obtained, divided by two, gives the per centage of silver for one decigramme. As the workable lead is a furnace product requiring a further treatment, in which a loss of silver also takes place, it is evident that the cupelling loss in this operation need not be taken into consideration. If, however, the per centage of silver is to be determined exactly, five decigrammes of such lead ought to be weighed, fused, and refined in two periods, and if, owing to too large a quantity of copper, the silver globule is not yet sufficiently pure, it is to be recupelled with one decigramme of proof lead. To the weight of the fine silver globule thus obtained, the amount of the cupelling loss, answering to the oxidized quantity of lead, is added, by which, and division of the sum by five, the real per centage of silver for one decigramme of the examined workable lead will be found. If revivified lead or bismuth is to be examined for silver, the former is cut into shreds like the workable lead, but the latter, on account of its brittleness, must be pulverized under the hammer. As no high per centage of silver can be expected in these compounds, an assay of five decigrammes ought generally to be employed. The weighed quantity of the alloy must either be immediately introduced, like workable lead, into a good cupel and fused, or in case the assay consists of a number of small particles, they should be first fused together on charcoal, and the globule removed from the support in two portions. The separation of the argentiferous bismuth bead, which is to be subsequently cupelled from the oxide of bismuth formed during the oxidation, should be effected with great care, in order to prevent particles of the brittle material remaining. The globule ought never to be raised from the accumulated oxide, but the latter always detached by means of the pincers. The pure globules obtained in the cupellation are to be measured in the scale, and the content of silver in one decigramme calculated from the weight found. e. EXAMINATION OF METALLIC COMPOUNDS IN WHICH The subsequent observations apply particularly to argentiferous antimony or zinc. If the antimony contains some per cents. of silver, it can be treated on charcoal in the oxidating flame; the antimony volatilizes, and the silver remains as a dull bead. If, however, this does not happen, the silver may probably have been carried away by the current of air, and consequently the following operation must be performed. Weigh one decigramme of the alloy and fuse it upon charcoal, in the reducing flame, with two decigrammes of proof lead, and a small quantity of borax, and subject the metallic button to the oxidating flame, to expel the antimonide of zinc. Oxidation of the lead does not commence until nearly the whole of these metals are volatilized. When the fumes cease, the blowing must be discontinued, the workable lead allowed to refrigerate, then separated from the scoria, and cupelled. This treatment is preferable for antimony, even when much silver is present, because, without an addition of lead, the last traces of antimony are expelled but imperfectly from the silver, and often a minimum of this metal might remain; besides, if other metals, as copper or iron, are present, these are segregated by the lead in cupellation. f. EXAMINATION OF METALLIC COMPOUNDS IN WHICH TIN IS THE PRINCIPAL INGREDIENT. Take for example Argentiferous tin. As tin cannot be separated from silver, either by cupellation or volatilization, upon charcoal, it must be treated in the following manner : Weigh, of the substance to be examined, one decigramme, and dress it with Envelop the assay in a cornet of soda paper, place the whole in a prepared charcoal cavity, and heat in a strong reducing flame, until the metals form an orbicular alloy, and the borax and soda, -the latter is employed to prevent an oxidation of the tin,-are vitrified. When this is accomplished, the metallic globule alone is to be touched with the blue flame, care being taken to prevent a violent oxidation of the tin, so that the glass may imbibe all the oxide formed. If reduced particles of tin appear upon the sides of the scoria, discontinue the blowing, and allow the assay to cool. The refrigerated globule, to which one decigramme of borax is to be added, must now be treated upon another piece of charcoal, first with the reducing flame, and, if perfect fusion is effected in the oxidating flame, until it presents a bright surface. The workable lead, rendered by this operation free from tin, is submitted to oxidation and cupellation, and the per centage of silver ascertained upon the balance, or, if too small, upon the scale. 9. EXAMINATION OF METALLIC COMPOUNDS, THE PRINCIPAL INGREDIENT OF WHICH IS MERCURY. To this class belong native and artificial amalgams of silver, and argentiferous mercury. One decigramme of the substance is weighed, placed in a small glass tube, with a bulb at one end, as in Fig. 44, page 202. The tube is held inclined, and heated gradually over a spirit-lamp. A very moderate heat volatilizes the greater part of the mercury, which deposits gradually, in a metallic state, upon the upper part of the tube. The bulb must now be heated to redness, and kept in this state until no more mercury is eliminated, when the tube is allowed to cool. After refrigeration, by slightly agitating the vessel, the globules of mercury readily cohere, and the main globule formed can be poured out, by gradually inverting the tube. If the substance submitted to this operation was an amalgam, the silver remains in the bulb as a porous bead, and can readily be extracted. This globule must be fused in the reducing flame with borax, and one decigramme, or, if it contains copper, two to three decigrammes, of proof lead, and the formed workable lead treated in the usual way. The weight of the silver globule obtained is then ascertained upon the balance, and, if a native amalgam has been employed, the cupelling loss deducted. However, if the distilled alloy be argentiferous mercury, the residuum in the tube is very inconsiderable, and cannot be detached from the bulb without great difficulty. In this case, the greater part of the tube must be cut off with a file, and what remains in the bulb dressed with one decigramme of lead, and 50 milligrammes of borax, and the whole placed in a charcoal cavity, and submitted to a strong reducing flame, until the metallic compound exudes from the glass. The argentiferous lead is, after cooling, easily separated from the glass and charcoal, and must be refined upon a well burned cupel of levigated bone ashes, and the resulting silver globule measured upon the scale. The cupellation is necessary in both cases; 1,-because the silver cannot be fused in the tube, and consequently all the mercury is not eliminated; 2,-because the amalgam obtained from amalgamated ores often contains various metals, which remain in the distillation, and can only be separated from the silver by cupellation. If, however, gold be extant in the assay, it cannot be segregated in this way, but must be treated in the manner given under the examination for that metal. h. EXAMINATION OF METALLIC COMPOUNDS HAVING IRON OR STEEL FOR A PRINCIPAL INGREDIENT. As iron or steel cannot be united immediately with lead in the Blowpipe flame, their union must be effected in an indirect manner. It is generally known that sulphide of iron combines with lead, if these bodies are treated with borax in the reducing flame; and ultimately, by heating in the oxidating flame, sulphur volatilizes, and the iron oxidizes and is absorbed by the borax, either in the state of a protoxide or a sesquioxide. Therefore, if iron or steel is united with sulphur, the silver contained in the mineral may be separated as easily as if argentiferous sulphide of iron, et cetera, were assayed. When hardened steel is to be examined for silver, it must first be submitted to a red heat, allowed to refrigerate, and, when cold, its surface cleansed, and about the necessary quantity for an examination taken off it with a file. One decigramme is then to be weighed and dressed with The whole must then be enveloped in a soda paper cornet, placed in a cylindrical charcoal cavity, and heated in the reducing flame until the assay forms a molten bead. By this treatment the sulphur combines with the lead and iron. As one decigramme of borax is not sufficient for imbibing the whole oxide of iron formed in the oxidation following the fusion, another decigramme of the flux is to be added, the whole re-fused, and submitted to a very strong oxidating flame until the impure lead begins to exude from the glass. The assay must now be held so that the lead is in contact with the flame,-the sulphur volatilizes, the iron oxidizes, and the formed oxide combines with the borax. After elimination of the sulphur, and segregation of the iron, the blowing is to be interrupted, and the lead, presenting a bright surface, and containing all the silver of the iron, allowed to refrigerate. If, when cold, it be of a whitish color, it must be treated like an ordinary workable lead, and the weight of the silver globule ascertained; if, however, it be brittle, and of a blackish aspect, it should, before cupellation, et cetera, be submitted to a re-oxidation. |