of coloring matter. By comparing fluids of an equal intensity of color, and taking the volume into consideration, a conclusion is formed as to the percentage of the coloring body which is contained in the one to be determined. The same manipulations occur here as in assays by gravimetric analysis, namely, solution, precipitation, etc., and in addition comparison of the colored assay solution with standard colored solutions contained in tubes or tapering glasses of known cross-sections, measuring the solutions in calibrated cylinders, etc.

III. Assay Furnaces.

9. GENERAL REMARKS.

The choice of an assay furnace will depend chiefly on the degree of heat to be obtained, and whether the substances are to be oxidized or reduced, or only calcined, fused, sublimed, or distilled. Furnaces, accordingly, are divided into muffle furnaces, draught or wind furnaces, blast furnaces, sublimation furnaces, and distillation furIn regard to their construction, they vary chiefly according to the fuel to be used (flaming or glowing fuel).

naces.

10. MUFFLE-FURNACES.1

The principal part of this is the muffle (Fig. 24). It is usually made of refractory clay, sometimes, though rarely, of iron. It is open in front, and closed at the rear; and the semi-cylindrical body is often provided along the sides with draft orifices, as shown. It is either connected

Fig. 24.

1 Engin. and Min. Journ. 1878, No. 26, p. 443. Silliman, Double MuffleFurnace, 1876, vol. xxii. No. 17.

with the bottom, or stands loose upon it. It serves for the reception of the assay charge, and is heated from the outside by a glowing or flaming fire. These furnaces are absolutely necessary for oxidizing processes (calcining, cupellation, refining), but they are also adapted for operations requiring only the production of a high temperature (glowing, reducing, and purifying fusion, etc.), that is to say, when only temperatures not exceeding the fusing point of gold and copper (about 1200° C., 2192° F.) are required (they are, therefore, not available for assays of cast iron). In the latter cases the fuel is not completely utilized, and besides, they are more difficult to attend than the wind and blast furnaces, where the crucibles, etc., are placed directly in the glowing fire, or come in direct contact with the flame.

The furnaces are either bricked in (for instance, large muffle-furnaces for burning coal), or they are portable. In the latter case, the furnace for receiving the muffle is constructed of fire-clay which is sometimes surrounded with a casing of sheet iron (mint furnaces). The work connected with the muffle-furnace consists chiefly in heating it, regulating the temperature (by reducing or urging the fire, regulating the admission of air, opening or closing the mouth of the muffle, by removing or piling up fuel, etc.), in stirring the fire regularly (in doing this the fuel must be piled chiefly upon the front part of the grate and only a thin layer upon the back part), in ventilating the grate frequently, in repairing (that is, lining defective places in the walls of the furnace, filling in of cracks in the bottom of the muffle with fire-clay, or scraping the bottom and lining it by strewing it with powdered fireclay, cupel ashes, chalk, pounded assay vessels, etc.), introducing and removing the assay vessels in the muffle, cleansing the furnace after the work is finished by draw

ing the glowing cinders from the grate and allowing the fire-door to remain open, etc.

α

Fig. 25.

m

According to the kind of fuel used, we may divide them into

1. Furnaces for solid, free-burning, flaming fuel.

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 furnace1), 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; l, 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; ƒ, fire-door; f', fire-box; g, refractory lining; h, i, channel for conducting the external air beneath

1 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; ', 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 regulated (the Schemnitz charcoal furnaces are of this con