etc.) of clay, porcelain, glass, or iron, in which the substances are heated without addition (as, arsenical pyrites, iron pyrites, amalgam), or with fluxes (preparation of realgar, separation of mercury from cinnabar, etc.). They are usually provided with a receiver, cooled off, for condensing the volatile products into solid bodies (sublimation), or to fluids (distillation).

Fig. 33 shows a sublimation furnace. a are clay tubes for the reception of the assay sample (as, for example, arsenical pyrites or iron and arsenical pyrites for the production of realgar); b, receiver for the sublimate (arsenic, realgar); c, the grate; d, flues.

Fig. 34 represents a distillation furnace. a, tube for the reception of the assay sample (for instance, gold or silver amalgam); e, grate; h, combustion chamber; g, chimney; c, pipe for carrying off vapors (of mercury) into the condensing pipe k, provided with a funnel 7, the edge of which is covered with a cloth q, upon which cold water flows from n, and overflows at o.-Or Fig. 35. a, a retort, with tubulure b; c, receiver, covered with paper or cloth kept cool by water which is allowed to trickle upon it.-Or Fig. 36, if a more thorough cooling off is required. h, a retort, from which the vapors pass through i into the cooling-pipe a. This rests upon a stand g, and is surrounded by a sheet-iron cylinder, into which cold water passes through e, and is discharged at cf, while the liquid formed from the condensed vapors is collected in b.

IV. Assay Vessels.

14. GENERAL REMARKS.

The form of the assay vessels, as well as the materials from which they are made, varies according to the object for which they are to be used. The principal distinction is, whether they are to be employed in the dry or wet method.

15. ASSAY VESSELS FOR THE DRY METHOD.

A. Clay vessels.'-These are required to be more or less refractory according to the heat to which they are to be exposed (their refractory quality depends on the proportion between silicic acid and alumina and the quantity of fluxing agents-ferric oxide, lime, alkalies, magnesia-which may be present). They must allow of being suddenly heated and cooled without cracking (fat, contracting clay requires to be mixed with quartz, chamotte,2 graphite). The vessels should be corroded as little as possible by the substances heated in them, but, as a general rule, this can never be entirely prevented. (It may be done to some extent by making the sides of the crucibles thicker, or by giving a finer grain to the stuff of which they are made. This should be made as compact as possible, by mixing the clay with chamotte instead of quartz. The interior of the crucible should be made very smooth, and it should be fired in the kiln as strongly as

' Kerl, Thonwaarenindustrie, 1879, p. 491. Percy, Metallurgy, vol. I. 1875, p. 111.

2 [Chamotte is a mixture of unburnt fire-clay and dust of fire-bricks, glass pots, or seggars.-TRANSLATOR.]

possible.) The vessels should further be very compact. This can be accomplished by giving a suitable grain to the mass, exercising great care in moulding and firing them strongly. The compactness of the vessels is tested by fusing metallic sulphides, such as galena, several times in them. They are made either by a plug and mould (roasting dishes and scorifiers, Upper Harz crucibles for lead smelting) or they are turned upon the potter's wheel (crucibles and larger melting pots).

The principal vessels, etc., are1. Vessels without feet.

a. Roasting dishes (Fig. 7).-They are flat, smooth inside, not very refractory, 8 to 10 millimeters (0.31 to 0.39 inch) deep and 50 to 80 millimeters (1.96 to 3.14 inches) wide. They are used in the manner indicated on p. 34.

Fig. 37.

b. Scorification or calcining vessels (Fig. 37).-They have a thick bottom and sides, very smooth interior, and are very compact. To avoid being corroded by lead oxide, they should be made of clay mixed with chamotte. They are 40 to 50 millimeters (1.57 to 1.96 inches) wide in the clear, 15 to 20 millimeters (0.58 to 0.78 inch) deep, with a bottom 10 millimeters (0.39 inch) or more thick.

Refining dishes.-They have either the same form as the flat roasting dishes, but are fire resistant and one edge is somewhat ground down, or they are made from fragments (Fig. 38) of crucibles (Fig. 42), and are then 70 to 80 millimeters (2.75 to 3.14 inches) long; or they are shaped like a flat saucer with feet. These are 30 millimeters (1.18 inches) wide, with a total height of 25 millimeters (0.98 inch) (Hungary).

d. Crucibles. These are of various forms and sizes, large and small (Figs. 39 to 41). They are respectively 32 and 45 millimeters (1.25 and 1.76 inches) high in the clear, and have a total height of 39 and 52 millimeters

(1.53 and 2.04 inches), a clear width of 33 and 43 millimeters (1.29 and 1.68 inches), and are not very refractory. Fig. 40 shows a large and a small Cornish crucible for the assay of copper. They are very refractory. Their respective dimensions are: diameter on the top 80 and 68 millimeters (3.14 and 2.67 inches), total height 84 and 60 millimeters (3.3 and 2.36 inches). Fig. 41 represents a crucible for iron. This is lined by means of a wooden plug with charcoal powder b (this is first moistened with starch paste, molasses, or clay), or it is lined with a mixture of 90 to 95 per cent. retort graphite, 5 per cent. rosin, and some petroleum, and burned with exclusion of air. They are covered with the perforated lid c. They are 37 millimeters (1.45 inches) high and 25 millimeters (0.98 inch) wide, and, after they have been lined, respectively 22 millimeters (0.86 inch) and 10 millimeters (0.39 inch). They are very refractory (Hessian pots). The French pots are especially refractory and smooth inside.

Graphite crucibles are made of graphite mixed with clay. They are very smooth inside, and very refractory. Those used in Cornwall for assays of tin are 80 milli