41B PRINCIPLES OF CHEMISTRY 



greater quantity of silver occurs in combination with sulphur, and 

 especially in the form of silver sulphide^ Ag 2 S, with lead sulphide 

 or copper sulphide, or the ores of various other metals. The largest 

 amount of silver is extracted from the lead in which it occurs. If this 

 lead be calcined in the presence of air, it oxidises, and the resultant 

 lead oxide, PbO (* litharge ' or ' silberglatte/ as it is called), melts into 

 a mobile liquid, which is easily removed. The silver remains in an 

 unoxidised metallic state. 14 This process is called cupellation. 



bright white globule. If the weight of the alloy taken and of the silver left on the cupel 

 be determined, it is possible tp calculate the composition of the alloy. Thus the essence 

 of cupellation consists in the separation of the oxidisable metals from silver, which does 

 not oxidise under the action of heat. A more accurate method, based on the precipitation 

 of silver from its solutions in the form of silver chloride, is described in detail in works 

 on analytical chemistry. 



14 In America, whence the largest amount of silver is now obtained, ores are worked 

 containing not more than p.c. of silver, whilst at p.c. its extraction is very profitable. 

 Moreover, the extraction of silver from ores containing not more than O'Ol p.c. of this 

 metal is sometimes profitable. The majority of the lead smelted from galena contains 

 silver, which is extracted from it.- Thus near Arras, in France, an ore is worked 

 which contains about 65 parts of lead and 0'088 part of silver in 100 parts of ore, which 

 corresponds with 186 parts of silver in 100,000 parts of lead. At Freiberg, in Saxony, the 

 ore used (enriched by mechanical dressing) contains about 0'9 of silver, 160 of lead, and 

 2 of copper in 10,000 parts. In every case the lead is first extracted in the manner 

 described in Chapter XVIII., and this lead will contain all the silver. Not unfrequently 

 other ores of silver are mixed with lead ores, in order to obtain an argentiferous lead as 

 the product. The extraction of small quantities of silver from lead is facilitated by the 

 fact (Pattinson's process) that molten argentiferous- lead in cooling first deposits 

 crystals of pure lead, which fall to the bottom of the cooling vessel, whilst the propoiv. 

 tion of silver in the unsolidified mass increases owing to the removal of the crystals' 

 of lead. The lead is enriched in this manner until it contains ^ part of "silver, and' 

 is then subjected to cupellation on a larger scale. According to Park's process, zinc is- 

 added to the molten argentiferous lead, and the alloy of Pb and Zn, which first separates 

 out on cooling, is collected. This alloy is found to contain all the silver previously con- 

 tained in the lead. The addition of O'i> p.c. of aluminium to the zinc (Bossier and Edelman) 

 facilitates the extraction of the Ag from the resultant alloy besides preventing oxida- 

 tion ; for, after re-melting, nearly all the lead easily runs off (remains, fluid), and 

 leaves an alloy containing about 80 p.c. Ag and about 70 p.c. Zn. This alloy may be used 

 as an anode in a solution of ZnCl 2 , when the Zn is deposited on the cathode, leaving the 

 silver with a small amount of Pb, &c. behind. The silver can be easily obtained pure by 

 treating it with dilute acids and cupelling. 



The ores of silver which contain a larger amount of it are : silver glance, Ag 2 S (sp. 

 gr. 7'2) ; argentiferous-copper glance, CuAgS ; horn silver or chloride of silver, AgCl ; 

 argentiferous grey copper ore; polybasite, M 9 RS 6 (where M=Ag, Cu 4 and R=Sb, As), 

 and argentiferous gold. The latter is the usual form in which gold is found in alluvial 

 deposits and ores. The crystals of gold from the Berezoffsky mines in the Urals contain 

 90 to 95 of gold and 5 to 9 of silver, and the Altai gold contains 50 to 65 of gold and 86 to 

 88 of silver. The proportion of silver in native gold varies between these limits in other 

 localities. Silver ores, which generally occur in veins, usually contain native silver and 

 Various sulphur compounds. The most famous mines in Europe are in Saxony (Frei- 

 fcerg), which has a yearly output of as much as 26 tons of silver, Hungary, and Bohemia 

 {41 tons). In Russia, silver is extracted in the Altai and at Nerchinsk (17 tons). The 

 richest silver mines known are in America, especially in Chili (as much as 70 tons), 

 Mexico (200 tons), and more particularly in the Western States of North America. The 



