stances is by using a wooden vanning trough (Figs. 4 and 5), from 420 to 470 millimeters (16.53 to 18.47 inches) long, 160 to 210 millimeters (6.29 to 8.26 inches) wide, Fig. 4.

Fig. 5.

1

and 35 to 40 millimeters (1.37 to 1.57 inches) deep; or they are washed upon a shovel 400 millimeters (15.74 inches) long, and of the same width (Cornish test of tin ore). The washed mass should be occasionally examined with a magnifying glass or blowpipe as to its physical characteristics (for instance, silver ore for the presence of metallic sulphides, antimony and arsenic in order to regulate the addition of lead before smelting).

4. WEIGHING AND MEASURING.

Before every weighing, the balance must be tested as to its equilibrium. The substance to be weighed should be cold, and must not be placed directly upon the scale-pan, but upon suitable smaller pans, watch-glasses, etc.; hygroscopic substances in closed tubes. The balance beam should be raised from the knife edge every time before a weight is put into or removed from the pan. The weights must not be put in the pan at random, but systematically, as this is the only way of saving time. The highest probable weight should be added first, then the next lowest, and so on until the equilibrium has been established. The pans should then be changed in order to test the correctness of the weight. Perfect equilibrium of the balance is, however, not absolutely essential, as a

correct weighing may be obtained by placing the substance to be weighed in one pan, and in the other pan, any convenient material as a make-weight, such as tinfoil, shot, granules of lead, etc., until equilibrium is established. The balance is then raised, the weighed substance is removed from the pan, and sufficient weights to counterpoise the balance are put in its place. The sum of these weights will give the correct weight of the sample.

1. Weighing: a. A pulverulent sample.-The dried sample is poured upon glazed paper and spread out in spirals with the spatula. It is then drawn together towards the centre by radial bands. Some of it is now taken with the spatula from the bottom to the top of the heap, the weights are placed upon the left pan of the balance, and the sample is continuously poured into a counterpoised saucer or watch-glass placed upon the other pan, by gently tapping on the handle of the spoon, until equilibrium has been established. In case too much has been poured in, some of it is removed, but the balance, while this is done, must be arrested. Large quantities are weighed upon a watch-crystal counterpoised by granules of lead or shot. Hygroscopic substances are conveniently weighed by filling a stoppered glass tube, 12 to 14 centimeters (4.72 to 5.51 inches) long, and 8 to 10 millimeters (0.31 to 0.39 inch) wide, with them, noting the weight, and, after pouring out the requisite amount of sample, again weighing the tube; the difference between the two weighings will give the weight of the quantity abstracted.

b. Alloys. They are converted either into granulated form or into small strips or splinters. These are collected in a glass or copper saucer and placed by means of the forceps in the right pan of the balance while the weights are placed in the left.

c. Fluxes. In weighing these, very great accuracy is not of so much importance. They are placed either directly upon the pan of the balance, provided it is not attacked by them, or otherwise upon a tared watch-crystal.

2. Weighing the button.-The button is taken hold of with the forceps and placed upon the left pan of the balance, and the weights are then put upon the right; but, as has been stated, the balance must always be arrested before the weights are put on or removed.

3. Measuring of fluxes.-Granulated lead (test lead) free from silver is measured with large, gauged, iron spoons numbered on the handles, or with a glass tube, one end of which is closed with a stopper, while in the other is a wooden cylinder provided with a scale.

5. MANNER OF CHARGING THE SAMPLE.

The sample is poured either directly into the crucible without any fluxes (as in roasting), or the fluxes are added in such a manner that—

1. The sample lies on the bottom of the crucible, and the fluxes are placed upon it in consecutive order without stirring the mass up. When this is done, the mass will not puff up as easily when it is heated (charges with carbonaceous mixtures, for instance, assay of lead with carbonate of potassa, flour and iron).

2. The sample is added to the fluxes already in the crucible, in cases where the puffing up of the charge on heating is not feared, and is intimately mixed together (for instance, assay of lead with potassium carbonate).

3. The sample is mixed with the fluxes before it is placed in the crucible. The mixing is done in a mixingscoop of copper (Fig. 6) by means of a spatula. The

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

Fig. 6.

Fig. 6a.

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.