contains, in addition to the sulphides of the platinum metals, a large proportion of sulphur and the sulphides of copper and lead. To get rid of these bodies, I have thought of concentrated sulphuric acid, which changes them to sulphurous acid and sulphates, while it does not act on the sulphides of the precious metals. This refining can be effected in an iron vessel, but Mr. Matthey, who neglects nothing to ensure the certainty and exactness of the results, makes use of platinum. When, after prolonged boiling, no more sulphurous acid is given off, the refining is complete. The mass of sulphides, diluted with a quantity of water, is thrown on filters, and thoroughly washed, until ammonia no longer finds any trace of copper or iron in the filtered liquid. At this point precious metals are entirely freed from iron, which is so detrimental to them, and from copper, and contain only a little sulphate of lead, which separates by itself during an ulterior reaction. They are then, moreover, in a condition to be dissolved by simple nitric acid or by aqua regia, and this is not their least valuable condition. "Treatment of the Sulphides.-The sulphides are next dissolved in aqua regia, which should not be previously prepared, because its action on sulphates is so sudden and energetic; it heats so rapidly, and the disengagement of gas is so great that, were it previously prepared, it would certainly be thrown from the vessels. 'I add then moderately strong cold nitric acid, and add it gradually, because its action is strong. A quantity of rutilant vapours are disengaged. Hydrochloric acid is added when the effervescence ceases. It is then gradually heated to boiling, which is necessary to obtain a complete solution. The solution is poured from the deposited chloride of lead, and the ordinary method with sal ammoniac is used to separate the different metals it contains. Experiments on large quantities of material have fully proved the advantages of this process. 641 CHAPTER XVIII. THE ASSAY OF BISMUTH. THE FOLLOWING varieties of bismuth ores are met with, but are somewhat rare : Oxide of Bismuth. Cupriferous Sulphide of Bismuth. Plumbo-cupriferous Sulphide of Bismuth. Lastly, we have Native Bismuth, which, although far from common, is the only mineral hitherto found to supply the wants of commerce with the pure metal; and the only products of it are bismuth slags and cupel bottoms, in which oxide of bismuth is present in lieu of oxide of lead; it sometimes happening that bismuth is employed instead of lead in cupellation (see Silver Assay). Native Bismuth possesses a tolerably bright metallic lustre; its colour yellowish-white, often iridescent. It fuses in the candle flame. It is generally found in small amorphous lamellar masses, yet it occasionally occurs in acute rhomboidal as well as cubical and octohedral crystals. This substance does not seem to form veins by itself, but generally accompanies other minerals, particularly those of cobalt, nickel, arsenic, and lead. Assay of Native Bismuth.-The assay of native bismuth may be done in the same way as that of Antimonium crudum, i.e. the bismuth is separated from the gangue in which it occurs by heating the mineral in closed vessels, as described at the Assay of Antimony. Assay of Bismuth Residues, Cupel Bottoms, &c.-These TT substances must be finely pulverised, and from 200 to 400 grains mixed with three times its weight of fused borax, their own weight of carbonate of soda, and from 100 to 200 grains of cyanide of potassium, and proceed with all the precautions above pointed out. In case the mineral contained besides bismuth other metals (they are mostly tin, copper, and lead), the resulting metal-button will contain part of those metals, and they must, be determined by the humid process. Determination of amount of Bismuth by the Humid Process.-Act on 50 grains of the finely powdered substance with strong nitric acid until all action ceases, evaporate to dryness, add from 50 to 100 drops of strong sulphuric acid, well mix with a glass rod, and evaporate to dryness; add water, with a few drops of sulphuric acid, and boil. Filter the solution, and to the filtered solution add excess of carbonate of ammonia. Collect the oxide of bismuth thus thrown down on a filter, wash, and dry; separate it carefully from the filter, ignite it, and weigh: every 100 parts correspond to 89.87 of bismuth. Or the bismuth may be obtained at once in the metallic state from the solution prepared as above: by adding to it metallic copper in the form of a small sheet, and gently heating, the bismuth will separate in the metallic state, and can be washed, dried, and weighed, as directed for copper, under the Assay of that metal. The high price of bismuth for some years past has induced M. Balard to undertake the search for this metal in old type metal. When it was cheaper, bismuth entered into the composition of the alloy for printing purposes. M. Balard proposes to effect this industrial analysis in the following way : 1. Dissolve the material in nitric acid, so as to transform all the tin into metastannic acid, which isolate by filtration. from the acid solution of nitrates of lead and bismuth; wash with acidulated water, dry, and reduce by charcoal. 2. Into the liquid, neutralised as much as possible, plunge plates of lead, which precipitate all the bismuth in a metallic state; dry, and melt with a reducing agent. 3. Precipitate the lead from the last liquid by carbonate of soda; separate, wash, dry, and reduce with charcoal. This way of operating gives the three metals in a metallic state; it may undergo several modifications for isolating the metals under another form according to the arrangement of the products. To obtain extremely pure subnitrate of bismuth, says M. Balard, it is necessary only to neutralise the liquid containing the soluble nitrates, and dilute with a large quantity of water naturally free from carbonates, chlorides, or sulphates. After again neutralising and diluting with water and repeating the operations several times, the greater part of this metal becomes separated in the state of white bismuth. Mr. R. W. Pearson has given the following process for the Assay of Bismuth by weight and by volume. Preparation of standard solution.-7135 grs. of pure crystallised bichromate of potash are dissolved in 100 grs. Call this solution the bichrome test A. of water. In a similar way, prepare a second solution, one-tenth the strength of bichrome test A; 07135 grs. of bichromate of potash diffused in 100 grs. of water, will furnish such a solution; call it the bichrome test B. Bichrome test C, onetenth the strength of solution B, is also prepared by dissolving 007135 grs. of the bichromate of potash in 100 grs. of water. These figures can be multiplied to any convenient number. These solutions will contain bichromate of potash, in 100 grs. of bichrome test A, equal to 1 gr. of bismuth; in 100 grs. of bichrome test B, equal to 0.1 gr. of bismuth; and in 100 grs. of bichrome test C, equal to 01 gr. of bismuth. The bismuth should be in the form of nitrate, and the solution kept hot during the experiment, as the precipitated chromate collects more readily than after comple precipitation of the bismuth; the solution will exhibit a characteristic colour, produced by excess of the bichromate of potash. By employing a standard solution of bismuth it has been ascertained that 71.35 parts of bichromate of potash are required to combine with 100 parts of bismuth. BLOWPIPE REACTIONS OF BISMUTH. NATIVE BISMUTH.-Alone, fuses, giving a weak arsenical odour. Otherwise, it presents the same phenomena as pure bismuth. In the open tube it gives a little arsenious acid. Cupelled, it tinges the bone ash pure orange-yellow. SULPHIDE OF BISMUTH.-Alone, in the tube, gives sulphurous acid and a white sublimate; heated to redness, it deposits oxide of bismuth round the assay, like pure bismuth. On charcoal it fuses with bubbling, throwing out small incandescent globules. This agitation lasts but a short time. OXIDE OF BISMUTH.-Alone, oxide of bismuth fuses readily on the platinum wire, forming a deep brown mass, which becomes yellow on cooling. If acted upon by a very intense flame, it is reduced, and perforates the platinum. It is reduced instantaneously on charcoal. With borax it fuses into a colourless glass in the oxidising flame. In the reducing flame it becomes greyish, owing to the dissemination of particles of bismuth. Microcosmic salt forms with it a brownish-yellow glass. In the reducing flame, particularly with tin, a glass is formed, which is clear and colourless while hot, but becomes greyishblack on cooling. Oxide of copper presents nearly the same phenomena under the same circumstances, but with this difference that tin produces a red colour. Owing to the facility with which bismuth may be reduced, it is nearly always on the metal that the assay is made; hence it becomes very important to distinguish it from the antimony and tellurium, with which it may be readily confounded. Firstly, in the matrass neither antimony nor bismuth sublime at a temperature the glass can bear. Tellurium, on the contrary, gives at once a little smoke (by means of the oxygen of the atmosphere), and finally, a grey sublimate of metallic tellurium is obtained. Secondly, in the open tube antimony gives a white vapour, which lines the interior of the tube, and which can be driven by heat from one part to another without leaving the least |