2t>4 



SILVER. 



acicular fibres; cleavage none; fracture more or 

 less perfect conchoidal; lustre resinous, passing 

 into adamantine; colour pearl-gray, passing into 

 lavender-blue, and some shade of green ; the colour 

 becomes brown on being exposed to light ; streak 

 shining; translucent, sectile; hardness about that 

 of talc; specific gravity 5-5. It occurs also in 

 rni!-ts and granular masses. It consists of silver 

 76-0, oxygen 7*1), and muriatic acid 16-4. It is 

 fusible in the flame of a candle, and emits fumes of 

 muriatic acid. Horn silver is most frequently 

 found in the upper parts of veins in clay-slate, but 

 occurs also in beds, generally along with other ores 

 of silver, or with iron-ochre. It is not abundant 

 in European countries, but occurs in large masses 

 in Mexico and Peru. It is used for extracting 

 silver. 



Such are the ores of silver which are properly so 

 called, and from which silver is chiefly extracted. 

 Besides these, however, argentiferous sulphurets 

 of lead and copper are sometimes smelted for the 

 small proportion of this precious metal which they 

 contain. 



We have now to allude to the methods employed 

 in obtaining the silver from its various ores. I 

 These are two in number, smelting and amalgama- 

 tion. The former is founded on the great affinity 

 of silver for lead, which, when fused with silver, 

 acts as a solvent, and extracts it from its union 

 with baser metals. The silver is afterwards sepa- 

 rated from the lead by the well-known process of 

 cupellation, which consists in exposing the alloy to 

 a stream of atmospheric air, by which the lead is 

 converted into an oxide or litharge, while the silver 

 remains untouched. The latter method depends 

 upon the property of mercury to dissolve silver 

 without the aid of heat. The first is called the 

 dry, the last, the wet way of treating silver ores. 

 One or the other process is employed, according to 

 the nature of the ores. The ores which are treated 

 in the wet way are usually those which consist 

 principally of argentiferous sulphuret of lead. The 

 first thing to be done, by this method, is to pul- 

 verize and roast the ore in a furnace, to expel the 

 sulphur. When the well or crucible is full of 

 metal, it is tapped and run off. It is now ready 

 for the process of refining, or cupellation. For 

 this purpose, a reverberatory furnace is employed, 

 the lower part of which is covered with wood 

 ashes and clay, so as to form a cupel. On one side 

 of the furnace there is a hole for the exit of the 

 litharge; and on the opposite side is another for 

 the admission of air to the surface of the metal, 

 which is introduced through an aperture above, to 

 which a cover is adapted. After the lead is melted 

 and brought to a red heat, the blast of air is ad- 

 mitted, and the scoria, as it collects, is removed. 

 When the litharge is formed, the heat is increased, 

 and the quantity becomes greater, and is withdrawn 

 through the opening in the furnace. At the same 

 time, some lead is volatilized. Towards the end 

 of the process, the litharge which comes off contains 

 a small quantity of silver, and is therefore kept 

 separate from the rest. After the whole of the 

 litharge is removed, and the surface of the metal in 

 the furnace becomes bright, a quantity of water is 

 poured on it, to keep it from spirting, which it is 

 apt to do when congealing. The metal thus ob- 

 tained is subjected to a similar operation for about 

 five hours, in a smaller furnace, and at a higher 

 temperature, by which it is completely freed from 

 the lead. With respect to the other method of re- 



duction or separation amalgamation the following 

 is an outline of the more important steps, of which 

 it consists. The ores best adapted to this process 

 are native silver and vitreous silver. The first 

 operation that requires description is the selection 

 of the ores to form a proper mixture, with reference 

 to the quantity of silver and sulphur they contain. 

 It has been observed that the amalgamation process 

 succeeds best when the silver produce is about 

 seventy-five ounces to the ton of ore ; at the same 

 time, regard being had to the quantity of sulphur 

 present, which is ascertained from the quantity of 

 sulphuret in the ore, previously learned by an assay 

 in the crucible. The sulphur is got rid of, by 

 adding to the mixture of raw ore ten per cent, of 

 common salt, by which, during the furnace opera- 

 tion, the sulphur becomes acidified, and the acid 

 thus formed, uniting with the base of the salt, 

 forms sulphate of soda; whilst the muriatic acid, 

 thus set free, combines with the silver in the ore, 

 that was not in the metallic state, and forms 

 muriate of silver. In this state, the ore is subjected 

 to various mechanical operations, with riddles, 

 mills, and sieves, until it is reduced to an impalpa- 

 ble powder. It is then submitted to the action of 

 mercury. This operation is performed in barrels, 

 which are arranged so as to revolve on their axes. 

 The mixture or charge in each barrel consists of 

 sifted calcined ore, mercury, metallic iron, and 

 water, in certain proportions. The ore is composed 

 of sulphate of soda, muriate of silver, and other 

 metals and earthy matters. By the process of 

 amalgamation, the barrels being made to revolve 

 during a period of sixteen or eighteen hours, the 

 muriate of silver becomes decomposed by the 

 action of the iron on its acid ; and the silver, thus 

 reduced to the metallic state, combines with the 

 mercury, forming what is termed amalgam, whilst 

 the sulphate of soda, the muriate of iron, and other 

 salts, become dissolved in the water. The silver 

 combined with mercury is then filtered, by which 

 the surplus metal is separated, and a compound 

 remains in the sack, consisting of six parts of mer- 

 cury and one of silver. This amalgam is subjected 

 to the action of heat in a distilling furnace, by 

 which the mercury is sublimated, and the silver 

 remains. Silver is also sometimes separated from 

 copper by the process of eliquation. The eliquation 

 is effected by means of lead, which, possessing a 

 greater affinity for silver than for copper, combines 

 with the former, when brought into fusion with 

 the alloy, and forms a new metallic compound. 

 The argentiferous lead, thus obtained, is subjected 

 to the usual processes of cupellation, and the coarse 

 copper, from which the silver has been separated, 

 is refined. 



We shall now take notice of the mines of silver 

 in different parts of the world, which furnish the 

 silver of commerce. Those of Mexico and South 

 America are incomparably more important than 

 those of all the rest of the world. Mexico alone 

 has above three thousand mines, or excavations for 

 silver ores, which produced annually, during the 

 last ten years of the seventeenth century, about 

 900,000. The mines of Guanaxuato yielded nearly 

 one quarter of this amount, while the single mine ot 

 Valenciana, situatedin the same district, has afforded, 

 for years together, during the last thirty years, 

 between two and four hundred thousand pounds. 

 Nor are the ores of Mexico, for the most part, 

 rich in silver; but the rich produce of that country 

 has depended upon their abundance, and the facility 





