282 



GOLD 



GOLDAU 



utilising them) is generally the first step, and 

 is most commonly performed in some kind of fur- 

 nace. This done, the ' sweet ' cinders are subjected 

 to the action of chlorine, which forms a soluble 

 chloride with the gold, easily separable by washing 

 with water. Sometimes the washing and chlorina- 

 tion are combined in one operation by placing salt 

 in the furnace ; but in many cases this has led to 

 enormous loss of gold by volatilisation. See Lock's 

 Practical Gold Mining (1889) and the bibliography 

 of the subject therein contained. Of late years 

 the ' cyanide process' an application of the solu- 

 bility of gold in a potassium cyanide solution has 

 come into very general use, resulting in an enormous 

 saving in the cost of extracting, and thus making 

 profitable the working of low-grade ores. 



The most important physical and chemical 

 properties of gold are as follow : In malleability 

 it stands first of the metals, and its ductility is 

 remarkable, hence it may be beaten into leaves 

 not exceeding inrrnnnr f an mcn thick, and quite 

 translucent, and 1 grain in weight may be made 

 to cover 56 square inches of surface, or drawn into 

 a wire 500 feet long. Its specific gravity is about 

 19*2 when fused, or 19'4 when hammered, being 

 less than platinum and iridium. Its colour and 

 lustre in the concrete form are sufficiently familiar, 

 but when thrown down from solution in a minute 

 state of division it appears brown, and seen by 

 transmitted light while held in suspension the 

 atoms exhibit a purple tint, as also when it is 

 volatilised. In softness it approaches lead, and in 

 tenacity it ranks below iron, platinum, copper, and 

 silver; yet a wire only jg^ of an inch truck will 

 support 150 Ib. It is an excellent conductor of 

 heat and electricity. Its fusing-point is 2016 by 

 Daniell's pyrometer. When pure it is difficult of 

 volatilisation, requiring the intense heat of an oxy- 

 hydrogen flame, or a strong electric current. It 

 was long thought to be practically non-volatile in 

 the heat of an ordinary furnace ; but, as has been 

 already stated, under certain conditions it is. very 

 readily vaporised, and immense losses have been 

 incurred in consequence. 



Having but little affinity for oxygen, gold is not 

 affected by exposure to the air ; but two oxides 

 may be formed artificially the protoxide, AuO, 

 by decomposing gold protochloride with a potassic 

 solution, and a teroxule, Au0 3 , or auric acid by 

 boiling terchloride with magnesia or carbonate of 

 soda. Silica, on the other hand, attacks it with 

 avidity, forming a silicate which is extremely in- 

 soluble in water, but decomposes with age. Sul- 

 phuretted hydrogen combines with gold at ordinary 

 temperatures to form a sulphide, which is soluble 

 in alkaline sulphides, and slightly so in pure water. 

 A bisulphide is obtained by passing sulphuretted 

 hydrogen through a cold solution of terchloride ; 

 and a double sulphide of gold and potash is pro- 

 duced by heating gold in a very fine state with 

 sulphur and carbonate of potash, constituting the 

 porcelain gilding known as 'Burgos lustre.' Gold 

 is affected by selenic acid, and is dissolved by 

 iodine and by hyposulphite of soda. It is not 

 affected by alkalies, ror by hydrochloric, nitric, or 

 sulphuric' acid alone ; but is rapidly dissolved by 

 aqua regia ( nitro-hydrochloric acid ), and by any 

 substance liberating chlorine. Two chlorides are 

 known : a proto salt, AuCl, and a ter salt, Au 

 C1 3 , the latter forming reddish-yellow solutions 

 with water, ether, and alcohol. Gold is volatile 

 in the presence of chlorine at all temperatures be- 

 tween boiling water and white heat, and cannot be 

 recovered by condensation, but only by decomposi- 

 tion of the volatile chloride. Gold chloride and 

 sulphide remain in solution in presence of excess 

 of sulphuretted hydrogen and an alkaline carbonate, 

 the gold gradually depositing as the carbonic acid 



escapes. Gold solutions are precipitated by oxalic, 

 tartaric, citric, and other organic acids ; also by 

 wood, bark, charcoal, and other organic matters, 

 the gold being thrown down in a pulverulent form, 

 and recoverable by burning. Gold is also precipi- 

 tated by iron sulphate, and by sulphur dioxide 

 in the presence of water, as a metallic powder ; 

 further, by copper sulphide, which, when converted 

 into sulphate, yields the gold in a metallic state 

 highly favourable for collecting. Mineral sul- 

 phides (e.g. pyrites) decompose gold solutions, 

 and collect the gold in a coherent form ; they 

 similarly attack gold chloride volatilised in the 

 roasting furnace, and absorb it. 



Gold forms many alloys with other metals. 

 Those occurring in nature have been already 

 mentioned ; their importance is very small in- 

 dustrially. But another alloy, that with copper, 

 is of prominent value, being the basis of gold 

 coinages. The admixture of copper lessens the 

 density, but increases the hardness and fusibility 

 of the alloy, rendering it better suited to the 

 purpose. The proportion of copper in standard 

 gold coin varies, being 8 '33 per cent, in Great 

 Britain, and 10 per cent, in France and the United 

 States. In Great Britain, since 1816, gold is the 

 only legal tender for sums above forty shillings ; 

 in many other countries gold coin is latterly coin- 

 ing into extended use where formerly silver only 

 was employed. The market price of gold bullion 

 varies with its purity : pure gold ( 24 carat ) is 

 worth 4, 4s. ll|d. per oz., while 22 carat fetches 

 only 3, 17s. 10|d., and 20 carat 3, 10s. 9d. (see 

 BIMETALLISM, CURRENCY, MONEY). The readi- 

 ness with which gold alloys with mercury is very 

 largely utilised in collecting the scattered fragments 

 of the precious metal, in treating auriferous sands 

 and rocks, and, on a smaller scale, in gilding. 

 The conditions governing perfect amalgamation 

 of crude gold demand most minute attention from 

 the miner. The fanciful alloys of gold made by 

 jewellers are chiefly : 



Red gold 75 parts fine gold + 25 parts copper. 



Dead leaf gold.. = 70 + 30 silver. 



Green gold = 75 .. u >, +25 n n 



Water green gold= 60 ,, +40 ., ,, 

 Blue gold = 75 M n n +25 H iron. 



See ALLOY, AMALGAM ; also ASSAYING, METAL- 

 LURGY, MINING. Gold may and often does cost 

 more to produce than it is worth. In Victoria, 

 where it is economically worked, the total average 

 of gold produced per head of all engaged in 

 gold-mining was in 1887 only 96, 17s. 2d. ; so 

 that the gold miner's wage may safely be set down 

 as lower than those given in the colony for many 

 other kinds of work. Among notable gold dis- 

 coveries are those in California in 1848 ; Australia 

 (New South Wales and Victoria) in 1851 ; British 

 Columbia, 1858; New Zealand and Nova Scotia in 

 1861; South Africa (Transvaal) and Sutherland- 

 shire, 1868 ; Western Australia, 1870 ; South Aus- 

 tralia, 1886 ; Klondike, 1896. The enormous out- 

 put of the Transvaal (q.v.) and Western Australia 

 (q.v. ) led, in 1895-96, to wild speculation. 



Fulminating gold is an extremely explosive green 

 powder made from teroxide of gold and caustic 

 ammonia. Purple of Cassius is a compound of 

 gold and tin used in colouring Glass ( q.v. ). Mosaic 

 gold is sulphide of Tin (q.v.). 



See, besides the writer's work above mentioned, T. K. 

 Rose, The Metallurgy of Gold (1894) ; H. Louis, Hand- 

 book of Gold Milling ( 1894 ) ; and works by T. S. G. 

 Kirkpatrick (1890) and Macdermott and Duffield (1890). 



Gold, FIELD OF THE CLOTH OF, the meeting 

 in 1520 between Henry VIII. (q.v.) and Francis I. 



Goldail* a small Swiss town behind the Rigi 

 and on the St Gotthard railway, was utterly de- 

 stroyed by a landslip, 2d September 1806 ; while the 



