GOLD AND SILVER.] 



CHEMISTRY. 



373 



The dust and nuggets picked up or mixed in the sand 

 of auriferous rivers, require little more than melting 

 before being introduced into commerce : when found 

 attached to quartz, the stone is broken by means of 

 powerful machinery ; and the powder thus obtained is 

 washed by an abundance of water until all the earthy 

 particles are removed. This is sometimes done by means 

 of a cradle, which being kept in continual motion, 

 permits the earth to be washed away ; whilst the heavy 

 gold falls to the bottom, and is subsequently collected by 

 the miners. In cases where the quantity of gold is com- 

 paratively small, the process of amalgamation is adopted ; 

 mercury is mi _>d with the golden sands, and it thus 

 dissolves the gold, just as sugar would be dissolved by 

 water ; the gold and mercury, forming the amalgam, 

 are put into a suitable vessel. Heat is employed, and the 

 mercury distilling over, leaves the gold behind in an 

 almost pure state : gold, thus procured, is generally cast 

 into blocks called ingots, and is then ready for the various 

 purposes to which we shall presently refer. 



Gold may be distinguished from other metals by the 

 following characteristics : In its pure state it is easily 

 cut by a knife ; it is insoluble in almost every liquid, 

 with the exception of nitro-hydrochloric acid : if it be 

 thus dissolved, it allbrds a rich purple colour on a solution 

 of tin being added thereto ; and sulphate of iron throws 

 the gold down, in the form of a dark-coloured powder, 

 in the metallic state : it may be distinguished from brass 

 by means of nitric acid, which dissolves the base metal, 

 and leaves the gold untouched. 



The uses to which gold is applied are very numerous ; 

 wherever magnificence, endurance, and costliness are 

 involved, the metal is called on to take a part. Pure 

 gold is never used for commercial purposes, being far too 

 soft. In the current coin of the realm, it is alloyed in 

 the proportion of twenty-two parts of gold to two of 

 copper. By these means it acquires sufficient hardness 

 to withstand the wear and tear to which it is exposed. 

 It is alloyed in various proportions, for the manufacture 

 of jewellery, watches, <bc. Metals are readily gilded by 

 being covered with an amalgam of gold ; which is made 

 by adding that metal to mercury. The latter is driven 

 oil' by heat, leaving the gold as a thin coating. Gold is 

 highly ductile and malleable ; it may be beaten out into 

 leaves which are so thin as to require between two and 

 three hundred thousand of them to form a pile one inch 

 thick. This is effected by beating the metal with heavy 

 hammers, between skins of parchment. Gold may be 

 drawn into wire as fine as a human hair. 



Like platiaa, its combinations are few, owing to the 

 weak affinity which subsists between it and chemical 

 reagents. It forms two combinations with oxygen : the 

 sub-oxide is obtained by precipitating the sub-chloride, by 

 means of a solution of potash. Auric acid, or the per- 

 oxide, is a compound of gold and oxygen ; and is obtained 

 from the perchloride by digesting it with magnesia, and 

 boiling the resulting mixture in nitric acid ; which affords 

 a yellow-coloured powder. 



The most important compounds of gold are the chlo- 

 rides and cyanides. The cliloride is obtained by dissolving 

 gold in iiitro-hydrocalorio acid ; and the dichloride by 

 heating the chloride so obtained to a temperature of 600 

 Full. Tliu cyanide is chiefly employed for the purposes 

 of electrotyping ; and for its description, manufacture, 

 and uses, we refer our readers to the section on Elec- 

 tricity (article Metallurgy) ; in which a full account may 

 be found. 



A kind of fulminating gold different to that which is 

 sometimes termed the fulminate of gold is produced by 

 adding a solution of strong ammonia to one of the chlo- 

 ri le, and by pouring more ammonia on the precipitate so 

 produced. This powder explodes on being rubbed by a 

 hard substance. The fulminates proper we shall descriln) 

 hereafter. lies' ''us the alloys with copper, which we 

 have already nn.-iitioiied, gold unites with silver, lead, 

 <tc., in various proportions. It may be obtained from 

 such alloys by the process tenned cupellatiou, which we 

 ha vo already d The purple powder, produced 



by precipitating a solution of cliloride of gold by means 



of the chloride of tin, is employed for the purpose of 

 colouring china ware. It is termed in the arts, the 

 "purple powder of Cassius." 



As it is often desired to distinguish gold readily from 

 the baser metals resembling it in colour, we may men- 

 tion that this may be done by rubbing the suspected 

 article on a rough hard stone, for which the " blood- 

 stone" is usually employed. A very minute portion 

 of the metal is thus removed to the stone ; and, on 

 the streak, a little nitric acid is to be dropped. If the 

 metallic appearance be lost, it is evident that gold is not 

 present, because that metal will not dissolve in nitric 

 acid. 



Steel articles may be easily gilded by dipping them 

 into a liquid, made by adding ether to the solution of 

 the chloride of gold. The ether takes up a portion of 

 the metal, from which the steel article removes it to its 

 own surface. Care should be taken that the metal does 

 not touch the acid solution resting beneath the ether. 



SILVER. 



SILVER, as we have previously remarked, belongs to a 

 class of metals not readily acted on by ordinary reagents ; 

 but being more so than those we have already described, 

 we have taken it last of the three. Its symbol is Ag ; 

 its equivalent 108 '1 ; and its specific gravity = 10 - 5. It 

 is found in a pure state in nature ; and by a late dis- 

 covery, which was accidentally made during an analysis 

 of a sheet of copper, taken from the hull of a vessel 

 which had been long at sea, it has been shown that the 

 ocean contains many millions of tons of the metal.- Sil- 

 ver is found in most of the lead ore of Great Britain, and 

 is separated therefrom as mucli as possible, because even 

 if not present in sufficient quantities to pay for extrac- 

 tion, it injures the lead by making that metal brittle. 

 The chief sources of silver for commercial purposes, are 

 Mexico, Chili, and Peru. It is sometimes found com- 

 bined with other metals, and occasionally with sulphur. 

 It is obtained from the ore by amalgamation, in a similar 

 manner to that we have described in respect to gold. 



Silver is easily distinguished from other metals. In a 

 pure state it has a white colour, and is susceptible of a 

 nigh polish ; it, however, is readily acted on by air con- 

 taining sulphur, and hence soon becomes tarnished in 

 rooms wherein coal-gas is burned. It is readily soluble 

 in nitric acid (aqua foriit); and if a solution of common 

 salt be added thereto, a white curdy powder falls down, 

 which turns to a dark colour on exposure to light. 



It is readily separated from other metals by cupellation ; 

 and it forms alloys with copper, gold, silver, <fcc. ; and an 

 amalgam with mercury. The latter mixture is much 

 employed for the purposes of plating, which, however, has 

 been now almost entirely superseded by electro-plating 

 processes, which we have fully described in the section 

 on Electricity ; article Metallurgy. 



Our remarks in reference to the uses of gold are 

 equally applicable to silver, which, to a considerable 

 extent, possesses the same qualities. For the purposes 

 of coin, silver is alloyed with copper ; one pound of 

 standard silver being coined into sixty-six shillings. It 

 is largely employed for making various domestic utensils 

 and ornaments. 



It may be beaten into leaves of a thickness not exceed- 

 ing riTuWrth f an m h each. It is also easily drawn 

 into fine wire ; and in this state is, like gold, much used 

 for embroidering drapery, <fec. From being unacted 

 upon by many chemical substances, it has been employed 

 to line cylinders in which coffee is roasted, and also in 

 vessels used in the manufacture of lemonade and soda- 

 water. 



Pure silver may be obtained by dissolving common 

 silver coin in nitric acid. To this solution, common salt 

 is to be added as long as the chloride falls down. The 

 precipitate is then to be dried and mixed with carbonate 

 of soda, and a small portion of powdered charcoal. It 

 is then to be heated in a crucible, in a wind-furnace, by 

 which the silver will be reduced bom the chloride to a 

 metallic state. Other methods for obtaining the same 



