Gold

GOLD

THE colour, lustre, and power of resisting oxidation, which this metal possesses, have caused it to be valued from the earliest ages. Allusions to gold are frequent in the Old Testament, and the refining of the precious metals by cupellation seems to have been a favourite illustration with the Jewish poets. Jewellery and vessels found in Egyptian tombs afford evidence of the perfection attained in working gold at a period earlier than the government of Joseph, and drawings on tombs of about this epoch clearly indicate the method of conducting the operations of washing, fusing, and weighing the metal.

Excavations in Etruria have brought to light beautiful ornaments of gold, enriched with minute grains of the metal, the workmanship of which was unrivalled until Castellani studied and revived the methods employed by Etruscan artists. The Greeks were familiar with natural alloys of silver and gold named electrum, rough nuggets of which were frequently stamped, and formed the earliest coins in Lydia. The colour of this electrum is pale yellow to yellowish white, and it contains from 20 to 40 per cent, of silver.

With regard to the history of the metallurgy of gold, it may be mentioned that, according to Pliny, mercury was employed in his time both as a means of separating the precious metals and for the purposes of gilding. Vitruvius also gives a detailed account of the means of recovering gold, by amalgamation, from cloth into which it had been woven.

Properties.— Gold is the only metal of a yellow colour, which is, however, notably affected by small quantities of other metals; thus the tint is sensibly lowered by small quantities of silver, and heightened by copper. The surface colour of particles of gold is often apparently reddened by translucent films of brown iron ore. It is nearly as soft as lead. The hardness varies, however, with the composition. Crystallized specimens from Oregon and Fraser River, containing respectively 835 and 910 parts of gold in 1000, are slightly harder than calc spar but sensibly softer than fluor spar, or much harder than the pure metal. When pure, gold is the most malleable of all metals. One grain may be beaten into leaves which cover a surface of 56 square inches, and are only -^.⅛yyth of an inch thick. Faraday has shown that the thickness of gold leaves may be still further reduced by floating them on a dilute solution of cyanide of potassium. When very thin, leaf gold appears yellow by reflected and green by transmitted light. If, however, certain gold films are heated, the light transmitted is ruby red; the pressure of a hard substance on the film so changes its state of aggregation that green light is again transmitted. The metal is extremely ductile·, a single grain may be drawn into a wire 500 feet in length, and an ounce of gold covering a silver wire is capable of being extended more than 1300 mile3. Gold can readily be welded cold, and thus the finely divided metal, in the state in which it is precipitated from solution, may be compressed between dies into discs or medals. According to G. Rose, the specific gravity of gold in the finely divided state in which it is precipitated from solution by oxalic acid is 19 - 49. The specific gravity of cast gold varies from 18'29 to 19*37, and by compression between dies the specific gravity may be raised from 19 - 37 to 19'41; by annealing, however, the previous density is to some extent recovered, as it then is found to be 19 , 40. Its atomic weight is variously given as follows:—196 - 67 (Berzelius), 196*3 (Levol), 196 ’ 5 (Wurtz), 196Ό (Watts). The number adopted in this work (Chemistry, vol. v. p. 528) is 196 ’ 2. Different observers have given the following temperatures as its melting point : —1425° C. (Daniell), 1200° C. (Pouillet), 1380° C. (Guyton de Morveau). Riemsdijk, after comparing the several results, concludes that it may be considered to be 1240° C. The electric conductivity is, given by Matthiessen as 73∙99 at 15 ’ 1° C., pure silver being 100; this depends greatly on its degree of purity,—the presence of a few thousandths of silver lowering its conductivity by 10 per cent. The specific resistance of the metal in electromagnetic measure, according to the centimetre-gramme-second system of units, is 2154. Its conductivity for heat is 53 ’ 2 (Wiedemann and Franz), pure silver being 100. Its specific heat is 0 ’ 324 (Régnault). Its coefficient of expansion for each degree between 0 and 100° C. is 0 , 000014661, or for gold which has been annealed 0Ό00015136 (Laplace and Lavoisier). The specific magnetism of the metal is 3 ’ 47 (Becquerel). Details as to its tenacity and rigidity are given in the article Elasticity. With regard to its volatility, Gasto Claveus states that he placed an ounce of pure gold in an earthen vessel in that part of a glass-house where the glass is kept constantly melted, and retained it in a state of fusion for two months without the loss of the smallest portion of its weight. Kunkel describes a similar experiment, which was attended with the same result. Homberg, however, observed that when a small portion of gold is kept at a violent heat, part of it is volatilized. Both Macquer and Lavoisier showed that when gold is strongly heated, fumes arise which gild a piece of silver held in them. Its volatility has also been studied by Elsher, and, in the presence of other metals by Napier. Hellot affirms that when an alloy of 7 parts of zinc and 1 part of gold is heated in air, the whole of the gold rises in the fumes of oxide of zinc which are produced. Gold is dissipated by sending a powerful charge of electricity through it when in the form of leaf or thin wire. In the gold spectrum Huggins has observed twenty-three lines, and the wave lengths of the three most important of these are 5231, 5835, and 6276 respectively. Some preliminary observations on the spectrum of the vapour at the temperature of the oxyhydrogen flame, made by Lockyer and Roberts, showed that there was a distinct absorption both at the blue and at the red end.

The solvents for gold are given in the article Chemistry, vol. v. p. 529. It may be added that finely-divided gold dissolves when heated with strong sulphuric acid and a little nitric acid. Dilution with water, however, precipitates the metal as a violet or brown powder from the solution so obtained. Gold is also attacked when strong sulphuric acid is submitted to electrolysis with a gold positive pole. W. Skey has shown that in substances which contain small quantities of gold, the precious metal may be removed by the solvent action of a tincture of iodine or bromine in water. Filter paper soaked with the clear solution is burnt, and the presence of gold is indicated by the colour of the ash.

Occlusion of Gas by Gold.— Graham has shown that gold is capable of occluding 0 ’ 48 of its volume of hydrogen, and 0 - 20 of its volume of nitrogen. Varrentrapp has also pointed out that “cornets” from the assay of gold may retain gas if they are not strongly heated. Artificial crystals of gold may be formed when the molten metal is slowly cooled.

Occurrence and Distribution.— Gold is found in nature chiefly in the metallic state, or as native gold, and less frequently in combination with tellurium, lead, and silver, forming a peculiar group of minerals confined to a few localities in Europe and America. These are the only certain examples of natural combinations of the metal,—the minute although economically valuable quantity often found in pyrites and other sulphides being probably only present in mechanical suspension, although for practical purposes it may be spoken of as combined. The native metal occurs tolerably frequently in crystals belonging to the cubic system, the octahedron being the commonest form, but other and complex combinations have been observed. Owing to the softness of the metal, large crystals are rarely well defined, the points being commonly rounded. In the irregular crystalline aggregates branching and moss-like forms are most common, and in Transylvania thin plates or sheets with diagonal structures are characteristic. These have recently been shown by Vorn Rath to be repeated combinations of distorted tetrahexahedra. During the preparation of a mass of pure gold in the Mint at London, some fine crystals which appear to be aggregations of octahedra were obtained; and dendritic crystals of gold, prepared artificially, have been described by Chester. It is possible also to obtain gold in crystals by heating its amalgam; according to Knaffl, an amalgam of 1 part of gold with 20 parts of mercury is maintained at a temperature of 80° C. for eight days. It is then heated to 80° C. with nitric acid of specific gravity 1 - 35, when dull crystals will be left, which become brilliant when more strongly heated. More characteristic, however, than the crystallized are the irregular forms, which, when large, are known as “nuggets” or “pepites,” and when in pieces below J to ⅜ ounce weight as gold dust, the larger sizes being distinguished as coarse or nuggety gold, and the smaller as gold dust proper. Except the larger nuggets, which may be more or less angular, or at times even masses of crystals, with or without associated quartz or other rock, gold is generally found bean shaped or in some other flattened form, the smallest particles being scales of scarcely appreciable thickness, which, from their small bulk as compared with their surface, subside very slowly when suspended in water, and are therefore readily carried away by a rapid current. These form the “float gold” of the miner. The physical properties of native gold are generally similar to that of the melted metal and its alloys as described above. The composition varies considerably in different localities, as shown in the following table :—

Analyses of Native Gold from various localities. [table]

Of the minerals containing gold the most important are sylvanite or graphic tellurium, of composition (AgAu) Te 2 , with 24 to 26 per cent.; calaverite, AuTe 2 , with 42 per cent.; and nagyagite or foliate tellurium, of a complex and rather indefinite composition, with 5 to 9 per cent, of gold. These are confined to a few localities, the oldest and best known being those of Nagyag and Ofenbanya in Transylvania; but latterly they have been found in some quantity at Red Cloud, Colorado, and in Calaveras county, California-—the nearly pure telluride of gold, calaverite, being confined to these places.

The minerals of the second class, usually spoken of as auriferous, or containing gold in sensible quantity, though not to a sufficient amount to form an essential in the chemical formulae, or even in many instances to be found in the quantifies ordinarily operated upon in analyses, are comparatively numerous, including many of the metallic sulphides. Prominent among these are galena and iron pyrites,—the former, according to the observations of Percy and Smith, being almost invariably gold-bearing to an extent that can be recognized in operating upon a pound weight of the lead smelted from it, the proportion increasing to some extent with the amount of silver. The second is of greater practical importance, being in some districts exceedingly rich, and, next to the native metal, is the most prolific source of gold. Magnetic pyrites, copper pyrites, zinc blende, and arsenical pyrites are other and less important examples,—the last constituting the gold ore formerly worked in Silesia. A native gold amalgam is found as a rarity in California, and bismuth from South America is sometimes rich in gold. Native arsenic and antimony are also very frequently found to contain gold and silver.

The association and distribution of gold may be considered under two different heads, namely, as it occurs in mineral veins, and in alluvial or other superficial deposits which are derived from the waste of the former. As regards the first, it is chiefly found in quartz veins or reefs traversing slaty or crystalline rocks, usually talcose or chloritic schists, either alone, or in association with iron, copper, magnetic and arsenical pyrites, galena, specular iron ore, and silver ores, and more rarely with sulphide of molybdenum, tungstate of calcium, bismuth, and tellurium minerals. Another more exceptional association, that with bismuth in calcite from Queensland, was described by the late Mr Daintree. In Hungary, the Urals, and northern Peru, silicates and carbonates of manganese are not uncommonly found in the gold and silver bearing veins. In the second or alluvial class of deposits the associated minerals are chiefly those of great density and hardness, such as platinum, osmiridum, and other metals of the platinum group, tinstone, chromic, magnetic, and brown iron ores, diamond, ruby, and sapphire, zircon, topaz, garnet, <fcc., which represent the more durable original constituents of the rocks whose disintegration has furnished the detritus. Native lead and zinc have also been reported among such minerals, but their authenticity is somewhat doubtful.

The distribution of gold-bearing deposits is world-wide; although the relative importance of different localities is very different, their geological range is also very extensive. In Europe the principal groups of veins are in slaty or crystalline schists, whose age, when it can be determined, is usually Palaeozoic, Silurian, Devonian, or Carboniferous, and less commonly in volcanic formations of Tertiary age. The alluvial deposits, being more extensive, are less intimately connected with any particular series of rocks. Few of either are, however, of much importance as compared with the more productive deposits of America and Australia. In the United Kingdom gold-bearing quartz veins were worked during the Roman occupation at Ogofau, near Llanpumpsant, in Carmarthenshire; and in the year 1863 as much as 5300 oz. was produced from similar veins in Lower Silurian slates at Vigra and Clogau mines, near Dolgelly. In 1875 the mine was reopened, and in 1878 it produced 720 oz. Tetradymite, native bismuth, and several other characteristic associates of gold were also found in small quantity. In Cornwall small pieces of native gold have at intervals been found in alluvial or stream tin works; and similar but more important finds have been made in the granite district of Wicklow, and more recently at Helmsdale, in Sutherlandshire. The largest nugget of British origin weighs under 3 oz.

On the continent of Europe the great rivers originating in the crystalline rocks of the Alpine region, such as the Rhine and Danube, are slightly auriferous in their alluvial deposits in several places; but the proportion of gold is extraordinarily minute, so that the working is only carried on by gipsies, or by the local peasantry at irregular intervals, the return for the labour expended being very small. The same remark applies to the Rhone and its affluents, and the rivers of the central granitic mass of France. In the Austrian Alps the gold quartz mines at the Rathausberg, near Gastein, at a height of about 9000 feet above the sea-level, and at Zell, in Tyrol, are of interest historically as having developed the system of amalgamation in mills, although they are economically of small importance at present. On the Italian side, in the Valanzasca and Vai Toppa above Lago Maggiore, a group known as the Pestarena mines have yielded from 2000 to 3000 ounces annually for several years past; and more recently a discovery of great interest of a highly auriferous copper ore has been made at Ollomont in the Vai d’Aosta. In Hungary the gold-bearing veins of Schemnitz occur in greenstones and trachytes of Tertiary age, the most powerful example, the Spitaler -gang, being filled with a mixture of quartz and brown iron ore known as zinnopal, and containing gold associated with silver ores, galena, and pyrites. In Transylvania, at Nagyag, the gold-bearing tellurium minerals previously noticed are found in small veins traversing greenstone trachyte. These are often very thin, as low as lth to τ 1 , r th of an inch, but each is carefully traced out, the rock being impregnated with gold and silver to a certain depth on each side. At Vυrospatak, another Transylvanian locality, gold with a very large proportion of silver and associateci with gypsum is worked in veins traversing a Tertiary sandstone, being almost the only known instance of such a mode of occurrence.

The Russian empire has the largest gold production among the countries of the Old World, most of the produce, however, being derived from its Asiatic territories. The more important localities are situated on the eastern slope of the Ural chain, extending in a nearly north and south line for more than 600 miles from 51° to 60° N. lat. The chief centres are Miask (55° N.), Kamensk (56° 30' N.), Berezovsk (57° N.), Nijne Tagilsk (58° N.), and Bogoslowsk (60° N.), the known deposits, which include both veins •and alluvial mines, extending for about one degree farther north. The geological age of the Ural veins is not very well defined—strata of the Silurian, Devonian, and Carboniferous periods, which form regular parallel alternations on the European slope, being present on the Asiatic side, but in much disturbed and contorted positions, in association with plutonic rocks, diorite, diabase, and granite, with which the gold veins are intimately connected. The latter are therefore of post-Carboniferous and probably of Permian date. At Berezovsk the mines cover an area of about 25 square miles, mainly composed of talcose, chloritic, and clay slates, vertical or sloping at high angles, and penetrated by dykes of beresite, a fine grained rock made up of quartz and white mica with some felspar and pyrites, the latter usually transformed into brown iron ore. These dykes, which have a general north-and-south direction are vertical, and are from 20 to 70 feet and upwards in thickness, are traversed perpendicularly to their direction by veins of quartz from the thinnest string to a maximum of 3’- or 4 feet thick, in which gold is associated with brown iron ore or ochres, resulting from the decomposition of pyrites. The workings being essentially shallow, none of the associated sulphides, galena, disulphide of copper, Ac., have as yet been found, as a rule, to be gold-bearing. The valuable parts of the veins are almost entirely restricted to the beresite dykes. The richest of the Ural mines are those of Smolensk, near Miask, and Ouspensk, near the village of Katchkar, in 52° N. The alluvial deposits which, though called sands, are but very slightly sandy clays, ex-, tend to the north beyond the inhabited regions, and to the south into the Cossack and Bashkir countries. The most valuable diggings are in the district of Miask, where the largest nuggets have been found, and in the Katchkar, which are remarkable for the great number of gems, pink topazes, emeralds, &c., found in connexion with the gold. Magnetite, quartz, and platinum are very common in all the Ural gold sands; less common are hematite, titaniferous and chromic iron, pyrites, garnet, and, least of all, zircon, kyanite, and diamond. These alluvial deposits are of later Tertiary age, some of them containing traces of prehistoric human work; others are post-Pliocene, with the remains of the mammoth, tichorrhine, rhinoceros, and other mammalian fossils. Somewhat similar conditions prevail in the alluvial gold region of the Altai. Besides the veins and alluvial deposits, the Ural rocks, such as serpentine, diorite, beresite, agrairite, Ac., are at times auriferous.

The gold deposits of the Caucasus, though immortalized in the tradition of Jason and the Argonauts, are now entirely abandoned, the last attempt at working them having been suspended in 1875.

In India gold is obtained in small quantities by native gold washers in various parts of the highlands of southern Bengal, and more recently quartz veins and alluvial deposits of considerable promise have been discovered in the district of Wynaad, in the southern part of the Madras presidency.

On the Atlantic slopes of North America the chief gold-bearing localities are on the Chaudière river, near Quebec, and in Nova Scotia. In both instances the quartz veins worked are contained in slates belonging to the Quebec group of the Lower Silurian period, those of the latter province being specially remarkable for their quasi-stratified character, as they penetrate the slates at a very low angle of inclination, and have been folded and corrugated together with the containing rocks by subsequent disturbances. Other deposits of old geological periods are found in Tennessee and North Carolina.

On the Pacific side of America gold is found under very different conditions, and on a much larger scale than on the Atlantic side. The whole distance from Mexico to Alaska may be said to be more or less auriferous, the most extensive deposits being in the great north-and-south valley of the Sacramento, which runs parallel to the coast, between the so-called Coast Mountains and the Sierra Nevada, the latter being distinguished further to the north in the Cascade range. Others of less extent are known in the Klamath, Columbia, and Fraser river basins; they extend in the last two far back into the interior, to the region between the Cascade range and the Rocky Mountains. In many of these valleys alluvial deposits are developed to an extent unparalleled elsewhere, the river channels being bordered by banks or benches of gravel and sand, rising in terraces to considerable heights on the flanks of the hills. For example, at the Methow a tributary of the Columbia, there are sixteen lines of such terraces, the highest about 1200 feet above the river; and at Colville, on the Columbia, traces of old terraces, much degraded by frost and rain, are seen at 1500 feet above the river. These gravels, which are of Pliocene and more recent origin, are in many places, though very unequally, auriferous, the richest points being found in the bars or shingle banks of the river after the summer floods, and in the channels of the smaller tributary streams, where the poorer material has been partially enriched by a process of natural washing. The most extensive, or rather the best known because most completely explored, deposits of this class are those of the Upper Sacramento valley, in California (see vol. iv. p. 701 ). Others of considerable importance are worked in the Cariboo district on the Upper Fraser river, yielding very coarse gold. Another discovery of a singular character, the produce being a regular gold gravel, was made some years back at Salmon river in Oregon, but the deposit, though exceedingly rich, was soon exhausted. Gold-bearing quartz veins are also common over a large part of California, notably in Grass Valley (vol. iv. p. 702), in strata that are supposed to be of Triassic age, the associated minerals being iron and arsenical pyrites, galena, àc. In Calaveras county, tellurium ores like that of Transylvania are characteristic of the gold veins. In the adjacent States of Nevada and Colorado, gold is so intimately associated with silver ores that it is for the most part only obtained from the ultimate process of refining the reduced silver. The same remark applies to the most of the mines of Mexico, and on the south-west coast of America, in Peru, Bolivia, and Chili. See Silver.

Very rich gold quartz has been brought from Carabaya on Lake Titicaca; and recently considerable deposits both alluvial and in veins have been opened at Caratal in Venezuela and at St Elie in French Guiana, which are interesting as proving the actual existence of Raleigh’s Eldorado.

In Brazil the principal gold mines are upon veins in clay slate, and a peculiar class of rocks known as Jacotinga or Itabirite, and which are mixtures of quartz, chlorite, and specular iron ore, the latter often occurring in large mirrorlike crystals several inches across. The gold occurs almost entirely in pyritic minerals, being most abundant in ordinary iron pyrites, and less so in magnetic and arsenical pyrites, free gold being rarely seen. See Brazil, vol. iv. p. 224.

In Africa the chief gold-bearing localities are on the west coast—gold dust derived from alluvial washings forming an article of export from many of the trading stations along the Guinea coast. Latterly, alluvial deposits have been worked in the mountains of Transvaal, in the Leydenburg district (25° S. lat., 31° E. long.), producing coarse nuggetty gold in masses up to 11 lb weight, and in a few cases gold-bearing quartz has been found in veins in talcose schist and quartzite, closely associated with eruptive masses of diorite. The age of these rocks is considered by Dunn l to be Silurian or Devonian, and the observed phenomena to be similar to those generally observed in Australia. The upper valley of the Nile produces a little gold in Abyssinia and Nubia, the latter being the land of gold of the old Egyptians. Very extensive ancient mines have been described by Linant Bey in the district known as Attaki or Allaki on the Red Sea, situated about 120 miles back from Ras Elba, the headland midway between Berenice and Sauwakin. These are probably the same mines that were described by Diodoru3 Siculus, and one of the oldest topographical documents known, a map or itinerary of the route to them from the Nile, is preserved at Turin. In the reign of Setee I., of the 19th dynasty, wells were opened along this route, in order that the mines, that were then of very great antiquity, might be reopened. Similar ancient gold mines have recently been discovered by Burton in the land of Midian, on the east coast of the Gulf of Akaba.

The gold districts of Australia cover a very considerable area, extending from the east side of the continent for about 20° of latitude (18° to 38’ S.), the more important deposits being those of Victoria in the south. The principal districts are in Victoria,—Ballarat, Castlemaine, and Sandhurst, lying west and north from Melbourne, and Beechworth near the Murray river to the north-east. In New South Wales the gold fields are scattered over the entire length of the colony from north to south, the more important districts lying between the 32d and 36th parallels of S. lat. on the western side of the Australian cordillera, on the upper tributaries of the Macquarie and Lachlan rivers, the centre being about the town of Bathurst. This is known as the western district. Another group, known as the northern district, is on the eastern side of the mountains near the Queensland boundary, in 29° S., Rocky River being the principal locality; while the southern district includes Braidwood, Adelaide, Tumbarumba, and other localities near the Murray river. In Queensland the chief localities are, commencing on the south, Gympie and Kilkevan near Maryborough, 26° S. lat.; a group extending about 50 miles north and south of Rockhampton, in 24° 30' S. lat., all near the coast; Eastern River, Hurley, and Peak Downs, about 300 miles inland on the 23d parallel; and Clomenny and Gilbert on a stream running into the Gulf of Carpentaria, besides numerous others. In all those localities two principal kinds of deposits are observed, namely, auriferous quartz veins traversing slates of Silurian and Devonian age, which are in intimate relation with masses of diorite and other eruptive rocks; and gold-bearing drifts of Miocene or even newer Tertiary date, derived from the degradation of the older strata. According to Daintree, no auriferous vein of any kind has been found in any Secondary or Tertiary-strata, or in the igneous rocks erupted through any such newer formations; and as a result of his experience the same observer gives the following as the modes of occurrence of gold in Australia :—(1) In pyritic diorites and felstones in Queensland, and their alluvial drifts; (2) in pyritic granites in New South Wales; (3) in drifts from auriferous serpentine in Queensland, also in the two northern colonies; (4) in more or less regular veins with quartz and calcspar in the preceding rocks; (5) in quartz and other veins in Devonian and Upper Silurian strata in proximity to similar igneous rocks, which is the general character of the Victoria quartz veins; (6) in veins of metamorphic rocks of unknown age in Queensland; and (7) in quartz veins in Lower Silurian strata, without any apparent connexion with igneous masses. The latter occur only in Victoria, and are of comparatively minor importance. In the northern territory of South Australia, alluvial gold mining has recently been developed to a considerable extent in the neighbourhood of Port Darwin in the Gulf of Carpentaria, the export being from 2000 to 3000 oz. monthly.

Statistics.— There are no means of stating exactly the total gold produce of the world for any particular year, as in many of the larger producing countries no systematic returns are obtained, and in others where such returns are collected their publication is often delayed for a considerable time. The following figures, mostly derived from a recent statistical work, A. Soetbeer, Edelmetall-Produktion, 1879, λvith some additions from late official sources, will give some idea of the relative importance of the different countries. Previous to 1837 the first place was held by Russia, and the estimated average annual yield from all sources was, in the decennial period 1841-50, 1,760,500 ounces.

The contributions of the different countries are as follows :— [table]

Since 1851 the yield has been very largely increased by the discovery of the Australian and Californian sources, the annual averages being— [table]

Proportion of Gold in Deposits.— A rich gold-bearing deposit is quantitatively very different from one to which the same term is applied when containing ores of other metals. In the latter the useful material must as a rule form a considerable proportion—one or more parts in a hundred—of the mass; while iu the former, owing to the superior value of the product, it rarely attains as much as 1 per cent., and is generally very much less, the amount of gold contained in easily worked alluvial deposits being often extremely small. For example, the yield of the Siberian gold washings ranges from 12 grains to 1 dwt. 12 grains per ton p while in the lodes, which are more difficult and expensive to work, the proportion is about 8 dwts. per ton. In the alluvial washings of California it is estimated at about two shillings worth, equal to about - 4 ⅛th of an ounce, per ton of gravel. In Australia the alluvial ground worked in the colony of Victoria in 1878 is returned as averaging 25 grains (1 dwt. 1 gr.) per ton, or about double the above quantity.

In vein mining, which is more difficult and costly, a larger yield is necessary, but probably 5 dwts., or about .£1 in value per ton, will in most places represent paying quantities from quartz containing free gold, i.e., not associated with pyrites. The proportional yield and quantities of the different kinds of auriferous materials treated in the colony of Victoria during the last three months of 1878 were — [table]

In the less tractable minerals, such as arsenical pyrites occurring in the lower portions of the veins, as much as 1⅛ to 3 oz. may be required for profitable working. When associated with the ores of other metals, such as silver, lead, and copper, the extraction of the gold is in most cases an incidental and final operation in their metallurgical treatment, and may therefore be best considered in the articles on these metals.

Mining.— The various deposits of gold may be divided into two classes—“veins” and “placers.” The vein mining of gold does not greatly differ from that of similar deposits of metals. It will only be necessary to refer here to certain details of the extraction of gold in such cases. In the placer or alluvial deposits, the precious metal is found usually in a water-worn condition imbedded in earthy matter, and the method of working all such deposits is based on the disintegration of the earthy matter by the action of a stream of water, which washes away the lighter portions and leaves the denser gold. In alluvial deposits the richest ground is usually found in contact with the “bed rock”; and, when the overlying cover of gravel is very thick, or, as sometimes happens, when the older gravel is covered with a flow of basalt, regular mining by shafts and levels, as in what are known as tunnel-claims, may be required to reach the auriferous ground. In the early days of gold washing in California and Australia, when rich alluvial deposits were common at the surface, the most simple appliances sufficed; the most characteristic being the “pan,” a circular dish of sheet-iron with sloping sides about 13 or 14 inches in diameter. The pan, about two-thirds filled with the “pay dirt” to be washed, is held in the stream or in a hole filled with water. The miner, after separating the larger stones by hand, imparts a gyratory motion to the pan by a combination of shaking and twisting movements which it is impossible to describe exactly, so as to keep its contents suspended in the stream of water, which carries away the bulk of the lighter material, leaving a black residue consisting of magnetic iron ore and other heavy minerals, together with any gold which may originally have been present in the mass. The washing is repeated until enough of the enriched sand is collected, when the gold is finally recovered by careful washing or “panning out” in a smaller pan. In Mexico and South America, instead of the pan, a wooden dish or trough, variously shaped in different districts, and known as “batea,” is used.

The “cradle,” a simple appliance for treating somewhat larger quantities, varies in length from 3 feet 6 inches to 7 feet, but the shorter length is that usually adopted. Its nature will be evident from fig. 1, in which a is a movable hopper with a perforated bottom of sheet iron in which the “pay dirt” is placed. Water is poured on the dirt, and the rocking motion imparted to the cradle causes the finer particles to pass through the holes in the hopper on to the screen b, which is of canvas, and thence to the base of the cradle, where the auriferous particles accumulate on the transverse bars of wood c, called “riffles.” Washing by the cradle, which is now but little used except in preliminary workings, is tedious and expensive.

The “tom” is a sort of cradle with an extended sluice placed on an incline of about 1 foot in 12. The upper end contains a perforated riddle plate which is placed directly over the riffle box, and under certain circumstances mercury may be placed behind the riffles. Copper plates amalgamated with mercury are also used when the gold is very fine, and even in some instances amalgamated silver coins have been used for the same purpose. Sometimes the stuff is disintegrated with water in a “puddling machine,” which is used, especially in Australia, when the earthy matters are tenacious and water scarce. The machine frequently resembles a brickmaker’s wash-mill, and is worked by horse or steam power.

In workings on a larger scale, where the supply of water is abundant, as in California, sluices are generally employed. They are shallow troughs about 12 feet long, about 16 to 20 inches wide, and 1 foot in depth. The troughs taper slightly so that they can be joined in series, the total length often reaching several hundred feet. The incline of the sluice varies with the conformation of the ground and the tenacity of the stuff to be washed, from 1 in 16 to 1 in 8.

Fig. 2 represents one of the simplest forms of sluice as used in river diggings in the north-west of America. A rectangular trough of boards, whose dimensions depend chiefly on the size of the planks available, is set up on the higher part of the ground at one side of the claim to be worked, upon trestles or piers of rough stone-work, at such an inclination that the stream may carry off all but the largest stones, which are kept back by a grating of boards about 2 inches apart at a. The gravel, which in this particular instance is from 12 to 16 feet thick, and with an average breadth to the river of 25 to 30 feet, is dug by hand and thrown in at the upper end, the stones kept back being removed at intervals by two men with four-pronged steel forks. The floor of the sluice is laid with riffles made of strips of wood 2 inches square laid parallel to the direction of the current (as at δ, and in cross section at c), and at other points d with boards having transverse notches filled with mercury. These were known originally as Hungarian riffles. The bottom of the working, which is below the drainage level of the valley, is kept dry by a Chinese bucket pump e, attached to a rough undershot wheel driven by the current in the sluice. The sluice boxes are made in lengths, and united together spigot and faucet fashion, so that they may easily be removed and re-erected as the different parts of the claim are progressively exhausted.

In the larger and more permanent erections used in hydraulic mining, the upper ends of the sluices are often cut in rock or lined with stone blocks, the grating stopping the larger stones being known as a “grizzly.” Inorder to save very fine and especially rusty particles of gold, so-called “under-current sluices” are used; these are shallow wooden tanks, 50 square yard3 and upwards in area, which are placed somewhat below the main sluice, and communicate with it above and below, the entry being protected by a grating so that only the finer material is admitted. These are paved with stone blocks or lined with mercury riffles, so that from the greatly reduced velocity of flow, due to the sudden increase of surface, the finer particles of gold may collect. In order to save finely-divided gold, amalgamated copper plates are sometimes placed in a nearly level position, at a considerable distance from the head of the sluice, the gold which is retained in it being removed from time to time. Sluices are often made double, and they are usually cleaned up,—that is, the deposit rich in gold is removed from them,—once a week. The gold is then recovered by “panning ' ’

The application of a jet of water to the removal of auriferous gravels by the so-called hydraulic system of mining has already been noticed at vol. iv. p 701. This method has for the most part been confined to the country of its invention, California, and the western territories of America, where the conditions favourable for its use are more fully developed than elsewhere,—notably the presence of thick banks of gravel that cannot be utilized by other methods, and abundance of water, even though considerable work may be required at times to make it available. The general conditions to be observed in such workings may be briefly stated as follows :—(1) The whole of the auriferous gravel, down to the “bed rock,” must be removed,—that is, no selection of rich or poor parts is possible; (2) this must be accomplished by the aid of water alone, or at times by water supplemented by gunpowder; (3) the conglomerate must be mechanically disintegrated without interrupting the whole system; (4) the gold must be saved without interrupting the continuous flow of water; and (5) arrangements must be made for disposing of the vast masses of impoverished gravel.

The general appearance of an hydraulic gold working is seen in fig. 3, the water being brought from a ditch on the high ground, and through a line of pipes to the distributing box, whence the branch pipes supplying the three jets diverge. The stream issues through a nozzle resembling that of a fire engine (fig. 4), which is movable in a horizontal plane around the vertical axis a, and in a vertical plane on the spherical joint and centre b, so that the direction of the jet may be varied through considerable angles by simply moving a handle. The material of the bank, being loosened by the cutting action of the water, crumbles into holes, or “caves in,” and the superincumbent mass, often with large trees and stones, falls into the lower ground. The stream, laden with stones and gravel, passes into the sluices, where the gold is recovered in the manner already described. Under the most advantageous conditions the loss of gold may be estimated at 15 or 20 per cent., the amount recovered representing a value of about two shillings per ton of gravel treated. The loss of mercury is about the same, from 5 to 6 cwt. being in constant use per mile of sluice. About 1 cwt. is added daily in at least two charges. The average half-yearly consumption is estimated at about one hundred flasks of 74 lb each, after allowing for the amount recovered in clearing up and distillation of the amalgam. The latter operation is performed at intervals of seven or fourteen days in the upper lengths of the sluice, and half-yearly in the lower parts.

The dressing or mechanical preparation of vein stuff containing gold is generally similar to that of other ores, except that the precious metal should be removed from the waste substances as quickly as possible, even although other minerals of value that are subsequently recovered may be present. This is usually done by amalgamation with mercury. In all cases the quartz or other vein stuff must be reduced to a very fine powder as a preliminary to further operations. This may be done in several ways, e.g., either (1) by the Mexican crusher or arrastra, in which the grinding is effected upon a bed of stone, over which heavy blocks of stone attached to cross arms are dragged by the rotation of the arms about a central spindle, motion being furnished by mules or other power, or (2) by the Chilian mill or trapiche, also known as the edgerunner, where the grinding stones roll upon the floor, at the same time turning about a central upright,—contrivances which are mainly used for the preparation of silver ores; but by far the largest proportion of the gold quartz of California and Australia is reduced by (3) the stamp mill, which is similar in principle to that used in Europe for the preparation of tin and other ores, but has received special modification in many details. Fig. 5 represents the ordinary Californian pattern of a stamp mill. The stamp is a cylindrical iron pestle faced with a chilled cast-iron shoe, removable so that it can be renewed when necessary, attached to a round iron rod or lifter, the whole weighing from 600 to 800 lb. The lift is effected by cams acting on the under surface of tappets a, and formed by cylindrical boxes keyed on to the stems of the lifter about one-fourth of their length from the top. As, however, the cams, unlike those of European stamp mills, are placed to one side of the stamp, the latter is not only lifted but turned partly round on its own axis, whereby the shoes are worn down uniformly. The bed or mortar A is of cast-iron. The height of lift may be between 8 and 10 inches, and the number of blows from 30 to 90 per minute. The stuff, previously broken to about 2 inch lumps in a Blake’s rock breaker, is fed in through the aperture n at the back of the “battery box,” a constant supply of water being given from the channel Ä-, and mercury in a finely divided state is added at frequent intervals. The discharge of the comminuted material takes place through the aperture d, which is covered by a thin steel plate perforated with numerous slits about th inch broad and ⅛th to ∣th inch long, a certain volume being discharged at every blow and carried forward by the flushing water over the apron or table in front, m, covered by copper plates filled with mercury Similar plates are often used to catch any particles of gold that may be thrown back, while the main operation is so conducted that the bulk of the gold may be reduced to the state of amalgam by bringing the two metals into intimate contact under the stamp head, and remain in the battery. The tables in front are laid at an incline of about 8 degrees, and are about 13 feet long; they collect from 10 to 15 per cent, of the whole gold; a further quantity is recovered by leading the sands through a gutter about 16 inches broad and 120 feet long, also lined with amalgamated copper plates, after the pyritic and other heavy minerals have been separated by depositing in catch pits and other similar contrivances.

When the ore does not contain any considerable amount of free gold, mercury is not, as a rule, used in the battery. The pulverized stuff is received upon blanket tables or sluices. These are inclined boards covered with coarse woollen cloth or sacking. The heavier particles become entangled in the fibres of the cloth, while the lighter deposits are carried forward by the current. At intervals of a quarter to half an hour the surface of the blanket is completely covered, when it is removed, and its contents are washed off in a tub of water and reserved for further treatment. This consists of amalgamation, in a contrivance analogous to the Hungarian mill subsequently described, and subsequent treatment in pan amalgamators somewhat similar to the arrastra in character, but with grinding surfaces of iron instead of stone.

At Schemnitz, in Hungary, quartz vein stuff containing a little gold, partly free and partly associated with pyrites and galena, is, after stamping in mills similar to those described above, but without rotating stamps, passed through the so-called Hungarian gold mill, fig. 6. This consists of a cast-iron pan a, having a shallow cylindrical bottom b, holding 50 lb of mercury, in which a wooden runner c, nearly of the same shape as the inside of the pan, and armed below with several projecting blades, is made to revolve by gearing wheels placed either above, or, as in the figure, below. The connexion of the runner with the driving shaft is effected by the three-armed crutch shown in plan at e, which sits on the square paît of the shaft. By means of set screws analogous to those of a flour mill, the runner is adjusted at such a height that the knives, just clear the surface of the mercury. The stuff from the stamps arrives by the gutter f and, falling through the hole in the middle of the runner, is distributed over the mercury, when the gold subsides in virtue of its superior density, while the quartz and lighter materials are guided by* the blades to the circumference and are discharged at g, usually into a second similar mill, and sometimes to a third, placed at lower levels, and subsequently pass over blanket tables. The most advantageous speed is from 12 to 14 revolutions per minute. The action of this so-called mill is really more nearly analogous to that of a centrifugal pump, as no grinding action takes place in it. The amalgam is cleaned out about once a month, The average amount of gold collected from 50 tons of stuff stamped, is about 6 oz. in the mills, and in the subsequent dressing processes 1 ft> of auriferous silver and 10 cwt. of lead. According to Rittinger, mercury that has been purified by distillation acts much more rapidly upon gold than such as has been saturated with the metal without losing its fluidity, although the amount that can be so dissolved is very small.

There are various forms of pan amalgamators of which space will not permit a description to be given. It may be stated, however, that experience of the great variety of pans that have from time to time been devised has led to the adoption of the more simple forms, in which the grinding is effected between horizontal flat surfaces instead of curved or conical bottoms, and in the pans now usually employed these flat grinding surfaces form an annular floor round a central cone through which a vertical shaft passes. The Knox pan, fig. 7, may be considered to be fairly typical. It is of cast-iron, 4 feet in diameter and 14 inches deep. It has a false bottom to form a hollow annular space through which steam can be introduced. The centre of the yoke d attached to the muller m, is keyed to a vertical wrought-iron shaft S, 2 inches in diameter, which can be brought in connexion with the driving gear G. The blocks r, r are of wood. In working the pan 100 lb of skimmings are introduced, and water added until the pulp will just adhere to a stick. After three hours grinding the pulp is heated with steam. About 5 lb of mercury are added for every charge, together with a cupful of equal parts of saltpetre and sal ammoniac. After three hours further working, water with a little caustic lime is added, and the pulp is discharged first through an upper and then through a lower hole.

One of the greatest difficulties in the treatment of gold by amalgamation, and more particularly in the treatment of pyrites, arises from the so-called sickening or flouring of the mercury; that is, the particles, losing their bright metallic surfaces, are no longer capable of coalescing with or taking up other metals. Of the numerous remedies proposed the most efficacious is perhaps sodium amalgam. It appears that amalgamation is often impeded by the tarnish found on the surface of the gold when it is associated with sulphur, arsenic, bismuth, antimony, or tellurium Wurtz in America (1864) and Crookes in England (1865) made independently the discovery that, by the addition of a small quantity of sodium to the mercury, the operation is much facilitated. It is also stated that sodium prevents both the “sickening” and the “flouring” of the mercury which is pιoduced by certain associated minerals. Cosmo Newberry has investigated with much care the action of certain metals in impeding amalgamation. Wurtz recommends two amalgams, one containing 2 and the other 4 percent, of sodium, and in practice 1 per cent, or less of these is added to the mercury in the amalgamator. Crookes employs three kinds, which he calls A, B, and C amalgams; each contains 3 per cent, of mercury, but the B variety has, in addition to the sodium, 20 per cent, of zinc, and C is mixed with 10 per cent, of zinc and 10 per cent, of tin. The addition, of cyanide of potassium has been suggested to assist the amalgamation and to prevent “flouring,” but Skey has shown that its use is attended with loss of gold.

Separation of Gold from the Amalgam.— The amalgam is first pressed in wetted canvas or buckskin in order to remove excess of mercury. According to Rittinger, mercury will dissolve from 0 ’ 05 to 0Ό8 per cent, of native gold of standard 650 to 850 without loss of fluidity, the solubility of the gold increasing with its fineness; and until the point of saturation is reached, no separation of solid amalgam is possible. Lumps of the solid amalgam, about 2 inches in diameter, are introduced into an iron vessel lined with a paste of fire-clay and wood ashes, and provided with an iron tube that dips below the surface of water. The distillation is then effected by heating, care being taken that the retort does not become visibly red in daylight. The amalgam yields about 30 to 40 per cent, of gold. In California the amalgam is retorted in cast-iron pans placed in cast-iron cylinders 11 inches in diameter, 4 feet 6 inches long, supported on brick work. The bullion left in the retorts is then melted in black-lead crucibles, with the addition of small quantities of suitable fluxes.

The extraction of gold from auriferous minerals by fusion, except as an incident in their treatment for other metals, is very rarely practised. It was at one time proposed to treat the concentrated black iron obtained in the Ural gold washings, which consists chiefly of magnetite, as an iron ore, by smelting it with charcoal for auriferous pig-iron, the latter metal possessing the property of dissolving gold in considerable quantity. By subsequent treatment with sulphuric acid the gold could be recovered. Experiments on this point were made by Anossow in 1835, but they have never been followed in practice.

Gold in galena or other lead ores is invariably recovered in the refining or treatment of the lead and silver obtained. Pyritic ores containing copper are treated by methods analogous to those of the copper smelter. This is extensively done. In Colorado the pyritic ores containing gold and silver in association with copper are smelted in reverberatory furnaces for regulus, which, when desilverized by ZiervogeΓs method, leaves a residue containing 20 or 30 ounces of gold per ton. This is smelted with rich gold ores, notably those containing tellurium for white metal or regulus; and by a following process of partial reduction analogous to that of selecting in copper smelting, “bottoms” of impure copper are obtained in which practically all the gold is concentrated. By continuing the treatment of these in the ordinary way of refining, poling, and granulating, all the foreign matters other than gold, copper, and silver are removed, and, by exposing the granulated metal to a high oxidizing heat for a considerable time, the copper may be completely oxidized while the precious metals are unaltered. Subsequent treatment with sulphuric acid renders the copper soluble in water as sulphate, and the final residue contains only gold and silver, which is parted or refined in the ordinary way. This method of separating gold from copper, by converting the latter into oxide and sulphate, is also used at Oker in the Harz.

Chlorination Process.— Plattner suggested that the residues from certain mines at Reichenstein, in Silesia, should be treated with chlorine after the arsenical products had been extracted by roasting. The process, which depends upon the fact that chlorine acts rapidly upon gold, but does not attack ferric oxide, is now adopted in Grass Valley, California, where the waste minerals, principally pyrites from tailings, have been worked for a considerable time by amalgamation. The roasting is conducted at a low temperature in some form of reverberatory furnace. Salt is added in the roasting to convert all the metals present, except iron, into chlorides. The auric chloride is, however, decomposed at the elevated temperature into finely-divided metallic gold, which is then readily attacked by the chlorine gas. The roasted mineral, slightly moistened, is next introduced into a wooden vat, pitched inside, and furnished with a double bottom, as is shown in fig. 8. Chlorine.

is lei from a suitable generator beneath the false bottom, and rises through the moistened ore, resting on a bed of broken quartz below the false bottom, converting the gold into a soluble chloride, which is afterwards removed by washing with water. The precious metal is then precipitated as metallic gold by sulphate of iron. The process has been greatly improved in America by Kiistel, Deetken, and Hoffmann; with proper care it is a very perfect one, and yields 97 per cent, of the gold originally present in the ore. It is stated not to cost more in California than 50s. a ton. Any silver originally present in the ore is of course converted into chloride of silver and remains with the residue, from which it may be extracted by the solvent action of brine or by amalgamation.

liefining or Parting Gold from other Metals.— Strabo states that in his time a process was employed for refining and purifying gold in large quantities by cementing or burning it with an aluminous earth, which, by destroying the silver, left the gold in a state of purity. Pliny shows that for this purpose the gold was placed on the fire in an earthen vessel with treble its weight of salt, and that it was afterwards again exposed to the fire with two parts of salt and one of argillaceous rock, which, in the presence of moisture, effected the decomposition of the salt; by this means the silver became converted into chloride. In a similar process still practised in New Granada the granulated argentiferous gold is mixed with one part of common salt and two parts of brick dust. In the presence of moisture, effected by the passage of aqueous vapour through the porous pots in which the mixture is heated, the salt acts on the brick dust, producing silicate of soda, and the evolution of hydrochloric acid affords a source of chlorine for the silver. The chloride of silver formed fuses readily and drops off, exposing a fresh surface of the alloy to the action of the gas.

Various methods for separating gold from silver or other alloys appear to have been in use from ancient times. Among these may be mentioned prolonged oxidation by exposure to air, and treatment with sulphur, sulphide of antimony, and corrosive sublimate. In the Harz, 2 ounces of the granulated alloy of gold and silver were mixed and heated with 1 ounce of sulphur, litharge being added to separate the gold remaining in the sulphide of silver.

Parting by Nitric Acid, the old process of refining, is now practised in England by only one firm, although in some refineries both the nitric acid and the sulphuric acid processes are combined, the alloy being first treated with nitric acid. It used to be called “quartation,” from the fact that 4 parts of the alloy best suited for the operation of refining contain 3 parts of silver and 1 of gold. The operation may be conducted in vessels of glass or platinum, and each pound of granulated metal is treated with a pound and a quarter of nitric acid of specific gravity 1’32. It is the method employed in the assay of gold (see Assaying). Defining by Sulphuric Acid is the process usually adopted for separating gold from silver on the large scale. It appears to have been proposed in France by Dizé at the beginning of the present century. It was actually in use in France in 1820, and was introduced into the Mint refinery, London, by Mr Mathison in 1829. It is based upon the facts that concentrated hot sulphuric acid converts silver and copper into soluble sulphates without attacking the gold, the sulphate of silver being subsequently reduced to the metallic state by copper plates with the formation of sulphate of copper.

About 80 ft) of the granulated alloy are boiled for three or four hours in a platinum vessel (fig. 9) with 2 ’ 5 times its weight of sulphuric acid of specific gravity 1∙84. The sulphurous acids which arise are partially condensed before being allowed to pass into the air. When the acid has ceased to act on the metal, a small quantity of sulphuric acid of specific gravity 1 *53 is added, and, after a second boiling, the contents of the vessel are allowed to settle. The supernatant liquid is then withdrawn from the gold, which falls to the bottom of the vessel, and is diluted until its density is 1'21 or 1 ’ 26. The silver is usually precipitated from solution by copper plates, but sometimes iron is used, and the silver is roughly dried and compressed by an hydraulic press before it is melted into ingots. The gold, which is often again treated with sulphuric acid, is then washed and melted into ingots that contain from 997 to 998 parts of gold in 1000. The operation of parting may be conducted in iron or platinum vessels; the use of the former was advocated by M. Tocchi, and they are still extensively employed. Magnificent vessels of platinum have, however, been made in England by Messrs Johnson, Matthey, & Co. The alloys best suited for the operation contain from 800 to 950 of silver and 50 to 200 of copper and gold, but the proportion of gold must not exceed 200 parts in 1000. Refiners obtain alloys in suitable proportions by mixing together auriferous silver and argentiferous gold, the proportions of the respective metals having been previously indicated by assay. By such an arrangement. silver which contains but the 0Ό004 part of gold, or 2 ’ 25 grains in the troy pound, may be profitably treated.

Cost of Defining.— The charge to the public for refining depends in a great measure on the amount of metal to be operated upon and its richness. In England, however, it may be considered to be about Id. per ounce for the silver and 4d. per ounce for gold. In France the charge is about 90 cents to 1 franc 25 cents for a kilogramme of silver.

The Lower Harz smelting works produce annually from 50 to 55 cwts. of test silver of an average fineness of 950 silver and 50 gold per 1000; the proportion of the latter metal is, however, variable, being lowest (3 per 1000) in the silver obtained from clean lead ores, and highest (10 per 1000) in that separated from argentiferous copper ores,—that from the mixed copper and lead ores being of intermediate richness. The silver, in quantities of 25 kilogrammes, is refined upon small tests in a muffle, and when sufficiently purified is granulated by ladling it into water, whereby thin flattened granules suitable for dissolving are obtained.

The parting vessels (fig. 10) are of porcelain which, to protect them against fracture by irregular heating, are covered with wire netting and plastered over with a mixture of clay and smithy scales. They are mounted in a frame and set loose in an iron pot with a hemispherical bottom, which is heated by a fire from below; the pot also serves to catch the contents of the porcelain vessel if the latter should be accidentally broken. The cover is perforated by a hole in the centre for the passage of a lead pipe to carry off the sulphurous acid fumes, and a smaller one at one side through which acid may be introduced. These, as well as other connexions on the pipes carrying off the vapours, are secured by water-joints. The charge of about 200 ounces (6 , 25 kilogrammes) of granulated silver is treated with twice its weight of sulphuric acid marking 66° Baumé, and, by careful firing, is dissolved in six hours. The proper management of the heat is of importance, as neglect in the conduct of the operation may easily lead to a breakage of the pot. When the charge is completely dissolved the liquid is allowed to settle for some time, and is then poured off into a lead pan, where the silver sulphate solidifies. This, when rcdissolved by an addition of water and careful warming, is treated with strips of copper, the separation of the silver being facilitated by agitating the liquid. When the latter is found to be completely free from silver the heating is stopped, and the contents of the pan are allowed to settle for eighteen hours, when the copper solution is drawn off by a siphon and sent to the vitriol crystallizers. In the precipitation of 100 kilogrammes of silver about 30 kilogrammes of sheet-copper are expended.

The precipitated silver is washed with water in a copper vessel upon a linen filter until the reaction of copper in the washings ceases, and then moulded in cylindrical blocks by screw pressure, to express the residual water. These when fire-dried are melted in black-lead pots, holding 75 B>, with the addition of a little soda nitre.

The parted gold remaining in the porcelain pot, though already sensibly finer than is usual when iron parting vessels are used, still contains silver, and is therefore boiled once more with sulphuric acid of 66° Baumé. Afterwards it is washed with water until silver can be no longer detected in the washings, when it is transferred to a porcelain dish and dried. When a quantity of about 10 lb of gold has been accumulated, it is mixed with a little borax glass, melted in a black-lead pot, and cast. The resulting bars average ⅛ 8 π ⅝ fine.

Refining by Chlorine Gas.— F. Bowyer Miller devised in 1867 the following method for separating silver from gold, The process, which is the one now adopted in the Australian mints, consists in converting the silver into chloride by the passage of a stream of chlorine gas through the molten alloy. Clay crucibles are employed after having been saturated with a strong solution of borax and allowed to dry. The chlorine is introduced through the gold by a clay pipe passing to the bottom of the crucible, and connected with the chlorine generator in which the necessary pressure is obtained by a pressure tube 8 feet high. The chloride of silver is easily poured off from the surface of the molten metal, and by carefully fusing with a little carbonate of soda, the small amount of gold it retains is separated and falls to the bottom of the crucible. The gold operated upon contains from 3 to 12 per cent, of silver, and the average fineness of the refined gold is 994. The operation is now conducted on a considerable scale in Australia, and in the years 1871 and 1872 no less than 1,100,000 ounces of gold were refined by its aid in Sydney alone. The absolute loss of gold does not execed 14 parts in 100,000.

Toughening Brittle Gold.— It will be seen from p. 751 that minute traces of certain metals, which do not exceed the τ ⅛ 7j th part of the mass, render gold brittle and unfit for coinage. Miller showed that the removal of the deleterious metals might be effected by converting them into volatile chlorides by a stream of chlorine gas. The process was introduced into the English mint by Roberts, who successfully treated over 40,000 ounces of brittle gold with but trifling loss of precious metal. Wagner has suggested that bromine may replace chlorine in Miller’s process. Brittle gold may also be toughened by throwing a small quantity of corrosive sublimate on to the surface of the molten metal, but this method is wasteful, and the fumes evolved are deleterious. The late MrWarington proposed to toughen brittle gold by the addition of about 10 per cent, of black oxide of copper. The process is efficacious, but the crucibles become much corroded and even perforated; the standard fineness of the gold is, moreover, lowered by such copper as is reduced to the metallic state. If gold is but slightly brittle, it may be toughened by pouring it in a thin stream through atmospheric air into a crucible lined with borax, or by the addition of a small quantity of chloride of copper.

Preparation of Pure Gold.— Chemically pure gold may be prepared by several methods. The metal, either in the form of powder or “cornets” from the purest gold that can be obtained, is dissolved in nitro-hydrochloric acid. The excess of acid is driven off, alcohol and chloride of potassium added to precipitate platinum, and the chloride of gold is then dissolved in pure distilled water, the solution being diluted until each gallon does not contain more than half an ounce of the precious metal. The solution is allowed to stand for several weeks, and the supernatant liquid is carefully removed by a siphon from any chloride of silver that may have fallen to the bottom of the vessel. The gold may then be precipitated by a stream of carefully washed sulphurous anhydride, or by the addition of oxalic acid, formic acid, or ferrous sulphate. The spongy gold is washed with dilute hydrochloric acid, distilled water, ammonia water, and again with distilled water, after which it is melted in a clay crucible with a little bisulphate of potash and borax, and poured into a stone mould. Roberts prepared by this method 70 ounces of gold of which the average purity was 999 ’ 96, the precipitant being oxalic acid. Gold precipitated by oxalic acid from an acid solution containing copper is always contaminated with cupric oxalate. E. Purgotti has, however, shown that by heating the solution with tho addition of potash, a soluble double oxalate of copper and potash is formed, and the gold is left in the pure state.

-Alloys of Gold.— The most important alloys are those with silver and copper. Those used for coinage at the present day contain from 800 parts of gold in 1000, the standard of the Norwegian 2-kroner piece, to 986'6, that of the Austrian reichsducaten, the alloying metal being mainly copper. In England, when gold, coins were first introduced by Henry III., in 1257, they were of pure gold. Edward III., in 1345, was the first to use a standard 994’8, and in 1526 Henry VIII. issued crowns of the double rose of the standard 916'6 for concurrent issue with sovereigns, and other coins of the original standard 994'8. In 1544 the standard for all gold coins was reduced to 916'6, and again in 1546 to 833’4, the lowest point ever reached in England. Mary restored the old standard 994’8. Elizabeth directed that coins of both standards, 916’6 and 994’8, should be issued, and the latter was employed at intervals until 1640. Since then the lower standard, 916’6, has been solely used, and, as is shown by the following extract from the Coinage Act of 1870, 33 Vic. c. 10, is the one now in use :— [table]

In America and in those countries which have formed the “Latin Convention,” the standard of gold coin is 900, with a “remedy” of ± τσ 2 δσ . Μ. Peligot suggested that by employing a triple alloy containing 58Ί per cent, of gold, 361 of copper, and 5’8 of zinc, a coin might be produced which, while being of the value of 25 francs, would have the decimal weight of 10 grammes. The alloy is perfectly malleable and of good colour. In England the following standards are used for plate and jewellery, 375, 500, 6’25, 753, and 916’6, the alloying metal being silver and copper in varying proportions. In France three alloys of the following standards are used for jewellery, 920, 840, and 750. A greenish alloy used by goldsmiths contains 70 per cent, of silver and 30 per cent, of gold. “Blue gold” is stated to contain 75 per cent, of gold and 25 per cent, of iron. The Japanese use for ornament an alloy of gold and silver, the standard of which varies from 350 to 500, the colour of the precious metal being developed by “pickling” in a mixture of plum-juice, vinegar, and sulphate of copper. They may be said to possess a series of bronzes, in which gold and silver replace tin and zinc, all these alloys being characterized by patina having a wonderful range of tint. The common alloy, Shi·ya-ku-Do, contains 70 per cent, of copper and 30 per cent, of gold; when exposed to air it becomes coated with a fine black patina, and is much used in Japan for sword ornaments. Gold wire may be drawn of any quality, but it is usual to add 5 to 9 dwts. of copper to the pound.

The “solders” used for red gold contain 1 part of copper and 5 of gold; for light gold, 1 part of copper, 1 of silver, and 4 of gold.

Alloys of Gold and Silver.— Electrum, the natural alloy of gold and silver, has already been described, p. 740. Matthiessen observed that the density of those alloys, the composition of which varies from AuAg 6 to Au 6 Ag, is greater than that calculated from the densities of the constituent metals. These alloys are harder, more fusible, and more sonorous than pure gold. The alloys of the formulae AuAg, AuAg 2 , AuAg 4 , and AuAg 20 are perfectly homogeneous, and have been studied by Levol. Hatchett has shown, by a series of careful experiments, that certain metals, even when present in such small quantities as the τ ⅛ j th part of the mass, render standard gold brittle and unfit for rolling. These metals are bismuth, lead, antimony, arsenic, and zinc.

Gold cmd Zinc.— With regard to the latter metal, it may be remarked that, although its presence in small quantities renders gold brittle, it may be added to gold in larger quantities without destroying the ductility of the precious metal, for, as has been already stated, Peligot proved that a triple alloy of gold, copper, and zinc, which contains 5’8 per cent, of the last-named, is perfectly ductile. The alloy of 11 parts gold and 1 part of zinc is, however, stated to be brittle.

Gold and 2‰.-Alchorne showed that gold alloyed with √ 7 th part of tin is sufficiently ductile to be rolled and stamped into coin, provided the metal is not annealed at a high temperature. The alloys of tin and gold are hard and brittle, and the combination of the metals is attended with contraction; thus the alloy SnAu has a density 14’243, instead of 14’828 indicated by calculation. Matthiessen and Bose obtained large crystals of the alloy Au 2 Sn 5 , having the colour of tin, which changed to a bronze tint by oxidation.

Gold and Iron.— Hatchett found that the alloy of 11 parts gold and 1 part of iron is easily rolled without annealing. In these proportions the density of the alloy is less than the mean of its constituent metals.

Gold and Palladium.— These metals are stated to alloy in all proportions. According to Chenevix, the alloy composed of equal parts of the two metals is grey, is less ductile than its constituent metals, and has the specific gravity ll ’ 08. The alloy of 4 parts of gold and 1 part of palladium is white, hard, and ductile. Graham has shown that a wire of palladium alloyed with from 24 to 25 parts of gold does not exhibit the remarkable retraction which, in ])ure palladium, attends its loss of occluded hydrogen.

Gold and Platinum.— Clarke states that the alloy of equal parts of the two metals is ductile, and has almost the colour of gold.

Gold and Rhodium.— Gold alloyed with ∣th or ⅞th of rhodium is, according to Wollaston, very ductile, infusible, and of the colour of gold.

Gold and Iridium.— Small quantities of iridium do not destroy the ductility of gold, but this is probably because the metal is only disseminated through the mass, and not alloyed, as it falls to the bottom of the crucible in which the gold is fused.

Gold and Nickel.— Eleven parts of gold and 1 of nickel yield an alloy resembling brass.

Gold and Cobalt.— Eleven parts of gold and 1 of cobalt form a brittle alloy of a dull yellow colour.

Assay of Gold.— It may be well to supplement the information given in the article Assaying with some additional details as to the assay of gold bullion, as practised in the Royal Mint, and of gold ores. The assay of bullion consists of six operations:—

(1.) The sample of metal taken for assay is flattened, and an assistant adjusts a portion of it to an exact weight by cutting or filing. This weight varies with different operators from 5 to 16 grains. The assayer then completes the adjustment on a more sensitive balance. The prepared assay piece is wrapped in lead foil, together with a certain amount of pure silver, which is generally equal to 2⅛ times the amount of gold assumed to be present. In the case of standard gold, the weight of lead employed is to the weight of the alloy taken for assay as 8 to 1, and the ratio of the weight of lead to the weight of copper present is 100:1. Much diversity of opinion exists as to the amount of lead that should be employed. The proportions recommended by D ’ Arcet are considerably less than those advocated by Kandelhardt; and it may be stated, with regard to the silver, that the last mentioned authority and Chaudet recommend the proportion of 1 of gold to 2⅛ of silver, but Pettenkofer states that the proportion need not exceed 1 to 1⅜, provided that the subsequent boiling in nitric acid is sufficiently prolonged. The amount of gold lost in cupellation has been shown by Rössler to increase with the amount of lead used, and to decrease as the amount of silver is increased.

(2.) The necessary number of cupels are arranged on the bottom of the muffle (fig. 2, Assaying), and the packets containing the silver and gold are transferred from a numbered wooden range to corresponding cupels. The furnace operations are then performed as is described in Assaying (p. 727), and the result is that each cupel contains a button of silver and gold.

(3.) The button a (fig. 11) is flattened by striking it with a hammer on a polished anvil, first in the centre, and then on the edge, a third blow being given on the opposite edge which elongates the metal. After annealing in an iron tray, the flattened buttons b are reduced by laminating rolls to the thickness of a visiting card c. They are again annealed and rolled into a spiral or cornet d.

(4.) These cornets are then treated with nitric acid of specific gravity 1 ’2, either separately in parting flasks, or together in cups of platinum, which are introduced into a suitable vessel of platinum, an arrangement by which it will be evident much time may be saved. The boiling is then continued for fifteen or twenty minutes, when the cornets are washed with distilled water, and treated with nitric acid of specific gravity 1’3, and in this the cornets remain for about the same period, after which they are again washed in distilled water and dried.

(5.) The cornets are annealed, separately, in little clay crucibles, or in the platinum cups in which they have been boiled, by heating them to bright redness. They then diminish considerably in bulk as e (fig 11), and are of a pure yellow colour.

(6.) The comets are then weighed in comparison with “check assays” made on pure gold. These “checks” are necessary, as the accuracy of the result of an assay is liable to be affected either by retention of silver or copper, or by loss of gold by volatilization in the muffle, solution in the acid, or retention in the cupel. The weight of gold, therefore, indicated by the balance, may be either less or greater than the amount originally present in the alloy. The correction to be applied to a gold assay will be evident from the following formula: —

Let 1000 be the weight of alloy originally taken; p the weight of the piece of gold finally obtained; X the actual amount of gold in the alloy expressed in thousandths; a the weight of gold (supposed to be absolutely pure) taken as a check, which approximately equals x; b the lo.ss or gain in weight experienced by a during the process of assay, expressed in thousandths; k the variation of “check gold” from absolute purity, expressed in thousandths; then the actual amount of fine gold in the check-piece = «[1- an d X the corrected weight of the assay will

— p - ± b; b being added or subtracted according as it is a loss or gain.

If α be assumed to be equal to x this equation becomes p ÷ 5

x = --—

1 + -A_

1000.

Example. — Let p == 901 - 1 thousandths.

a = 920 Ό ,, b = 0 - 3 ,, gain in weight.

h = o∙ι

Then by the first formula—

x = 901T - —-Î-— 1 - o·3; 1000

F or, as δ is a gain in weight, it must be deducted, hence x = 901T - 0Ό92 - 0'3

= 900 - 708.

And by the second formula—

o , _ 901∙l-0∙3

1 + -θl 1000

= 900’708

Assay of Gold Ores. — 500 grains of the finely powdered sample, which must be taken with the greatest care and accuracy, is passed through a sieve of fine wire gauze with at least 80 meshes to the linear inch. Any residue there may be of flattened particles of gold is set aside for subsequent treatment, usually by direct cupellation. Assay of the ore by fusion with litharge is best suited to «res which do not contain much iron pyrites. For auriferous quartz

500 grains of the ore are fused with 500 grains of red lead, 300 grains of sodic carbonate, 20 grains of powdered charcoal, and 250 grains of borax. The mixture is introduced into a clay crucible, which it should half fill, and is fused in an air furnace. The button of reduced lead may be removed, either by pouring the contents of the crucible into a mould, or by breaking the crucible when cold. If the ore contains much iron pyrites, or is of the nature of “sweep,” the name given to carbonaceous residues which accumulate in mints and goldsmiths’ shops, it will be necessary to roast it in a shallow fire-clay dish placed in a muffle. In the case of pyrites containing about 7 dwts. to the ton, the operation would be conducted on about 1000 grains. The roasted ore is then fused with about the same mixture of fluxes as has been given for quartz.

Assay by Scorification.— Scorification resembles cupellation, but the oxide of lead produced in the operation, instead of sinking into a porous cup, is held in a fiat saucer of fire-clay, and dissolves the earthy constituents of the ore, leaving the precious metal to pass into another portion of lead which remains in the metallic state. About 200 grains of the roasted ore are placed in the scorifier, intimately mixed with 500 grains of granulated and 50 grains of borax lead; 500 grains of lead are then distributed over the surface of the mixture; the contents of the scorifier are fused in a muffle; air is admitted to oxidize the greater portion of the lead; and, at the conclusion of the operation, the litharge should be perfectly fluid and cover the molten lead. The slag may be freed from particles of precious metal by the addition at the conclusion of the operation of a small quantity of powdered anthracite, which reduces a portion of the litharge to metallic globules, which fall through the slag and unite with the lead button. The gold is then separated by cupellation, and the silver with which it is nearly always associated is removed by parting in nitric acid.

Assay by means of the Spectroscope.— Lockyer and Roberts state, as the result of a careful spectroscopic investigation of the alloys of gold and copper, that it is possible to distinguish between alloys of these metals which only differ in proportion by τδδδδ th part. Their experiments have been repeated in America by A. E. Outerbridge. (W. C. R.—H. B.)

It will be convenient to give here, in connexion with the article Gold, rather than in their proper alphabetical place, the articles Goldbeating and Gold Lace.