struction), and where the stoke-hole is in the rear, or the firing is done through two channels on the sides.

Fig. 28.

C

b

Assay furnaces for charcoal. — Fig. 28 shows such a furnace. b, muffle of fire-clay, 14 centimeters (5.51 inches) long, 7.5 centimeters (2.95 inches) high, 9 centimeters (3.54 inches) wide, with walls 8 millimeters (0.31 inch) thick, and resting upon two rails passing through openings in the iron casing. The inside of the casing is lined with fire-clay from 15 to 20 millimeters (0.59 to 0.79 inch) thick. In front of the muffle is a shelf of sheet iron resting upon the rails supporting the muffle; c is the mouth of the furnace through which the charcoal is fed, and the products of combustion escape into a hood or through a sheet-iron smoke

stack.

a

The mouth of the muffle and the flues above and below it can

be closed by dampers; a, the cupel.

The

3. Gas furnaces (coal-gas).-By using these furnaces the work can be carried on in a very cleanly manner, and the temperature can be very perfectly regulated. gas is introduced either by means of burners placed at the rear wall (Perrot's furnace, used in the Berlin School of Mines), or from below through four straight burners standing alongside each other beneath a slit in the bottom (furnaces of Lenoir and Forster of Vienna, used in the laboratory of the Schemnitz School of Mines, etc.), or through curved burners arranged in the form of a circle beneath the furnace (furnace of the Société genevoise pour la construction d'instruments de physique à Genève, used in the Berlin School of Mines). The oil furnaces of Andouin-Deville of Paris (using the vapors of crude petroleum) are said to be cheaper in operation than the

gas furnaces just described. The oil trickles from funnels upon the hot grate-bars set obliquely and channelled. There it is instantly vaporized and burns.'

Fig. 29 shows Perrot's gas muffle-furnace. a, muffle of fire-clay, with refractory coating and movable cover b; e, f, g, furnace walls of sheet iron with refractory lining; A, burner, with chamber p, into which coal gas enters at o, from the pipe u, provided with mano

meter v. From here it passes through narrow channels into the burner-tubes q and r, which are provided below with openings furnished with valves t for regulating the admission of air; w, w, the nozzles from and which the flame passes through d into the space around the muffle, escapes through the flue k into the chimney 7, in which is a damper m. The chimney also receives, through the pipe n, the fumes which may escape from the mouth of the muffle.

11. DRAUGHT OR WIND FURNACES.

These consist—

1. In case carbonized fuel (coke, charcoal) is used, of a round, rectangular or oblong fire-place, separated from

1 Ztschr. des Ver. deutsch. Ingen. xxi. 225.

Fig. 30.

the ash-pit at the base by a grate, and provided with a fire-clay or cast-iron cover or top-plate. A lateral flue connects the fire-place with the chimney. The furnace is either bricked in (Fig. 30) or is portable. In the latter case, the body of the furnace is made of a sheet-iron cylinder lined with refractory material. It is also a very good plan to set a furnace of this kind into brick-work. leaving an intermediate space between the two, in which case the usual binding with strapiron may be dispensed with. The cover or top-plate over the

fire-place consists of two fire-tiles provided with some convenience for easily removing and replacing them. It is best to place a small carriage in the ash-pit for receiv ing and removing the ashes (Berlin School of Mines). The degree of temperature possible to attain, depends on the height of the shaft between the grate and the flue, the height of the chimney, and the quality of the fuel used (coke will give a higher temperature than charcoal). The temperature can be increased with the aid of a flue leading from the ash-pit into the open air, or by an undergrate blast, and is regulated by a damper fixed in the door of the ash-pit, or in the flue or chimney. The highest temperature is found at about 4 to 6 centimeters (1.57 to 2.36 inches) above the grate, which should be taken into consideration in placing the crucibles in the furnace.

The labor attending these furnaces consists of

a. The placing of the assay vessels in the furnace by

hand. If it is necessary to look into them during the operation (assay of lead in iron crucibles, Cornish roasting assay of copper), they are placed in a hollow made in the fuel, generally coke; or, if this is not required (assays for lead, copper, tin, iron, etc., in clay crucibles), the assay vessels are placed immediately upon the grate, leaving sufficient space between them for the necessary fuel, and in such a manner that the part of the vessel which is to be heated the strongest stands about 4 to 6 centimeters (1.57 to 2.36 inches) above the grate. If, therefore, vessels with feet are used, they must be placed directly upon the grate, while those without feet (crucibles) are supported on a block or stand of fire-clay.

b. Firing. This, as a general rule, is done from below by putting glowing coals between the assaying vessels, filling up the shaft with fuel, and then gradually closing the top-plate of the furnace. But if the heating must take place very slowly, the firing is done from above, by placing the glowing coals on top of the fuel with which the shaft is filled. The fire, when the mouth of the furnace is closed, will then gradually work down. (In the Schembelow the grate

nitz laboratory, the lateral flue is placed and the air required for combustion is introduced from above.) The temperature is regulated in the manner indicated on p. 56, and, if necessary, fuel is added from time to time, but, before this is done, the glowing coal must be poked down to do away with empty spaces.

c. Taking the vessels from the furnace.―This is done by lifting the vessels out at the top of the furnace, by means of crucible tongs (Fig. 47), either out of the coke, or from the grate, after the fuel has burned down, or, in the latter case, it may be more convenient to remove them through an opening in the side (t in Fig. 30), but this must be closed up during the operation of the furnace.

Either the contents of the crucibles are poured out and the crucibles while still glowing placed back in the furnace and again charged from the mixing capsules (Fig. 6) (assay of lead in an iron crucible), or the clay crucibles are allowed to cool off and are then broken up.

A furnace for 10 lead assays is 35.4 centimeters (13.93 inches) high, 33.7 centimeters (13.26 inches) long, and 36.3 centimeters (14.29 inches) wide. It has seven bars. The door of the ash-pit is 15.3 centimeters (6.02 inches) wide, and 14.2 centimeters (5.58 inches) high, 14.2 centimeters (5.58 inches) below the grate. The chimney is from 1.75 to 2.34 meters (5.74 to 7.67 feet) high, with a diameter of 10 to 12 centimeters (3.93 to 4.72 inches). A furnace tapering towards the upper end and intended for 1 lead assay in an iron crucible has the following dimensions: 18 centimeters (7.08 inches) wide, 12 to 14 centimeters (4.72 to 5.51 inches) high, with walls 6 millimeters (0.23 inch) thick. The ash-pit is 12 centimeters (4.72 inches) high, and 14 centimeters (5.51 inches) wide. For assays of copper, the shaft of the furnace is a few centimeters higher and the chimney from 1 to 1.5 meter (3.28 to 4.92 feet) higher than that for lead. The Cornish furnace for assays of copper is 40 centimeters (15.74 inches) high, and 20 to 26 centimeters (7.87 to 10.23 inches) wide. For iron assays, the shaft is 55 centimeters (21.65 inches) high, and 33 centimeters (12.99 inches) wide. The ash-pit is from 29 to 30 centimeters (11.41 to 11.81 inches) high; the flue 33 centimeters (12.99 inches) long, and 11 centimeters (4.33 inches) high; the chimney 10 to 12 meters (32.8 to 39.37 feet) high, and 20 centimeters (7.87 inches) wide. The dimensions of the furnaces used in the Berlin School of Mines are: for assays of lead, 20 centimeters (7.87 inches) high, and 34 centimeters (13.38 inches) wide; for assays of copper, 26 centimeters (10.23 inches) high, and 34 centimeters (13.38 inches) wide; for assays of iron, 35.5 centimeters (13.97 inches) high, and 34 centimeters (13.38 inches) wide, with a chimney 10 meters (32.8 feet) high, provided with a damper for regulating the draught.'

2. Wind furnaces for free-burning (flaming) coal.— The assay vessels stand over the grate upon a tile of fireclay in the same manner as in Plattner's furnace, except that there is no muffle. (Freiberg.)

B. u. h. Ztg. 1878, p. 83.