Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/774

Rh 700 and a sm-aller one at one side through which acid may be introduced. These, as well as other connexions on the pipes carrying off the vapours, are secured by waterjoints. The charge of about 200 ounces (0'2-3 kilogrammes) of granulated silver is tre-.1te-1 with twice its weight of sulphuric acid m-.irl<ing 66° Baumé, and, by care- ful ﬁring, is dissolved in six hours. The proper management of the heat is of im- portance, as neglect in the conduct of the oper-ation may easily lead to a breakage of the pot. When the charge is completely dissolved the liquid is allowel to settle for some time, and is then poured off into a lead pan, where the silver sulphate solidiﬁes. This, when relissolved by an ad- dition of water and careful warming, is treated with strips of copper, theseparation ,’ / of the silver beinrr ._ ..;~-. ;~.r;‘-' ~ ;j§ facilitated by agitai ing the liquid. When Fig. 10. the latter is found to be completely free from silver the heating is stopped, and the contents of the pan are allowed to settle for eighteen hours, when the copper solution is drawn off bya siphon and sent to the vitriol crystallizers. In the precipitation of 100 kilogrammes of silver about 30 l{llOgl‘.1l11tll8S of sheet—copper are expended. The precipitated silver is washed with water in a copper vessel upon a linen ﬁlter until the reaction of copper in the washings ceases, and then moulded in cylindrical blocks by screw pressure, to express the residual water. These when fire-dried are melted in black-lead pots, holding 75 It), with the addition of a little soda nitre. The parted gold remaining in the porcelain pot, though already sensibly ﬁner than is usual when iron parting vessels are used, still contains silver, and is therefore boiled once more with sulphuric acid of 66° Baumé. Afterwards it is wasnel with water until silver can be no longer detected in the washings, when it is transferred to a porcelain dish and dried. When a quantity of about 10 lb of gold has been accumulated, it is mixed with a'little borax glass, melted in a black-lead pot, and cast. The resulting bars average T°U",;‘U ﬁne. ' Ilq/1'rn'ny by C’/zlorine G'as.—F. Bowyer Miller‘ devised in 1867 the following method for separating silver from gold. The process, which is the one now adopted in the .-Jstralian mints,” consists in converting the silver into chloride by the passage of a stream of chlorine gas through the molten alloy. Clay crucibles are employed after hav- ing‘ been saturated with a strong solution of borax and allowed to dry. The chlorine is introduced through the gold by a clay pipe passing to the bottom of the crucible, and connected with the chlorine generator in which the necessary pressure is obtained by a pressure tube 8 feet , // /'3 1., 1 C/zeuz. Soc. Journ., v. xxi.. 1868, p. 506. 9 I-‘our!/z A.'L71?tt'l Rnp-"t If Drputy-_lI:1strr If Jlint, 1873, p. 62. GOLD high. The chloride of silver is easily poured off from the surface of the molten metal, and by carefully fusing with a little carbonate of soda, the small amount of gold it retains is separated and falls to the bottom of the crucible. The gold operated upon contains fro111 3 to 12 per cent. of silver, and the average ﬁneness of the reﬁned gold is 994. The operation is now conducted on a considerable scale in Australia, and in the years 1871 and 18722 no less than 1,100,000 ounces of gold were reﬁned by its aid in Sydney alone. The absolute loss of gold does not execed 14 parts in 100,000. To-ug/zem'n_q Brittle G'oltI.—-It will be seen from p. 751 that minute traces of certain metals, which do not exceed the Tfﬁth part of the mass, render gold brittle and unﬁt for coinage. Miller showed that the removal of the deleterious metals might be effected by converting them into volatile chlorides by a stream of chlorine gas. The process was introduced into the English mint by Roberts,” who successfully treated over 40,000 ounces of brittle gold with but triﬂing loss of precious metal. Wagner has suggested4 that bromine may replace chlorine in _liller’s process. Brittle gold may also be toughened by throwing a small quantity of corrosive sublimate on to the surface of the molten metal, but this method is wasteful, and the fumes evolved are deleterious. The late Mr Waringtcn-" proposed to toughen brittle gold by the addition of about 10 per cent. of black oxide of copper. The process is eﬁicacious, but the crucibles become much corroded and even perforated; the standard fineness of the gold is, more- over, lowered by such copper as is reduced to the metallic state. If gold is but slightly brittle, it may be toughened by pouring it in a thin stream through atmospheric air into a crucible lined with borax, or by the addition of a small quantity of chloride of copper. I’reparatz'on of I’-are GoltI.—Chemically pure gold may be prepared by several methods. The metal, either in the form of powder or “ cornets ” from the purest gold that can be obtained, is dissolved in nitro—hydrochloric acid. The excess of acid is driven off, alcohol and chloride of potas- sium added to precipitate platinum, and the chloride of gold is then dissolved in pure distilled water, the solution being diluted until each gallon does not contain more than half an ounce of the precious metal. The solution is allowed to stand for several weeks, and the supernatant liquid is carefully removed by a siphon from any chloride of silver that may have fallen to the bottom of the vessel. The gold may then be precipitated by a stream of carefully Washed sulphurous anhydridc, or by the addition of oxalic acid, formic acid, or ferrous sulphate. The spongy gold is washed with dilute hydrochloric acid, distilled water, ammonia water, and again with distilled water, after which it is melted in a clay crucible with a little bisulphate of potash and borax, and poured into a stone mould. lloberts“ prepared by this method 70 ounces of gold of which the average purity was 90006, the precipitant being oxalic acid. Gold precipitated by oxalic acid from an acid solu- tion containing copper is always contaminated with cupric oxalate. E. Purgotti7 has, however, shown that by heating the solution with the addition of potash, a soluble double oxalate of copper and potash is formed, and the gold is left in the pure state. Alloys of Gold. —The most important alloys are those with silver and copper. Those used for coinage at the present day contain from 800 parts of gold in 1000, the standard of the Norwegian 2-kroncr 3 First and Second Annual Reports of Dq)ut_1/-Jlrzstcr of Jlint. p 1870- 2, p. 93 and 34 respectively. “ Bull. Chem. Soc. Paris, t. xxv., 1876, p. 138. 5 Chem. Soc. Journ., xiii. 1860, p.31. 5 Fourth Annual Report of Royal .1! int, 1874, p. 46; C'0mm[ttcc of Iiritish Association Report, 1873, p. 219. 7 Zcitsrlzr. Anr/I. C'h('::2., ix. 127.