The mineral to he examined, after being carefully separated from all foreign matter, is broken up small and finely powdered in an agate or steel mortar. Small portions of it may then be subjected to various simple tests, the results being carefully noted. If mercury, sulphur, or arsenic be present, they are r( adily detected by the formation of a sublimate when the sub stance is heated in a clean, dry test-tube, and by the odour of the vapours which are evolved. A number of metals, such as lead, copper, iron, antimony, lithium, &c., may be recognized by the characteristic, colours which they impart to the flame, when a small portion is held in it on the end of a platinum wire. Others, such as iron, cobalt, manganese, chromium, &c., are detected by the power they possess of colouring a bead of borax or microcosmic salt. This is effected by forming a loop at the end of a platinum wire, and heating a fragment of borax in it until it runs to a clear glass ; a minute quantity of the substance under examination is then added to the fused bead, which is again heated before the blowpipe, when the metallic oxide is dissolved ; the following results must be observed : the colour of the bead in the reducing and oxidizing flames respectively ; and whether the colour so imparted be altered when the bead has become cool. Care must be taken only to add enough of. the substance to give a slight tinge to the bead, or the colour may be too intense to be distinguish able. When microcosmic salt, which is a combination of phosphate of soda and ammonia, is used, it should be fused upon platinum foil to expel the water and the excess of ammonia contained in it ; it is then used upon platinum wire in the same way as borax. The assayer should also make himself familiar with the incrustations formed by different metals when heated upon charcoal. To obtain these, a small hollow is cut in a block of close-grained pine-wood charcoal ; a small portion of the powdered mineral is placed in the hollow and heated iu the reducing flame, with a mixture of carbonate of soda and cyanide of potash ; lead, antimony, bismuth, zinc, and cadmium are indicated in this way. He should be able also to distinguish between the beads of metal obtained by heating many metallic salts on charcoal with reducing agents.
We have now to point out the most characteristic tests for the more commonly occurring metals. Detection of Antimony.—When reduced on charcoal with carbonate of soda, it yields a brittle bead of metal, the charcoal at the same time being coated with a white incrustation of oxide. On continued heating, the bead volatilizes completely. A further test may be made by acting upon the metal bead with a small quantity of nitric acid, by which it is converted into the oxide ; this substance is insoluble in pure hydrochloric acid, but dissolves readily if a few drops of nitric acid be added ; on diluting the solution with water, a white precipitate is formed, which redissolves on addition of excess of hydrochloric acid. If a stream of sulphuretted hydrogen (formed by the action of sulphuric acid upon fragments of iron pyrites) be passed through the solution of the bead in hydrochloric acid, a brick-red precipitate of sulphide of antimony is thrown down. This precipitate dissolves readily on treatment with sulphide of ammonia, but is repreeipitated on addition of BM acid.
Detection of Bismuth.—Native bismuth fuses readily in either the oxidizing or reducing blowpipe flame, coating the charcoal with a yellowish-brown incrustation, which is somewhat darker in colour than that obtained from lead. The following is the most delicate test for the presence of bismuth :—A small quantity of the powdered mineral is heated in the reducing flame on charcoal, with pure carbonate of soda ; the metallic bead thus obtained will be observed to be of reddish white lustre, and, when cold, exceedingly brittle. It is dissolved in a little warm, dilute nitric acid. To the solution is then added a few drops of a liquid obtained by adding caustic soda to a solution of stannous chloride (the " tin-salts" of commerce), until the precipitate formed at first is redissolved. The presence of bismuth is shown by a black precipitate of oxide of bismuth. Fused in a borax bead on platinum wire, bismuth imparts to the bead a yellow colour while bot, which disappears on cooling. With microcosmic salt, used in precisely the same manner, only adding slightly more of the assay, bismuth gives a yellow bead which is also colourless, or sometimes enamel-white, when cool. The borax test for bismuth ought never to be relied on by itself, but only employed as confirmatory of the above method in the wet way.
Detection of Cadmium.—When heated on charcoal with reducing agents, cadmium compounds give a brownish-yellow incrustation of oxide, which appears the moment the heat is applied ; this serves to distinguish it from that obtained from zinc compounds, which takes a much longer time to form. With borax or microcosmic salt cadmium compounds form a yellow bead, becoming colourless when cool. On passing a stream of sulphuretted hydrogen through an acid solution of cadmium, a bright yellow precipitate of sulphide is formed, which, on standing, darkens a little in colour. This sulphide may be distinguished from other yellow sulphides, thrown down by sulphuretted hydrogen, by its complete insolubility in sulphide of ammonia.
Detection of Chromium.The presence of this metal is generally determined by means of the blowpipe. All the salts, when fused with borax in either blowpipe flame, yield a beautiful emerald-green bead. The production of this bead in both flames serves to distinguish chromium from the metals vanadium and uranium, which yield a green bead only when heated in the reducing flame. All compounds of chromium, ignited with a mixture of nitre aud an alkaline carbonate, form a chromate of that alkali ; if this be dissolved out with water and neutralized with acetic acid, the solution will give with lead salts a splendid, yellow precipitate of chromate of lead. The mineral k-nown as chrome-iron ore, which is the commonest source of the metal, is generally recognized by its brownish-black appearance.
Detection of Cobalt. —Compounds of cobalt reduced on charcoal with carbonate of soda, or other reducing agent, give a grey, magnetic powder of metallic cobalt. Fused with borax in either flame of the blowpipe, cobalt gives a splendid blue bead ; this is a very characteristic test, and serves to detect the most minute quantities. If iron be present in large quantity, the bead will be tinged with green. Only a minute trace of the assay should be employed, or the colour will be so intense as to appear black.