QUANTITATIVE ANALYSIS WITH THE BLOWPIPE. Preparation of the Substance, the Metallic Constituents of which are to be ascertained. BEFORE it is possible to proceed with the investigation of a mineral, ore, furnace product, et cetera, in order to ascertain the different metals present, they must undergo a certain treatment, which is termed "Preparation of the Assay." The antecedent labors are desiccation of those substances containing mechanically mixed water; and pulverization, when required in a finely divided state. Brittle substances, difficultly reducible to powder, may be crushed under the hammer upon an anvil but malleable bodies can be first laminated, and then cut into shreds with a strong scissors. It frequently occurs that ores, prepared on a larger scale, are apparently dry, although some per cents of water may be mechanically held between their layers; in other cases, ores kept, after desiccation, in damp apartments, or in open vessels, reabsorb moisture from the air. If, therefore, the substance under examination contains mechanical moisture, a quantity greater than is requisite for two assays should be heated in a porcelain basin over a spirit-lamp, to expel the water, and the desiccated mass then triturated. During the desiccation, care must be taken not to raise the temperature so high as to roast the specimen. Minerals and furnace products received for investigation in a dry, but not pulverized state, may be broken into fragments upon the anvil, and afterwards, if they be fit for powdering, triturated in the agate mortar. Great exactness in the results cannot be expected, unless a portion eight or ten times greater than is required for one experiment be prepared,-except in the case of pure crystals and homogeneous specimens of minerals. It is impossible to represent the average constituents of any substance, as a rich metallic ore associated with extraneous nonmetallic constituents : for by taking a small portion for investigation, it is probable that either too small or too large a per centage of the metals is obtained, according to the excess of the metallic or nonmetallic ingredients present. Therefore, of minerals dressed on a large scale, an ounce should be selected from various parts of the mass, pulverized in an iron mortar, and, if possible, reduced to powder. A satisfactory result will be obtained, if 8 or 10 decigrammes, the quantity necessary for an experiment, be employed. DESCRIPTION OF THE VARIOUS QUANTITATIVE ANALYSIS. All minerals, ores, and furnace products must be specified under various heads, videlicet : TA. Ores, Minerals, and Products of Smelting Works,--the subdivisions of which are: a. Such as contain volatile ingredients; b. Such as contain no volatile ingredients except chlorine; d. Such as are irreducible with borax and workable lead. ¶ B. Metallic Compounds, the principal ingredients of which are: a. Silver, b. Gold, c. Copper or Nickel, d. Lead, e. Antimony or Zinc, f. Tin, g. Mercury, h. Iron or steel. I. THE SILVER ASSAY. The Silver Assay with the Blowpipe, which HARKORT has published, is one of the most important quantitative analysis that can be performed with this instrument. It affords the means not only of ascertaining the proportion of silver in any ore, mineral, or production of smelting works, et cetera, but also of determining its quantity with sufficient accuracy. However, to obtain satisfactory results, it must be considered what ingredients besides silver are combined in the body under exami nation. TA. Ores, Minerals, and Furnace Products. a. EXAMINATION FOR SILVER IN THOSE MINERALS CONTAINING VOLATILE INGREDIENTS. To this class belong, besides the ores prepared on a greater scale, and containing large quantities of Sulphides of Iron, Copper, Arsenic, Antimony, and Zinc, the following minerals: Vitreous Silver, Sulphide of Silver,-Antimonial Silver, Melan Glance, light and dark Ruby Silver, Telluric Silver, Arsenical Silver, Argentiferous Sulphide of Copper, Miargyrite, Fahl Glance, Bismuthic Lead Ore, Vitreous Copper, Bournonite, Tin Pyrites, Galena, et cetera; and of the furnace products, Rohstein, Bleistein, Kupferstein, Kupferleg, Tutty, Lead Speiss, Cobalt Speiss, et cetera. After preparation in the manner given on the two previous pages, the following procedure is to be undertaken : Weighing and Dressing of the Assay. The weight is to be ascertained either on a balance, or upon the scales, described after the Gold Assay, and at the end of this Section. Of rich silver ores associated with nonmetallic constituents, and therefore prone to represent very varying contents, two, or if necessary, three samples, of one decigramme, ought to be weighed twice; but poor silver ores, crystallized minerals, and also products of smelting works, which generally form a homogeneous mass, may be weighed once only. However, if the experimenter be not accustomed to Blowpipe examinations, even ores which generally do not materially differ should be weighed and examined twice. A sample of one decigramme being weighed, it is to be poured from the basin of the balance, the adhering particles can be removed with the hair brush, so as to incur no loss,-into the mixing capsule, and dressed with borax and proof lead. The requisite proportion of borax varies according to the quantity and fusibility of the substance. In most cases, one decigramme is sufficient even for a difficultly fusible alloy; if, however, during the operation, the mass with this quantity appears intact when submitted to a strong heat, another small portion of borax should be added. For ores easily fusible, particularly for such as are not associated with earths, and consist of sulphides which unite quickly with lead, but do not oxidize so readily as this metal, a small quantity only of borax, 50 to 75 milligrammes, ought to be taken. However, if argillaceous constituents be in excess, or the assay contains much iron, cobalt, or tin, one decigramme of the flux will be necessary. With regard to the lead, it should first be considered what other metals besides silver are extant in the alloy, for if an ore, mineral, or furnace product contains no more than 7 per cent. of copper, or 10 per cent. of nickel, five decigrammes of lead will be sufficient for one decigramme of the ground ore; but, on the contrary, if the substance contains more than the above cited per ONE DECIGRAMME OF centage of copper or nickel, the quantity of lead must be increased. As it is impossible always to have prescience of the per centage of these metals in a mineral or ore, the safest plan to adopt is to have a slight excess of lead, otherwise the copper cannot be perfectly detached from the silver, and also, a fusion of a nickeliferous workable lead is nearly impossible. The annexed table shows the proportions of lead to the substance in some of the most common minerals and furnace products, consisting partly of copper and nickel. |