Detection of Copper.—All compounds of copper tinge the flame green. Fused with borax in the oxidizing flame of the blowpipe, copper-salts impart a bright green colour to the bead while hot, turning to a beautiful blue when cold. Reduced on charcoal, before the blowpipe, with carbonate of soda or cyanide of potash, a bright red bead of metallic copper is obtained. The oxides and carbonates are reduced to metal without the aid of reducing agents. The metallic bead, thus obtained, should be dissolved in a small quantity of dilute, warm nitric acid, in a test-tube, and the solution, which is of a bluish colour, diluted with water until perfectly colourless. The addition of ammonia in excess now produces a deep blue coloration, which is obtained if the slightest trace of copper be present.
Detection of Gold.—Gold compounds heated on charcoal with borax, in the inner flame, yield a yellow, very malleable bead of metallic gold, which may be readily recognized. Native gold is generally koown by its colour and weight ; it is soluble in aqua regia, the dilute solution affording a purple precipitate (purple of cassius) on treatment with chloride of tin ; this reaction will indicate the presence of one part of gold in 64,000 parts of liquid, by a faint tinge of the purple colour.
Detection of Iron.—When a very minute quantity of a powdered iron ore is heated with borax in the outer blowpipe flame, a yellow bead is produced, which becomes colourless on cooling; if a little more iron be added to the bead, it becomes red, turning yellow when cool. In the inner flame, the bead is of a greenish colour. The presence of cobalt, chromium, and other metals interferes materially with these reactions ; when the presence of iron is doubtful, it is advisable therefore to have recourse to the wet methods of analysis. The principal ores of iron are, however, so well known, that it is seldom necessary to submit the mineral to any test for this metal previous to its quantitative determination.
of Lead.—Whcn heated on charcoal with reducing agents in the inner flame, lead cora peunds give a bluish-white, very malleable bead of metallic lead ; around the head is formed a yellow coating of oxide of lead, which becomes lighter in colour on cooling. The bead of metal is readily soluble in nitric acid, but with difficulty in sulphuric and hydrochloric acids, unless strong and boiling ; aulphuretted hydrogee passed through the solution produces a black precipitate of sulphide of lead, insoluble in weak, cold acids. Sulphuric acid produces a white, heavy precipitate of sulphate of lead, soluble in hot, concentrated nitric acid, and in solutions of salts of ammonia.
Detection of Manganese.—The presence of manganese may readily be detected by the borax method. In the outer flame of the blowpipe, a bead is obtained of a beautiful and very characteristic amethyst colour, which, when heated in the inner flame, becomes colourless. Only a trace of the powdered mineral should be introduced into the bead or the colour will be too intense. Another teat, which is more delicate still, may be made by heating a mixture of the powdered mineral with twice its weight of carbonate of soda and a little nitre on platinum foil in the blowpipe flame, which is preferably directed upon the foil from beneath. If the least trace of manganese be present, it is et once converted into mangauate of soda, which may readily be recognized by its bright green colour.
Detection of Mercury.—Native mercury is, of course, recognized at once by its appearance, and cinnabar may also he known by its weight and colour. If a mineral containing mercury, powdered and quite dry, be placed in a narrow tube open at one end, and heated gently, it is decomposed with sublimation of metallic mereory in the upper part of the tube. When only a small quantity of mercury is present, the sublimate is not easily distinguished from those of antimony and arsenic without the aid of a lens; examined in this way, the sublimate will he seen to consist of minute globules of metallic mercury.
Detection of Nichel.—It is difficult to detect the presence of nickel in its ores by the borax method, owing to the deep blue colour imparted by cobalt, which is almost invariably present also. When heated on charcoal with carbonate of soda in the reducing flame of the blowpipe, a grey, metallic powder is produced, which is highly magnetic. The fused mass should be powdered under water and stirred up with a magnet, or with a magnetized pen-knife; the powder is attracted and may be withdrawn from the water adhering to the magnet, and then scraped off with a knife. When free from cobalt, nickel compounds heated with borax in the oxidizing flame yield a reddish bead which turns yellow on cooling.
Detection of Potassium.—The estimation of the alkalies, potash and soda, is described separately in the article on Alkalimetry.
Detection of Silver.—When mixed with carbonate of soda and heated on charcoal in the reducing flame, silver compounds are reduced to a bright white bead of metallic silver. This bead may be dissolved in pure, warm, dilute nitric acid. Hydrochloric acid added to this solution produces a white, curdy precipitate of chloride of silver, by which the least trace of silver may readily be detected. This precipitate gradually turns violet on exposure to daylight, ultimately becoming black ; it is easily soluble in solution of ammonia.
Detection of Tin.—Heated on charcoal with carbonate of soda, tin ores yield a bright, malleable, metallic bead, no coating being formed around it. This bead is soluble in hydrochloric acid ; the solution, when treated with mercuric chloride, gives a white precipitate of mercurous chloride; this precipitate becomes grey on beating, from reduction of metallic mercury.
Detection of Tungsten.—When heated with borax in the inner flame, tungsten compounds yield a yellow bead, if sufficient quantity has been added ; if not, the bead is colourless. With microcosmic salt, in place of borax, the bead is blue, unless iron be present, the latter turning it blood-red. The addition of a small quantity of metallic tin to the bead reproduces the blue colour immediately.
Detection of Zinc.—Heated with carbonate of soda on charcoal in the inner flame, all zinc com pounds give a yellow incrustation of oxide, which turns white on cooling, and does not disappear when heated in the outer flame. With borax, they form a yellowish bead, turning white when cool, and opaque if much zinc be present. If the white incrustation or the white bead be moistened with a dilute solution of cobalt nitrate and then heated strongly in the outer flame, a rich green colour is produced.