When an open grate is used for chemi cal purposes, it should he provided with cranes, to support the vessels operated in, that they may not be overturned by the burning away of the fuel.
Eta-races. In all these, the principal objects are, to produce a sufficient de gree of heat, with little consumption of fuel, and to be able to regulate the degree of heat. An unnecessary expenditure of fuel is prevented by forming the sides of the furnace of very imperfect conductors of caloric, and by constructing it so, that the subject operated on may be exposed to the full action of the fire. The degree of heat is regulated by the quantity of air which comes in contact with the burning fuel. The quantity of air is in the corn pound ratio of the size of the aperture through which it enters, and its velocity The velocity is increased by mechanical means, as by bellows, or by increasing the height and width of the chimney. The size and form of furnaces, and the materials of which they are constructed, are various, according to the purposes for which they are intended.
The essential parts of a furnace are, a body for the fuel to burn in ; a grate for it to burn upon ; an ash-pit to admit air; and receive the ashes ; a chimney for car rying off the smoke and vapours.
The ash-pit should be perfectly close, and furnished with a door and register plate, to regulate the quantity of air ad mitted. The bars of the grate should be triangular, and placed with an angle point ed downwards, and not above half an inch distant. The grate should be fixed on the outside of the body. The body may be cylindrical or elliptical, and it must have apertures for introducing the fuel and the subjects of the operation, and for conveying away the smoke and va pours. When the combustion is support ed by the current of air naturally excited by the burning of the fuel, it is called a wind-furnace ; when it is accelerated increasing the velocity of the current by bellows, it forms a blast-furnace ; and when the body of the furnace is covered with a dome, which terminates in the chimney, it constitutes a reverberatory furnace.
Furnaces are either fixed, and built of fire-brick, or portable, and fabricated of plate-iron. When of iron, they must be lined with some badly conducting and refractory substance, both to pre vent the dissipation of heat, and to defend the iron against the action of the fire. A mixture of scales of iron and powdered tiles, worked up with blood, hair, and clay, is much recommended ; and Pro fessor Hagen says, that it is less apt to split and crack when exposed at once to a violent heat, than when dried gradually, according to the common directions. pr.
Mack employed two different coatings. Next to the iron he applied a composition of three parts by weight of charcoal, and one of fine clay. These are first mixed in the state of fine powder, and then worked up with as much water as will permit the mass to be formed into balls, which are applied to the sides of the fur nace, and beat very firm and compact, with the face of a broad hammer, to the thickness of about one inch and a half in general, but so as to give an elliptical form to the cavity. Over this, another lute, composed of six or seven parts of sand, and one of clay, is to be applied in the same manner, to the thickness of about half an inch. These lutes must be allowed to become perfectly dry before the furnace is heated, which should at first be done gradually. They may also he lined with fire-bricks of a proper form, accurately fitted and well cemented to gether before the top plate is screwed on.
The general fault of furnaces is, that they admit too much air, which prevents us from regulating the temperature. It either becomes too violent and unma nageable, or when more cold air is admit ted than what is necessary for supporting the combustion, it carries off heat, and prevents us from raising the temperature as high as we otherwise would. The su perior merit of Dr. Black's furnace con sists in the facility with which the admis sion of air is regulated ; and every at tempt hitherto made to improve it, by in creasing the number of its apertures, have in reality injured it.