scoop is about 140 millimeters (5.51 inches) long and 40 millimeters (1.57 inches) wide.

The mixture is poured into the crucible through the spout of the scoop, about 20 millimeters (0.78 inch wide), a brush being used to brush out the last traces of the mixture; or, in case a very vigorous chemical reaction is desired, the sample and fluxes are first intimately rubbed together in a mortar of stone (porcelain, serpentine, agate), or of metal (steel, cast iron, brass). Open mixingscoops of copper provided with a handle (Fig. 6a) are used for mixing the charge, or for receiving, in consecutive order, the substances constituting it, or for pouring them into the glowing crucible standing in the heated furnace (assays of lead, English assay of copper).

II. Chemical Operations.

6. CLASSIFICATION.

These operations are divided into those by the dry and those by the wet method, and are either preliminary (roasting, etc.), or capital operations (smelting, etc.).

7. WORKING BY THE DRY METHOD.

These operations are carried on, either below the fusing point (ignition, carbonizing, calcining, roasting), with or without admittance of air; or at a fusing heat (smelting); or volatile substances are to be expelled by

heat and their vapors condensed to the liquid state (distillation), or to the solid state (sublimation).

1. Ignition, carbonizing, calcining.'-Heating without fusing

a. In a neutral atmosphere, to drive out volatile substances (for instance, water and carbonic acid from iron ores), or to change their molecular condition (for instance, annealing gold and silver alloys before rolling them out, etc.).

b. With exclusion of air in covered pots or crucibles, to decompose metallic sulphides and arsenides (iron and arsenical pyrites in the dry assay of blende), to effect reduction (ignition of tin ore with charcoal), or to arsenize or dearsenize the substances (assay of nickel and cobalt).

c. With admission of air in roasting dishes (determination of ash in fuel, combustion of bitumen in copper schist and black-band iron ore, oxidation of cement copper, etc.).

d. With reagents for decomposing substances insoluble in acids (for instance, silicates with four times their weight of a mixture of 13 parts of potassium carbonate and 10 parts of anhydrous sodium carbonate in a platinum crucible).

2. Roasting.-Metallic sulphides, arsenides, and antimonides are heated in presence of air to a temperature insufficient for fusion, but which permits of their oxida tion; metallic oxides are, therefore, produced, while sulphurous, arsenious, and (sometimes) antimonious acids are volatilized..

The process is as follows: The powdered sample is spread out in a shallow, smooth roasting dish. This is

1 Gaslampen in Dingler, ccxxiv. 617; ccxxv. 83 (Müncke). Fresenius' Ztschr. 1879, p. 257 (Ebell). Bunsenbrenner von Glas in Dingler, ccxxvii 85, 398.

about 50 to 52 millimeters (1.96 to 2.04 inches) wide in the clear, and 8 to 10 millimeters (0.31 to 0.39 inch) deep, made of not too refractory clay, and has rather thin sides. It is lined with reddle, chalk, or oxide of iron, and, if necessary, in order to increase the surface, the lined sides are marked with a spatula in such a manner that radial furrows, running from the

Fig. 7.

centre towards the edges, are formed. It is then placed in the muffle of a muffle-furnace (Fig. 25) and heated at a gradually increasing temperature until it glows; the heating must be the more gradual the more fusible the sample. (Antimonial and arsenical metals are more easily fused than metallic sulphides, antimony glance, lead sulphate, and “fahlerz" containing mercury). The mouth of the muffle is left open, with the exception of a low layer of pieces of wood charcoal touching each other, and continued in a forward direction. These pieces in a glowing state heat the oxidizing air current. The roasting dish must occasionally be turned around during the operation. It is taken from the muffle when the mass has ceased to burn, and oxidation is complete. This is indicated by the heated mass ceasing to fume and no longer emitting odors of sulphurous or arsenious acid, and by the metallic lustre having been replaced by an earthy appearance. If this should not be the case, the roasting dish must be placed back into the muffle until these signs make their appearance. The now roasted sample may be somewhat sintered together. It is then rubbed with the iron knob, b, of a wooden-handled spatula, after being loosened from the edges of the roasting dish with the knife's edge, a, of the rod. This tool (Fig. 8) is about 195 millimeters (7.67 inches) long, and consists of the iron head b, about 16 millimeters (0.63 inch) diam., and

the steel knife-blade a, set in a wooden handle.

The

roasting is repeated once or several times, but the mass must be rubbed up previous to each roasting.

pow

Fig. 8.

It is then mixed with 1 or 2 volumes of dered wood charcoal, or 20 to 25 per cent. of graphite, and the roasting dish with its contents is again placed in the muffle and brought to a glow. By this process the sulphates, antimoniates, and arseniates formed during the oxidizing period are reduced to metallic sulphides, antimonides, and arsenides, while the volatile products of oxidation escape (reducing roasting). These compounds when all the carbon has been consumed (which may be readily recognized by the manner of glowing) will be again converted into oxides; sulphurous, arsenious, and antimonious acids being evolved in the operation. But new sulphates, antimoniates, and arseniates will constantly be formed, and these, if the sample is to be roasted as completely as possible (for instance, copper ores, but lead ores in a less degree), can only be removed by repeating the rubbing up of the assay sample twice or three times, mixing it with charcoal powder, and glowing until the coal is completely consumed, although even after this small quantities of sulphates will nevertheless remain. When the roasted sample has become sufficiently cold, it is placed in an iron mortar and mixed with 20 to 50 per cent. of ammonium carbonate. A small conical heap of the mixture is formed in the roasting dish, this is covered with an empty roasting dish and quickly ignited until the odor of ammonia can no longer be detected. When this is the case, the last traces of sulphuric acid in the roasted sample will have been volatilized in the form of ammonium sulphate. (Lead and bismuth sulphates are only incompletely decomposed by ammonium carbonate.) The

roasting dish is now taken from the muffle and allowed to cool off. The sample is then placed in a mortar and rubbed up.

Modifications. When the ores are refractory (for instance, copper pyrites), powdered charcoal or graphite is added to the sample before roasting, in order to shorten the time required for the operation. Very fusible substances which evolve vapors (such as "fahlerz” containing mercury) must be heated very gradually. To diminish the loss of metal (for instance, of silver and gold) the temperature must not be raised higher than is absolutely necessary. The loss from this cause is greatest with ores containing antimony, arsenic, zinc, etc.

3. Fusion. The sample is brought into a liquid state, either by itself, or with fluxes. During this process the resulting products (metal button or regulus, speiss, matt, slags) arrange themselves in layers according to their specific gravities, and are separated from each other, either by breaking to pieces the clay assay-vessels in which they have been fused, after they have become cold, or they are poured out while still in a fluid state, into iron or bronze moulds, where the separation then takes place. Sometimes the fluid, oxidized substances are absorbed by the porous sides of the assay-vessel, leaving the metal button behind (cupellation of lead, and refining copper on the cupel). The following distinctions are made according to the object of the fusion:

a. Oxidizing fusion.-In this process the following may serve as oxidizing agents: the oxygen of the air, demanding open vessels for the operation (cupels, calcining and roasting dishes), which must be heated in the mufflefurnace (for instance, cupellation of lead, refining copper, assay of cobalt and nickel); or fluxes yielding oxygen, and then open or covered assay-vessels (pots, crucibles),