Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/765

Rh G O L D 74] Vith regard to the history of the metallurgy of gold, it I vessel in that part of a glass-house where the glass is kept 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. Propez-lies.—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 11early as soft as lead. The lzarclness varies, however, with the composi- tion. 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 p11re, 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 -,_«,—,_,‘__.'1(,—?,——(jf;l1 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 aggrega- tion that green light is again transmitted.‘ The metal is extremely duct-ile; 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 miles. Gold can readily be welded cold, and thus the ﬁnely divided metal, in the state in which it is precipitated from solution, may be eompressed between dies into discs or medals. According to G. Rose? the speciﬁc _r/ra.vz'ty of gold in the ﬁnely divided state in which it is precipitated from solution by oxalic acid is 1949. The speciﬁc gravity of cast gold varies from 1829 to l9'37, and by compression3 between dies the speciﬁc gravity may be raised from l9'37 to 19°41 ; by annealing, however, the previous density is to some extent recovered, as it then is found to be l9'40. Its atomic 'u'el[/let is variously given as follows :—~196'67 (Berzelius), 1963 (Level), l96'5 (Wurtz), 1960 (Watts). The number adopted in this work (CnEMIsTRY, vol. v. p. 529) is 196'2. Different observers have given the following temperatures as its melting point :——1425° C. (Daniell), 1200” C. (Pouillet), l380° C. (Guyton de Morveau). Riems- dijk,‘ after comparing the several results, concludes that it may be considered to be 1240° C. The electric con- ilartivity is given by Matthiessen as 7399 at 151° 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 specz'._/ic 7'esz'stance of the metal in electromagnetic measure, accord- ing to the centimetre—gramme—second system of units, is 2154. Its conclnctiv2‘l_1/for heat is 53'2 (Viedemann and Franz), pure silver being 100. Its speczﬁc heat is 0324 (Regnault). Its coe[ﬁc2'ent of expansion for each degree be- tween 0° and 100° C. is 0000014661, or for gold which has been annealed 0'O00015136 (Laplace and Lavoisier). The speciﬁc ma_gnetz'sm of the metal is 3'47 (Becquerel). De- tails as to its tenacz'l_7/ and 7-z'_qz'r.l£ly are given in the article ELASTICITY. With regard to its 2'ol_a(z°l1'lg/, Gasto Claveus 5 states that he placed an ounce of pure gold in an earthen 1 Phil. Trans., 1857, p. 145. 1’og_r/. .-17221., vol. lxxiii. p. 1, and lxxv. p. 408. ' Ih._r/lil_IL Ann. Report of Deputy Jlastcr of the Jlliut, 1877, p, 4] _ ‘ .-lrclzrrcs Nécrlavztlaises, 1:. iii., 1868. "' Quoted by l)r T. Tliomson, System of CI2emz'.9l7'_2/, 5th edition, 1817, vol. i. p. 484. 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, ob- served 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.7 Hellot aﬂirms 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 oxy~ hydrogen ﬂame, made by Lockyer and I{oberts,3 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, pre- cipitates 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.9 W. Skey has shown 10 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 G'ol¢l.—Graham has shown 11 that gold is capable of occluding 048 of its volume of hydrogen, and 020 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 Dz'slr'lbz(tion..—Gol(l is found in nature chieﬂy in the metallic state, or as native gold, a11d 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 n1oss—like forms are most common, and in Transylvania thin plates or sheets with diagonal structures are character- istic. These l1ave recently been shown by Vom 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, 5 Jfcin. I’(I..1'is Academy, 1702,11. 147. 7 Chem. Soc. ./ourn., vol. X. p. 2:39, vol. xi. p. 168. 3 [’-roc, Roy. Soc,’ 1875, p. 344. 9 Spiller, Chem. .'czI_'s, X. 173. 1° Il/z'd., xxii. 245. “ 1’lu'l. Trans, 1866, 433.