METALS METALLURGY. 



been employed to extract gold, and any silver asso- 

 ciated with it as chlorides, from which both metals 

 are precipitated together by metallic copper or 

 other agent. Gold is separated from the silver 

 by several processes, but most largely by what is 

 called the ' wet way.' By this method the alloy, 

 previously brought to the proportion of three parts 

 of silver to one of gold, may either be treated 

 with boiling nitric acid (quartation), which dis- 

 solves the silver, leaving the gold unattacked ; or, 

 a similar alloy being prepared in which, how- 

 ever, the exact proportion of gold matters less 

 it may be subjected to the action of boiling sul- 

 phuric acid, which also dissolves the silver, and 

 so parts it from the gold. 



Gold is the only yellow metal, and it is the most 

 malleable of all. It can be beaten out into leaves 

 not exceeding 57^7 of an inch in thickness, while 

 still thinner is the coating of gold on silver wire 

 used for gold-lace ; and, in this case, the gold and 

 silver are drawn out together from a rod of the one 

 metal coated with the other. This proves that it 

 is also very ductile, a fact otherwise shewn by the 

 extreme fineness into which wire from gold itself 

 can be drawn. Pure gold is nearly as soft as lead, 

 and does not oxidise or tarnish in air or water. It 

 is quite insoluble in any simple acid except selenic, 

 but it dissolves in aqua-regia, which is a mixture 

 of hydrochloric and nitric acids. In this case, the 

 active agent is the liberated chlorine, the metal 

 dissolving in solutions containing free chlorine or 

 bromine. The specific gravity of gold is 19-5, 

 being less than platinum and one or two other 

 metals. 



Gold intended for coinage or jewellery is always 

 alloyed with copper or silver to harden it. Standard 

 gold, of which British sovereigns are made, con- 

 tains 22 parts of gold to 2 of copper ; that is, 

 taking the number 24 to represent pure gold, such 

 coinage is 22 carats fine. Jewellery, watch-cases, 

 chains, and like articles, are manufactured of 

 alloys, which contain as a minimum one-third of 

 their weight of gold, or 8 carats fine. The richest 

 of these rarely exceed 18 carats fine, and by a 

 special treatment these finer qualities take on a 

 rich and characteristic colour. It was long 

 believed that this colour could not be given to less 

 than 15-carat gold, but a process has lately been 

 discovered on the continent by which an alloy of 

 13-carat can be 'coloured.' Large quantities of 

 gold are consumed in gilding, which is done in 

 various ways, such as by the use of gold-leaf, by 

 the electrotype process, and by painting on a 

 mixture of its powder with varnish, which is after- 

 wards burnished. The fine ruby and crimson 

 colours given to glass and porcelain are produced 

 by a compound of gold and tin. 



Silver. 



Silver does not stand second to gold in intrinsic 

 value, but it ranks next to it in beauty, in the 

 length of time it has been known, and among the 

 precious metals in the variety and importance of 

 its applications. It is found native rather exten- 

 sively, and sometimes in crystallised or arborescent 

 pieces of great beauty. Most native silver, how- 

 ever, occurs disseminated in small fragments 

 through argentiferous ores and their associated 

 rocks. Occasionally a very large piece is met 

 with. One, for example, was found at Kongsberg, 



in Norway, weighing 560 Ibs. ; another at Huantaya 

 in Peru, weighing 800 Ibs. ; and a third in Mexico 

 weighing 2700 Ibs. We have already referred to 

 native alloys of gold and silver with an excess of 

 gold, and may now state that such alloys are also 

 found in which the silver greatly predominates. 

 The ores of silver are numerous, but the great 

 majority of them consist of compounds of sulphur 

 and silver, some other metal being also frequently 

 present. Silver likewise occurs in combination 

 with chlorine, bromine, and iodine. It is scarcely 

 ever absent from galena, the chief ore of lead, and 

 though usually present in very small proportion, 

 yet, from this source alone, a very large quantity 

 of silver is now annually obtained. Of silver ores 

 proper, the most important are silver glance, or 

 vitreous sulphide of stiver, containing, when pure, 

 87 per cent, of the metal; stephanite, or brittle 

 sulphide of silver, containing, when pure, 70 per 

 cent, of silver ; pyrargyrite, or ruby silver, which, 

 when pure, contains 59 per cent.; and chloride of 

 silver, or horn silver, consisting of 75 per cent of 

 silver and 25 of chlorine. 



Mexico, whose silver mines were discovered in 

 the 1 6th century, has long stood at the head of 

 silver-producing countries ; but the extraordinary- 

 discoveries of ore in Nevada and adjoining states, 

 which began so recently as 1859, will probably 

 soon bring their produce abreast of the Mexican 

 yield. Even now, these Pacific States of the 

 Union are not far behind, and the annual produce 

 of the two regions may be roundly taken as three- 

 fourths of all the silver obtained in the world. 

 Next in importance are the ancient mines of Peru 

 and Bolivia, and the later, though by no means 

 recently discovered, deposits of Chili. In Peru, 

 the principal mines are at Cerro de Pasco, a town 

 I 3>^73 feet above the sea, and containing some 

 18,000 inhabitants. There are two principal veins, 

 one 9600 feet long and 400 broad, the other 6400 

 feet in length and 380 in breadth. The mines 

 have been badly worked, and but for the draw- 

 backs attending mining operations in such high 

 and desolate regions, could probably be made to 

 exceed those of all other districts in extent of pro- 

 duction. In Europe, Spain is the largest producer 

 of silver, Austria is next, and then follow Saxony, 

 Prussia, and Great Britain. The other countries 

 yield comparatively little, although Norway has a 

 rich but limited mining district at Kongsberg. 



Silver is extracted from its ores by several pro- 

 cesses. One of the oldest was introduced into 

 Mexico in the i6th century, and is still practised 

 both there and in South America. It consists in 

 grinding the ore to a fine powder ; spreading it in 

 heaps in the state of pasty slime on a stone floor ; 

 and adding first a small quantity of common salt, 

 and then copper pyrites (the sulphide of iron 

 and copper), which becomes converted into the 

 sulphate. Mules now tread the mass for several 

 hours, after which mercury is evenly spread over 

 a heap, and the animals then continue to tread it 

 repeatedly every other day till the whole is well 

 mixed. The nature of the chemical reactions in 

 the process is disputed ; but the most recent view 

 is that an oxychloride of copper is formed, which 

 then combines with the sulphur of the ore, leaving 

 the silver free to amalgamate with the mercury. 

 The amalgam is separated from the slime by 

 washing, and after the quicksilver is distilled off 

 by heat, the silver is obtained in a spongy state 



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