These are generally used with large muffles, and with such fuel the heat can be better regulated than in furnaces heated by a glowing fire, but they require more care in attending them. Stoking is done from the front (Plattner's furnace'), or from the back (Schemnitz, Pribram2). With the latter arrangement the operator, working in front of the furnace, is not exposed to the direct heat, but it also prevents him from giving immediate attention to the firing should the assay require it.

Fig. 25 represents Plattner's muffle-furnace for coal, with the stokehole in front. a, muffle of fire-clay, 36.6 centimeters (14.4 inches) long, 17.6 centimeters (6.92 inches) high, and 34.2 centimeters (14.17 inches) wide, with an ascending slope of 2.4 centimeters (0.93 inch). It rests upon the support b, and three legs c; d is the vault. There is a space of 4.9 centimeters (1.92 inches) between it and the walls of the furnace. e, the chimney, 14.7 centimeters (5.73 inches) wide, and 3 to 4 meters (9.84 to 13.12 feet) high. f, mouth of the muffle, 12 centimeters (4.72 inches) wide, and 14.6 centimeters (5.74 inches) high, which can be closed by the fire-clay door g. Another door h is used for covering a slit sometimes provided over the muffle (for heating plates of metal, etc.), but it is usually omitted; i is the grate, 26.8 centimeters (10.55 inches) wide, and 51.4 centimeters (20.23 inches) long, 28.1 centimeters (11.06 inches) below the muffle ; k, the stoke-hole, 22 centimeters (8.66 inches) high, and 26.8 centimeters (10.55 inches) wide; 7, fire-door; m, ash-pit, 76.8 centimeters (30.25 inches) long, and 26.8 centimeters (10.55 inches) wide; n, a channel, 22 centimeters (8.66 inches) wide, communicating with the open air for conducting air under the grate through the flue o, p, 9.8 centimeters (3.85 inches) wide, which is provided with a damper q; r, ash-pit door, 26.8 centimeters (10.55 inches) wide, and 34.2 centimeters (13.46 inches) high.

Figs. 26 and 27 represent a muffle-furnace with the stoke-hole at the back. a, muffle, resting upon the supports c and c'; b, mouth of the muffle; d, front wall; e, grate; f, fire-door; f', fire-box; g, refractory lining; h, i, channel for conducting the external air beneath

Freiberger, Jahrb. 1842, p. 1.

Plate 12, Figs. 3 to 5.

Ztschr. des Ver. deutsch. Ingen. 1877,

2 Rittinger's Erfahr. 1857, p. 29. B. u. h. Ztg. 1876, p. 353; 1876, p. 61.

the grate; k, damper; 7, ash-pit door; l', ash-pit; m, fire-space surrounding the muffle; m', chimney (it is better to place it nearer d), with damper n, n', and lever o, for regulating the same; p, brickwork of the chimney, with flues r, for carrying off the fumes coming from the mouth of the muffle; q, hooping.

2. Charcoal and coke furnaces.-Coke as a general rule requires a grate under the muffle, and a strong draught. With charcoal this arrangement is not so essential, though in order to secure a more uniform supply of air a grate is usually provided. The ashes from coke are more difficult to remove and attack the walls of the furnace more than wood ashes. Smaller furnaces of this kind are much used for assaying gold and silver; and also larger ones, in which the heat can be better regu lated (the Schemnitz charcoal furnaces are of this con

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.

α

The mouth of the muffle and the flues above and below it can be closed by dampers; a, the cupel.

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. The 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 which the flame passes through d into the space around the muffle, and 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

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

the ash-pit at the base by a grate, and provided with a fire-clay or cast-iron cover or top-plate. A lateral

Fig. 30.

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 receiving 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