Within this frame a wrought
iron rim S is placed, for the purpose of enveloping the sand which constitutes the sand-bath. There are rims of different depths, according to the size of the vessels to be placed in the sand.
Figs. 5, 6, and 7, are two sections and a plan of a rever beratory furnace. A is the ash-pit, B the fire place, g the grate; F, in the plan Fig. 7, is the opening for the intro duction of fuel, which is pit-coal. The flame plays along the hearth H, and passes into the chimney C. 0 is an opening for the introduction of the substances to be enter ed or melted, which are placed upon the hearth. If the substance is to be melted, it runs down to the opening 1', where it is taken out. D is the damper for regulating the fire. This furnace is more particularly adapted for melt ing than for calcining, on account of its incliniti`g hearth, and the opening at 'I'. The construction is a little varied for the latter purpose. The hearth is nearly horizontal, and there is generally but one opening, which is in the middle.
Figs. 8 and 9, are two sections of an enamelling furnace. A is the ash-pit, g the grate, B the body-furnace, where the fuel is contained ; Al is the muffle, an eat then vessel more plainly seen in Plate CXLIII. Fig. 9. It contains the substances to be operated upon, and is for the purpose of defending them from smoke or flame, and admitting a supply of flesh air ; K is the cover, T the throat, and C the chimney. This furnace is also employed for assaying metals by cupellation.
Figs. 10, 11, and 12, contain a view and section of a muffle furnace, for producing very intense heats. The body of the furnace, shewn at AA, is in the form of an ob long coffer, swelling out in the middle. The grate is shewn at C, standing over the ash-pit F. The hole for the muffle is scut at E; and the dome, or upper part of the furnace, is seen at BB, having a very large door I), for the purpose of introducing the Fuel. This furnace was em ployed by Pott, and afterwards by D'Arcet, in their expe riments on the habitudes of earths and stones, when ex posed to a violent and long continued heat. Figs. 13, 14, 15, 16, represent fire-tongs for various purposes.