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...