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.1

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.)

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

3. Wind furnaces for illuminating gas.-These furnaces are easily attended, the work can be carried on in a very cleanly manner, and at the same time with the greatest accuracy, as the assayer can conveniently look into the crucible during the operation.

Fig. 31 represents Perrot's furnace. a, the outer shell, with cover b, and sight-hole c; d, the crucible, upon a movable stand, e; f, inner

shell; g, pipe, with manometer h, and cock i, for conveying gas into the annular chamber k, from which it passes through the pipes 7,

1 B. u. h. Ztg. 1873, p. 284 (Perrot); Dingler, ccvi. 360 (Wiessnegg); Dingler, clxxx. 220; clxxxix. 376; Oestr. Jahrb. v. Hauer, 1878, p. 123 (Schlösing). Mitchell, Pract. Assaying, 1868, pp. 81 and 73. Hempel's Gasofen mit Oxydationsvorrichtung, z. B. zum Abtreiben, in Fresenius' Ztschr. xvi. 454; xviii. 404 (may be had of Desaga in Heidelberg).

through the annular opening m, into the inner space of the furnace, where flame plays around the crucible d, and finally escapes through the upper opening of the inner chamber into the exterior annular space, and is carried off below through the pipe n, leading to the. chimney; o, openings for admitting the air required for combustion, which is mixed in the pipes 7, with the gas. The admission of air is regulated by a cut-off. p is a cup for the reception of any metal which may overflow and escape from the crucible.

Wiessnegg's gas furnaces are of simpler construction, and consume less gas. The flame plays around the crucible in the form of a spiral. By this means air and gas are more intimately mixed, and higher temperatures can be obtained. The same result is attained in Schlösing's furnace. The attainable heat of Perrot's furnace is about 1560° C. (2840° F.).

12. BLAST FURNACES.

These are low cylindrical shaft furnaces, constructed of fire-resisting material, or of a sheet-iron cylinder lined with refractory clay. At some distance above the hearth of the furnace, are one or several tuyeres symmetrically arranged. The mouth of the furnace is provided with a movable sheet-iron chimney. The furnace, in order to increase the temperature, is well supplied with air heated in the space between the two iron shells surrounding the shaft (Sefström's furnace) or in a reservior below the per forated hearth of the furnace (Deville's furnace).1 The hearth in Welch's furnace may be easily separated from the furnace body.2 The fuel used in these furnaces should be in lumps about the size of a walnut and uniform in size. A very high temperature can be produced in a shorter time, and with the consumption of less glowing fuel than in wind furnaces, but a certain amount of power is required for operating the blast (bellows, fan, Root's blower),3

1 Kerl, Thonwaarenindustrie, 1879, p. 65.

2 Dingler, ccxxix. 159.

3 Root's blowers are well adapted for this purpose.

the furnace. time, each is

and the fire must be frequently stirred and fuel added. If only one crucible is used, it is placed in the centre of But if more are placed in the furnace at one placed at the same distance from one of the tuyeres. Raschette's furnace furnishes very high temperatures; it has an oblong cross-section, and the tuyeres are arranged alternately in rows. Munscheid's1 gas blast furnace gives also very high temperatures. Gas and air mixed, are drawn into it by means of an exhaust fan.

α

Fig. 32.

α

Fig. 32 shows Sefström's furnace. b is the space between two sheetiron cylinders closed on top by a. The inner cylinder is lined with at fire-resisting material, c (1 part clay and 3 to 4 parts quartz sand). The air enters at d, and, after having been heated in the intermediate space, is carried through the tuyeres o. The dimensions of a furnace for six small iron crucibles are as follows: 18 centimeters (7.07 inches) in diameter; total height 15 centimeters (5.9 inches); a collar 7 centimeters (2.75 inches) high, upon which sits the sheetiron chimney; width and height in the clear, 10.5 centimeters

b

(4.14 inches); thickness of the refractory lining 2.5 centimeters (0.98 inch); distance between the two cylinders, on the sides 1.2 centimeters (0.47 inch), and on the bottom 2.5 centimeters (0.98 inch). The manometer is placed on the exterior shell. Lang's blast-furnace2 for larger masses has an annular air-conduit.

13. FURNACES FOR SUBLIMATION AND DISTILLATION.

These consist of a fire-space to be heated, for the reception of variously shaped vessels (tubes, retorts, boilers,

1 B. u. h. Ztg. 1878, p. 361. 2 Kärnthn. Ztschr. 1879, No. 8, p. 287.