along with its own weight of granulated assay lead, and with half its weight of anhydrous carbonate of soda. Upon cooling, the globule of silver-lead thus obtained is cupelled as usual. e. Compounds chiefly consisting of Iron: argentiferoussteel; cast-iron; bears from smelting furnace.-Compounds consisting principally of iron with a small percentage of silver, although occasionally produced in the arts intentionally, as, for example, the so-called silver-steel, are commonly found on the blowing-out of furnaces used in the smelting of silver and copper ores, and are frequently rich in silver, as is the case with the bears from the silver furnaces at Kongsberg in Norway. An alloy of iron with silver is occasionally also found appearing in small quantities on the surface of melted silver in the process of casting, and in some cases at least this may be due to the action of the melted silver on the iron rods used for stirring up the molten metal. As iron cannot be made to alloy itself with lead before the blowpipe, it becomes necessary to extract the silver by a more indirect process than is used in the case of other alloys containing that metal. In order to remove the iron the alloy must first be converted into sulphide of iron and silver, and to effect this the iron or steel must be reduced to powder, or fragments none greater than about a quarter of a grain in weight; for which purpose steel when hardened may require to be softened previously. One part of the finely-divided iron or steel is now mixed with 0.75 part sulphur, eight parts granulated assay lead, and one part pulverised borax-glass; the mixture after being placed in a soda-paper cornet is carefully fused in a cavity on charcoal in the reducing flame, until the whole appears as a fluid globule containing both the lead and iron in combination with the sulphur. Without removing either this globule or the glass surrounding it from the charcoal, an amount of borax-glass in one or more fragments (in all about equal in weight to the original amount of iron employed), is now added (in order to combine with and slag off the whole of the iron), and fused along with the former globule, after which the whole is submitted to a strong oxidating flame until the impure lead globule shows itself protruding from the slag. : The charcoal is then inclined, so that the lead is alone subjected to the action of the outer flame, in order to volatilise the sulphur, and at the same time oxidise the iron which goes into the slag this operation is continued until the globule of lead appears with a bright metallic surface; should it on cooling, however, be found to possess a black colour, and to be brittle, it must be still further oxidised as before described. The silver-lead thus obtained will now be found to contain all the silver, and at the same time to be free from both iron and sulphur, and can be cupelled as usual. No notice is here taken of alloys of silver and gold, since these metals cannot be separated before the blowpipe by any process yet known; and in all cases where gold may be present in an alloy, treated as here directed for obtaining its contents in silver, the gold also will be found to follow along with the silver, and must be parted from that metal by the humid method, in order to enable the true amount of silver present in the substance to be ascertained. CHAPTER XVI. THE ASSAY OF GOLD. FOR the purposes of assay, all substances containing gold may be divided into two classes, as in the case of silver. The First Class comprises all substances containing gold in a minute state of division; such, for instance, as those which, suitably pulverised, completely pass through a sieve of 80 holes to the linear inch. It often happens, however, that these substances contain fragments of gold of such magnitude as will not allow them to pass through the sieve in such cases, that which passes through belongs to the first class, and that which remains on the sieve to the second class. The Second Class comprises all alloys of gold, native or otherwise. The name of substances belonging to this class is legion, for an extended examination shows that nearly every mineral substance contains more or less gold. The most common are-gold quartz, auriferous gossans, sulphides of iron (mundic), blende, copper pyrites, many antimonial minerals, galena, and nearly all the primitive rocks. All auriferous slags, amalgamation residues, and tailings, belong to this class. Assay of substances of the First Class.-This assay is conducted in precisely the same manner as that of the corresponding silver class, which see. In case, however, the amount of gold present in the sample is small, as much as 2,000 grains, with flux suitably increased, may be employed. In case any metallic gold is left in the sieve, its amount is to be calculated as that of silver (see pages 476 and 477). It may here be mentioned, that if silver or platinum coexist with the gold in the mineral subjected to assay, t will be found combined with the gold obtained by cupellation; and all gold so obtained must be submitted to the parting process,' which see under the head' Assay of Auriferous Substances of the Second Class.' It may here be mentioned also, that the metallic gold left on the sieve must be thus operated on, as well as that obtained by fusion of the sieved ore and consequent cupellation, before the calculation given at pages 476 and 477 be entered into. When gold is associated in quantity with quartz, its percentage can be approximatively ascertained in the same manner as that of pure tin-stone when mixed with quartz (see pages 414 and 415). If possible, a fragment of the gold must be detached from the quartz, and its specific gravity taken if this be not possible, and the gold is nearly fine, the number 19 may be adopted. It is better, however, to determine experimentally the specific gravity of both quartz and gold. Substances of the Second Class. Native gold. Aurides of silver (native). Gold and rhodium. Gold and palladium. Argentiferous telluride of gold. Native Gold and Aurides of Silver (Native), Au and AuAg", are found in variously contorted and branched filaments, in scales, in plates, in small irregular masses, in the crevices or on the surface of common ferruginous and other quartz. In Devonshire, at the Britannia Mine, it has occurred in pipes or veins, and disseminated in a compact hard gossan, one specimen of which was found to contain 27 per cent. of fine gold; or, as in Wales, it largely accompanies blende and galena: it also occurs in a pyritous quartz; and it has been found in Scotland and Ireland. In the latter locality it occurred in the beds of streams as small scales and rolled masses, and nearly up to the present time this has been the most frequent mode of occurrence; but now, however, by the aid of improved machinery, rocks and minerals containing a comparatively small quantity can be profitably worked; and from this source, the greatest part of the gold poured into commerce is now extracted. Composition of several varieties of Native Gold, by Boussingault, the chief part from Central America. |