GOLD

Au. Syn. Aurum ; Or , Fr.; Gold , Ger.) Gold is the most valuable and, probably, the longest known of all the metals. From the remotest period it has been esteemed for its beauty and permanence, and has been taken as the standard measure of value amongst all civilised nations. An account of the uses of gold in the arts, and its influence on society in all ages, as a symbol of wealth and an article of ornament and utility, would embrace the whole history of mankind. At the present day it alike contributes to the conveniences, comforts, and luxuries of life; as often exciting the baser pa ions of the human heart as promoting the cause of benevolence and virtue. Gold is found almost invariably in the metallic state. It occurs as gold dust in the sands of various rivers, and in the alluvial soil of auriferous districts, from both of which it is obtained by the simple proce of washing. Traces of it are constantly found in the iron and other pyrites of the more ancient rocks. Sometimes it occurs beautifully crystallised in the cubic form, a ociated with quartz, oxide of iron, and other substances, in regular veins. In the gold fields of California and Australia lumps of nearly pure gold have been discovered in abundance during the last few years. In the former country a ma of gold weighing 28 pounds was found, whilst in our own colonies one weighing 106 pounds was dug out of a quartz rock, near Bathurst. The latter contained upwards of 91% of pure gold, and nearly 8 1 ⁄ 2 % of silver; being as pure as the English sovereign, or, in trade language, ‘22 carats fine.’ Prep. This consists merely in the separation of the gold and its subsequent purification. Formerly, the auriferous sulphides, if very poor, were first roasted, then fused into ‘mattes’ and again roasted; they were next melted with lead, and the alloy thus obtained was refined by cupellation. When the ores were very rich, the preliminary calcination and fusion were omitted, and the alloy of lead at once formed. This method (by fusion) does not answer well with auriferous copper pyrites or ores very poor in gold. At the present time the method of amalgamation is principally followed. When a ‘vein-stone’ is to be wrought for gold, it is reduced to powder (on the small scale by hand, on the large scale in stamping mills), and is shaken in a suitable apparatus with water and mercury; an amalgam of gold is formed, which is then separated from the mixture, and its mercury removed by distillation. The gold is next cast into ‘ingots.’ Refining. —Gold obtained by the first method usually contains a little copper and silver, and frequently tin or iron. Tin may be removed by adding a little corrosive sublimate or nitre to the gold melted in a crucible. The proce by amalgamation commonly leaves no other alloy than silver. This metal is removed either in the ‘dry way,’ by fusing the gold with sulphur or sulphide of antimony; or in the ‘wet way,’ by ‘quartation’ and ‘parting.’ At the Royal Mint, “when gold ingots contain a certain quantity of silver” (say 2% or 3%), “instead of leaving it, as formerly, to constitute a part of the standard alloy, it pays to extract it, and to substitute copper in its place. To get the silver out of the said ingots, they are melted with about 3 parts of silver—the resulting alloy is granulated and boiled with sulphuric acid—the gold remains untouched, and all the silver is di olved and converted into sulphate.... The sulphate of silver is then decomposed by the immersion of copper plates; the silver is precipitated in a fine, crystalline powder, washed, pre ed into ma es, and melted, and so affords PURE SILVER , which is afterwards made standard by alloying it with copper, and is used for coinage. The resulting sulphate of copper (which exists in the solution) is then crystallised, and sold.” (Brande.) “By first exhausting the gold with nitric acid, and then boiling it in sulphuric acid, some two or three thousandth part of silver which escaped the action of the nitric acid is di olved out, and perfectly pure gold is obtained.” (Ure.) By a foreign invention, patented in 1851 by Mr W. E. Newton, the operations of ‘separations’ and ‘refining’ are conducted by one proce . The argentiferous substance, whether in the state of ore or bullion, is reduced to a granulated or spongy state, by fusion along with zinc, or some other metal cheaper than silver, and the zinc is subsequently removed, by digesting the resulting granulated, laminated, or pulverulent alloy, in dilute sulphuric acid, or other acid. The zinc, ., is recovered by the usual means. This proce , carefully conducted, produces metal of great ductility and purity, containing 99% to 99 1 ⁄ 2 % of pure gold. Chemically pure gold is obtained by di olving the metal in nitro-hydrochloric acid, adding a solution of pro to sulphate of iron, and collecting and washing the precipitate. In this state it is a brown powder, which acquires a metallic lustre by friction or heat. Prop. The most marked properties of gold are its rich yellow colour, its ductility, malleability, insolubility in all menstrua except ‘aqua regia’ (nitro-hydrochloric acid), aqueous chlorine, and hydrofluoric acid, and its very slight affinity for oxygen. It melts at a bright red heat (2316° Fahr.—Daniell), and the fused metal has a brilliant green colour. It forms compounds with chlorine, iodine, oxygen, sulphur, . Sp. gr. of native gold, 13·3 to 17·7; of pure gold, 19·3 (average); its greatest density is 19·5. Tests. Metallic gold is characterised by its yellow colour, insolubility in nitric acid, and its ready solubility in aqua regia, forming a rich yellow or amber-coloured liquid, which stains the skin purple. Solutions of gold exhibit the following reactions:—Pro to sulphate of iron gives a brown precipitate, which acquires a metallic lustre when rubbed;—Pro to chloride of tin (preferably containing a little perchloride) gives a violet, purple, or blackish precipitate, insoluble in hydrochloric acid;—Sulphuretted hydrogen and hydrosulphide, of ammonia give a black precipitate, insoluble in simple acids;—Ammonia gives a reddish-yellow precipitate (‘fulminating gold’), with tolerably concentrated solutions, either at once or on boiling the liquid;—Liquor of pota a gives a reddish-yellow precipitate with neutral solutions of gold, insoluble in exce . Estim. 1. In the dry way;— The quantity of gold in an ALLOY is usually estimated by ‘a aying’ the sample. Before proceeding to the a ay, it is nece ary to form some estimate of the quantity of other metals (copper or silver, or both) in the specimen to be examined, in order to employ the proper proportion of lead in the ‘cupellation.’ The experienced a ayer commonly does this by the ‘a ay of the touch,’ and, in certain cases, by a rough preliminary a ay. The quantity of lead employed may be about 16 times the weight of the copper present in the sample, and when the alloy contains silver an additional allowance of lead, equal to 1 ⁄ 10 th of its weight, is made on that account. When no silver is present, or it is not required to be estimated, a much larger proportion of lead may be employed. The weight taken for the a ay (‘a ay pound’) is usually 12 or 6 gr. The alloy and dose of lead being accurately weighed and separately wrapped in small pieces of paper, the a ay may be at once proceeded with. α. Cupellation. This operation, the most important of the whole, has been already described. Unlike silver, gold will bear the highest heat of the furnace without ‘vegetating,’ ‘fuming,’ or being absorbed by the cupel. The lo of weight gives the amount of copper in the alloy. β. Quartation. The cupelled sample is fused with three times its weight of pure silver (called the ‘witne ’), by which the gold is reduced to one fourth of the ma , or le , and in this state may be easily removed. γ. Parting. The alloy, after quartation, is hammered or rolled out into a thin strip or leaf, curled into a spiral form, and boiled for a quarter of an hour, in a small flask, with about 2 1 ⁄ 2 to 3 oz. of nitric acid (sp. gr. 1·3); and the fluid being poured off, it is again boiled in a similar manner with 1 1 ⁄ 2 to 2 oz. more of nitric acid (sp. gr. 1·2), after which the gold is carefully collected, washed in pure water, and dried. When the operation of ‘parting’ is skilfully conducted, and the acid not too strong, the metal preserves its spiral form; otherwise, it falls into the state of flakes or powder. The second boiling or digestion is technically termed the ‘reprise.’ The lo of weight by ‘parting,’ after deducting that of the ‘witne ,’ corresponds to the quantity of silver originally in the specimen. δ. Annealing. This consists in putting the pure gold obtained by the last proce into a small porous crucible or cupel, and heating it to redne in the muffle. ε. Weighing. This must be done with the utmost accuracy. The weight, in grains troy, doubled or quadrupled, as the case may be, gives the number of carats fine of the alloy examined, without calculation. According to the ‘ old French method ’ of a aying gold, the following quantities are taken:—For the a ay pound, 12 gr.; fine silver, 30 gr.; lead, 108 gr. These having been cupelled together, the (perfect) button is rolled into a leaf (1 1 ⁄ 2 × 5 inches), twisted on a quill, and submitted to parting with 2 1 ⁄ 2 oz. and 1 1 ⁄ 2 oz. of nitric acid, sp. gr. 1·16 (20° Baumé). The remainder of the proce is similar to that above described. Two a ays are made in the same manner, with a third on pure gold or gold of a known finene ; and no conclusion is drawn, unle the a ay of the latter comes out accurately, and that of the first two correspond to each other. For alloys containing platinum, which usually consist of copper, silver, platinum, and gold, the method of a aying is as follows:—The alloy is ‘cupelled’ in the usual way, the lo of weight expre es the amount of copper; and the button, made into a riband and treated with sulphuric acid, indicates, by the portion di olved, that also of the silver present. By submitting the residuum to quartation, the platinum becomes soluble in nitric acid. The lo after digestion in this menstruum expre es the weight of that metal, and the weight of the portion now remaining is that of the pure gold. Gold containing palladium may be a ayed in the same manner. 2. In the wet way: The richne in gold of any substance, whether liquid or solid, when the quantity is small (and indeed in all other cases), is most simply and economically performed by the common method of chemical analysis. The gold may be thrown down from its solution by adding a solution of pro to sulphate of iron; the precipitate, after being washed, dried, and gently heated, may be weighed as pure gold. Pois., . The soluble preparations of gold (chlorides) are violent poisons. The symptoms resemble those occasioned by corrosive sublimate, but are somewhat le violent. Metallic gold in a minute state of division is also capable of producing very unpleasant consequences, and even endangering life. The antidote is iron filings or a solution of sulphate of iron, given conjointly with an emetic. Uses. The numerous applications of gold in the arts and the daily transactions of life need only be alluded to here. In medicine , gold has been given in the form of powder, in scrofula and syphilis, by Chrestein, Niel, and others, with apparent advantage.— Dose , 1 ⁄ 4 gr. to 1 gr., 3 or 4 times a day, in pills, or as a friction on the tongue and gums. An ointment made of 1 gr. of powdered gold and 30 gr. of lard has been applied by Niel to the skin deprived of the epidermis (endermic ally) in the above diseases. The more important chemical compounds containing gold, the alloys and commercial forms of the metal, together with certain factitious substances popularly called ‘gold’, are noticed in alphabetical order below :—