is treated with aqua regia, this alloy is left undissolved, together with grains of chrome-iron ore and titanic iron. To extract the osmium from this residue, it is heated in a porcelain tube through which a current of dry air is passed, when the osmium is converted into osmic acid, the vapour of which is carried forward by the current of air and condensed in bottles provided to receive it. The osmic acid forms colourless prismatic crystals which fuse and volatilise below the boiling-point of water, yielding a most irritating vapour resembling chlorine. It is very soluble in water, giving a solution which exhales the odour of the acid and stains the skin black; tincture of galls gives a blue precipitate with the solution. Its acid properties are feeble, for it neither reddens litmus nor decomposes the carbonates, and its salts are decomposed by boiling their solutions. By adding hydrosulphuric acid to a solution of osmic acid, the tetrasulphide of osmium (OsS) is obtained as a black precipitate, and if this be carefully dried and heated in a crucible made of gas-carbon, metallic osmium is obtained as a brittle mass (sp. gr. 21-4), which is not fused even by the oxyhydrogen blowpipe, and is not soluble in acids. When obtained by other processes in a finely divided state, osmium oxidises even at the ordinary temperature, and emits the odour of osmic acid. In this state, also, it may be dissolved by nitric acid, which converts it into osmic acid. By dissolving osmic acid in potash and adding alcohol, the latter is oxidised at the expense of the osmic acid, and rose-coloured octahedral crystals of osmite of potash (KO. OsO,, 2Aq.) are obtained; the osmious acid has not been isolated. Á protoxide and a binoxide of osmium have been obtained. Osmium appears to form four chlorides-protochloride (OsCI), sesquichloride (Os,Cl), bichloride (OsC1), and terchloride (OsCl). The protochloride and bichloride are formed by the direct combination of chlorine with osmium; the former sublimes in green needles, which yield a blue solution in water, soon absorbing oxygen from the air and becoming converted into bichloride. By heating a mixture of pulverulent osmium with chloride of potassium in a current of chlorine, a double chloride of osmium and potassium (KCl, OsCl2), is obtained which is sparingly soluble, and crystallises in octahedra like the corresponding salt of platinum. When decomposed with nitrate of silver, it gives a dark green precipitate (AgCl, OsCl2). 303. RUTHENIUM.-In the process for extracting osmium from the residue left on treating the platinum ore with aqua regia, by heating in a current of air, square prismatic crystals of binoxide of ruthenium (RuO2) are deposited, nearer to the heated portion of the tube than the osmic acid, for the binoxide is not itself volatile, being only carried forward mechanically in company with the osmic acid. When binoxide of ruthenium is heated in hydrogen, metallic ruthenium is obtained as a hard, brittle, almost infusible metal, which is scarcely affected even by aqua regia. The protoxide of ruthenium (RuO) is a dark grey powder insoluble in acids. The sesquioxide (Ru,O) and the binoxide (RuO2) have feebly basic properties. The sesquioxide is not decomposed by heat. The anhydrous binoxide is a greenish blue powder. Ruthenic acid (RuO3) is known only in combination with bases. 304. IRIDIUM, named from Iris, the rainbow, in allusion to the varied colours of its compounds, has been mentioned above as occurring in the insoluble alloy from the platinum ores. It is also sometimes found separately, and occasionally alloyed with platinum, the alloy crystallising in octahedra, which are even heavier than platinum (sp. gr. 22.3). If the insoluble osmiridium alloy left by aqua regia be mixed with common salt and heated in a current of chlorine, a mixture of the sodio-chlorides of the metals is obtained, and may be extracted by boiling water. If the solution be evaporated and distilled with nitric acid, the osmium is distilled off as osmic acid, and by adding chloride of ammonium to the residual solution, the iridium is precipitated as a dark red-brown ammonio-chloride (NH3, HCl, IrCl2) which leaves metallic iridium when heated. Like platinum, it then forms a grey spongy mass, but is oxidised when heated in air, and may be fused with the oxyhydrogen blowpipe to a hard brittle mass (sp. gr. 21-2), which does not oxidise in air. Like rhodium it is not attacked by aqua regia, unless alloyed with platinum. The product of the oxidation of finely divided iridium in air is the sesquioxide (Ir2O3), which is a black powder used for imparting an intense black to porcelain; it is insoluble in acids. The protoxide (IrO) is also more easily acted upon by alkalies than by acids; its solution in potash becomes blue when exposed to air, from the formation A new mineral found in Borneo, and named laurite, contains sulphides of ruthenium and osmium. It forms small lustrous granules. 402 OCCURRENCE OF GOLD IN NATURE. of the binoxide (IrO2). The teroxide (IrO3) is green. The protochloride (IrCl) and bichloride (IrCl) of iridium resemble the corresponding chlorides of platinum in forming double salts with the alkaline chlorides. There is also a sesquichloride (IrCl), the solution of which has a green colour, and gives a yellow precipitate with mercurous nitrate, and a blue precipitate, soon becoming white, with nitrate of silver. Iridium resembles palladium in its disposition to combine with carbon when heated in the flame of a spirit-lamp. 305. The following table exibits a general view of the analytical process by which the remarkable metals associated in the ores of platinum may be separated from each other, omitting the minor details which are requisite to ensure the purity of each metal. The group of platinoid metals exhibits some very remarkable features, and it is to be regretted that it is comparatively imperfectly known in consequence of the difficulty and expense attendant upon the purification of the metals. Its members may be arranged in two divisions, the metals in each agreeing closely in their equivalent weights and specific gravities. as NaCl.IrCl Chrome iron, Through osmium, this group of elements is connected with the group containing antimony, arsenic, and phosphorus, which osmium resembles in the facility with which it is oxidised, and in the volatility of the oxide formed. Palladium connects it with mercury and silver by its solubility in nitric acid, and its special attraction for cyanogen and iodine. GOLD. 306. Gold is one of those few metals which are always found in the metallic state, and is remarkable for the extent to which it is distributed, though in small quantities, over the surface of the earth. The principal supplies of this metal are derived from Australia, California, Mexico, Brazil, Peru, and the Uralian Mountains. Small quantities have been occasionally met with in our own islands, particularly at Wicklow, at Cader Idris in Wales, Leadhills in Scotland, and in Cornwall. The mode of the occurrence of gold in the mineral kingdom resembles that of the ore of tin, for it is either found disseminated in the primitive rocks, or in alluvial deposits of sand, which appear to have been formed by the disintegration of those rocks under the continued action of torrents. In the former case, the gold is often found crystallised in cubes and octahedra, or in forms derived from these, and sometimes aggregated together SMELTING OF GOLD ORES. 403 in dendritic or branch-like forms. In the alluvial deposits, the gold is usually found in small scales (gold dust), but sometimes in masses of considerable size (nuggets), the rounded appearance of which indicates that they have been subjected to attrition. The extraction of the particles of gold from the alluvial sands is effected by taking advantage of the high specific gravity of the metal (19.3), which causes it to remain behind, whilst the sand, which is very much lighter (sp. gr. 2.6), is carried away by water. This washing is commonly performed by hand, in wooden or metal bowls, in which the sand is shaken up with water, and the lighter portions dexterously poured off, so as to leave the grains of gold at the bottom of the vessel. On a somewhat larger scale, the auriferous sand is washed in a cradle or inclined wooden trough, furnished with rockers, and with an opening at the lower end for the escape of the water. The sand is thrown on to a grating at the head of the cradle, which retains the large pebbles, whilst the sand and gold pass through, the former being washed away by a stream of water which is kept flowing through the trough. When the gold is disseminated through masses of quartz or other rock, much labour is expended in crushing the latter before the gold can be separated. This is effected either by passing the coarse fragments between heavy rollers of hard cast-iron, or by stamping them, with wooden beams shod with iron, in troughs through which water is kept continually flowing. In some cases it is found advantageous to smelt the ore by fusing it with some substance capable of uniting with the gold, and of being afterwards readily separated from it. Lead is peculiarly adapted for this purpose; the crushed ore, being mixed with a suitable proportion, either of metallic lead, or of litharge (oxide of lead) and charcoal, or even of galena (sulphide of lead), together with some lime and oxide of iron or clay, to flux the silica, is fused on the hearth of a reverberatory furnace, when the fused lead dissolves the particles of gold, and collects beneath the lighter slag. The lead is afterwards separated from the gold by cupellation (see p. 353). In smelting the ores of gold in Hungary, the metal is concentrated by means of sulphide of iron. The ore consists of quartz and iron pyrites (bisulphide of iron) containing a little gold. On fusing the crushed ore with lime, to flux the quartz, the pyrites loses half its sulphur, and becomes sulphide of iron (FeS), which fuses and sinks below the slag, carrying with it the whole of the gold. If this product be roasted so as to convert the iron into oxide, and be then again fused with a fresh portion of the ore, the oxide of iron will flux the quartz, whilst the fresh portion of sulphide of iron will carry down the whole of the gold contained in both quantities of ore. This operation having been repeated until the sulphide of iron is rich in gold, it is fused with a certain quantity of lead, which extracts the gold and falls to the bottom. The lead is then cupelled in order to obtain the gold. When the ores of lead, silver, or copper contain gold, it is always found to have accompanied the silver extracted from them, and is separated from it by a process to be presently noticed. Gold is sometimes separated from the impurities remaining with it after extraction by washing, by the process of amalgamation, which consists in shaking the mixture with mercury in order to dissolve the gold-dust, and straining the liquid amalgam through a chamois leather, which allows the excess of mercury to pass through, but retains the solid portion containing the gold, from which the mercury is then separated by distillation.* In the Tyrol, this process is adopted for separating the gold from an auriferous iron pyrites by grinding it in a mill of peculiar construction, with water and a little mercury, the latter being allowed to act upon successive portions of ore until it becomes sufficiently rich to be strained and distilled. Gold, as found in nature, is generally alloyed with variable proportions of silver and copper, the separation of which is the object of the gold refiner. It may be effected by means of nitric acid, which will dissolve the silver and copper, provided that they do not bear too small a proportion to the gold. Sulphuric acid, however, being very much cheaper, is generally employed. The alloy is fused and poured into water, so as to granulate it and expose a larger surface to the action of the acid; it is then boiled with concentrated sulphuric acid (oil of vitriol), which dissolves the silver and the copper in the form of sulphates, with evolution of sulphurous acid gas, whilst the gold is left untouched. In order to recover the silver from the solution, scraps of copper are introduced into it, when that metal decomposes the sulphate of silver, producing sulphate of copper, and causing the deposition of the silver in the metallic state. Finally, the sulphate of copper may be obtained from the solution by evaporation and crystallisation. This process is so effectual when the proportion of gold in an alloy is very small, that eventh part of this metal may be profitably extracted from 100 parts of an alloy, and much gold has been obtained in this way from old silver-plate, coins, &c., which were manufactured before so perfect a process for the separation of these metals was known. On boiling old silver coins or ornaments with nitric acid, they are generally found to yield a minute proportion of gold in the form of a purple powder. But this plan of separation is not so successful when the alloy contains a very large quantity of gold, for the latter metal seems to protect the copper and silver from the solvent action of the acid. Thus, if the alloy contains more than th of its weight of gold, it is customary to fuse it with a quantity of silver, so as to reduce the proportion of gold below that point, before boiling it with the acid. Again, if the alloy contains a large quantity of copper, it is found expedient to remove a great deal of this metal in the form of oxide by heating the alloy in a current of air. Pure gold, like pure silver, is too soft to resist the wear to which it is subjected in its ordinary uses, and it is therefore alloyed for coinage in this country with th of its weight of copper, so that gold coin contains 1 part of copper and 11 parts of gold. The gold used for articles of jewellery is alloyed with variable proportions of copper and silver. The alloy of copper and gold is much redder than pure gold. The degree of purity of gold is generally expressed by quoting it as of so many carats fine. Thus, pure gold is said to be 24 carats fine; English standard gold is 22 carats fine, that is, contains 22 carats of gold out of the 24. Gold of 18 carats fine would contain 18 parts of gold out of the 24, or ths of its weight of gold. Pure gold is easily prepared from standard or jeweller's gold, by dissolving it in hydrochloric acid mixed with one-fourth of its volume of nitric acid, evaporating the solution to a small bulk to expel excess of acid, diluting with a considerable A small quantity of sodium dissolved in the mercury has been found very materially to facilitate the amalgamation of gold and silver ores. quantity of water, filtering from the separated chloride of silver, and adding a solution of sulphate of iron, when the gold is precipitated as a dark purple powder, which may be collected on a filter, well washed, dried, and fused in a small crucible with a little borax, the crucible having been previously glazed with borax to prevent adhesion of the globules of gold. The action of the sulphate of iron upon the terchloride of gold is explained by the equation— AuCl + 6(FeO.S03) = Au + FeCl3 + 2(Fe2O3. 3SO3) . By employing oxalic acid instead of sulphate of iron, and heating the solution, the gold is precipitated in a spongy state, and becomes a coherent lustrous mass under pressure. The metal is employed in this form by dentists. When standard gold is being dissolved in aqua regia, it sometimes becomes coated with a film of chloride of silver which stops the action of the acid; the liquid must then be poured off, the metal washed, and treated with ammonia, which dissolves the chloride of silver; the ammonia must be washed away before the metal is replaced in the acid. In the case of jeweller's gold, it is advisable to extract as much silver and copper as possible by boiling it with nitric acid, before attempting to dissolve the gold. Gold lace should be incinerated to get rid of the cotton before being treated with acid. The genuineness of gold trinkets, &c., is generally tested by touching them with nitric acid, which attacks them if they contain a very considerable proportion of copper, producing a green stain, but this test is evidently useless if the surface be gilt. The weight is, of course, a good criterion in practised hands, but even these have been deceived by bars of platinum covered with gold. The specific gravity may be taken in doubtful cases; that of sovereign gold is 17.157. In assaying gold, the metal is wrapped in a piece of thin paper together with about three times its weight of pure silver, and added to twelve times its weight of pure lead fused in a bone-ash cupel (see page 355) placed in a muffle (or exposed to a strong oxidising blowpipe flame), when the lead and copper are oxidised, and the fused oxide of lead dissolves that of copper, both being absorbed by the cupel. When the metallic button no longer diminishes in size, it is allowed to cool, hammered into a flat disk, which is annealed by being heated to redness, and rolled out to a thin plate, so that it may be rolled up by the thumb and finger into a cornet, which is boiled with nitric acid (sp. gr. 1·18) to extract the silver; the remaining gold is washed with distilled water, and boiled with nitric acid of sp. gr. 1.28 to extract the last traces of silver, after which it is again washed, heated to redness in a small crucible, and weighed. The stronger nitric acid could not be used at first, since it would be likely to break the cornet into fragments which could not be so readily washed without loss. The addition of the three parts of silver (quartation) is made in order to divide the alloy, and permit the easy extraction of the silver by nitric acid, which cannot be effected when the gold predominates. 307. The physical characters of gold render it very conspicuous among the metals; it is the heaviest of the metals in common use, with the exception of platinum, its specific gravity being 19.3. In malleability and ductility it surpasses all other metals; the former property is turned to advantage for the manufacture of gold leaf, for which purpose a bar of gold is passed between rollers which extend it into the form of a riband; this is cut up into squares, which are packed between layers of fine vellum, and beaten with a heavy hammer; these thinner squares are then again cut up and beaten between layers of gold-beater's skin until they are sufficiently thin. An ounce of gold may thus be spread over 100 square feet; 282,000 of such leaves placed upon each other form a pile of only one inch high. These leaves will allow light to pass through them, and always appear green or blue when held up to the light, though they exhibit the ordinary colour of gold by reflected light; extremely thin leaves of gold, obtained by partially dissolving ordinary gold leaf by floating it on solution of cyanide of potassium, transmit a violet or a red light, according to their thickness, though they still appear yellow by reflected light, and if taken up on a glass plate and heated to about 600° F. they lose their |