Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/766

Rh 742 prepared artiﬁcially, have been described by Chester. It is possible also to obtain gold in crystals by heating its aiualgain ; according to Knaﬁl, an amalgam of 1 part of ' gold with 20 parts of mercury is maintained at a tempera- ture 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 moi'e 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 } to gounce weight as gold dust, the larger sizes being dis- tinguished as coarse or nuggety gold, and the smaller as gold dust proper. Except the largei' 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 f orm, 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 coin- position varies considerably in different localities, as shown in the following table :— A mil 3/503 of .'ali'i'c Gold from 'various local z'ti'cs. Locality. : Gold. l Silver I Ii'on. ‘Copper. Autliorit_v. EFROPE. British Isles— l Vigra re Clogau...' 90'l6 l 9-26 trace trace Forbes. Vicklow(rivei')... 9-2-3-2; 6'17 -73 Mallet. Transylvania. ...... .. 60' 49 3874 . 0'77 G. Rose. ASIA. l Russian Empire- , Brezovsk ........ .. 91'88 l 8'03 trace '09 G. Rose. El(ate1'inburg.....l 9S'96| 0'16l -05 -35 . AFRICA., _ I Ashantec . . . . . . . . . ..| 90'05 ' 9'94 A.iEI:icA. , Brazil .............. .. 94'0 5'85 D'Arcct. Central America 88 05 11'96   Funny and ( I clouze. Titiribi ........... ... 7641 2312 0'87 Rose. California... 9012 9'01 Mai-iposa ........... .. 81'00 1S'70 F. Claudet. Cariboo .. .. .. 84 '25 14'90 '03 Claudet. AUS'[‘I'..I.I.-1. I South Australia. . 87'78 6'07 6'15 . A. S. Thomas. Ballarat .......... .. ' 99'25 0'65 - I Claudet. Of the iuiiierals containing gold the most important are sylvanite or graphic tellurium, of composition (AgAu) Tea, with 24 to 26 per cent.; calaverite, AuTe2, 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 N agyag and Ofenbaiiya in Transylvania; but latterly they have been found in some quantity at Red Cloud, Colorado, and in Calaveras county, Cilifornia—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 quanti;ies ordinarily operated upon in analyses, are comparatively numerous, including many of the metallic sulphides. Prominent among these are galena and iron pyrites,——tlie 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 pounrl weight GOLD of the lead smelted from it, the proportion increasing to some extent with the amount of silver.‘ The second is of greater practical import-ince, being in some districts exceed- ingly rich, and, next to the native metal, is the most prolific source of gold. Magnetic pyritcs, copper pyrites, zinc blende, and arsenical pyrites are other and less important examples,—tlie 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 soinetiines 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 coii- sidered 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 cliloritic 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 tel- luriuiii 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, osiuiriduni, and other metals of the platinum group, tiustone, chromic, magnetic, and brown iron ores, diamond, ruby, and sapphire, zircon, topaz, garnet, &c., 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 worldwide; 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 iiiti— mately 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 IRTS it produce:l 720 oz. Tetradymite, native bismuth, mid 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 extraordinarilyminute, so that the working is only carrierl on by gipsies, or by the local peasantry at irregular inter- vals, the rcturn for the labour expended being very small. The same remark applies to the Rhone and its 1 ['}u'l. .lI.rI_9, vii., 1854, p. 126.
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