SILVER. 



«old-duft brought to the wellern coall of Africa, leads us 

 to believe, that the countries fouth of the Niger contain 

 large quantities of this metal, though ve have httle inform- 

 ation refpcfting the filver of the African continent. 



A^ay and Analytis of Silver-Ores. — Pure native filver re- 

 quires no other allay than fiifion, with a little potalh to free 

 it from its earthy matter. In the humid way the filver may 

 be diffolved in nitric acid, and precipitated by common fait. 

 The precipitate may then be fufed with ioda in a crucible, 

 by which the filver is obtained pure, and the muriate of 

 foda fublimed. The auriferous filver-ores may be treated 

 with potafh, by fufion in a crucible : the alloy of filver and 

 gold is firft obtained, and the two metals may be fepa- 

 rated by the procefs of parting. See RefinIiS'g and As- 

 saying. 



Thofe ores which confift of filver combined with anti- 

 mony or arfenic, or both, are firft roafted, to drive off the 

 arfenic and antimony, the filver remaining pure. The pro- 

 cefs is much facilitated by the ufe of nitre, for the pur- 

 pofe of oxydating the metals to be dilTipated. 



The humid analyfis of this ore requires more particular 

 treatment. 



The ore commonly called arfenical is firft to be coarfely 

 powdered, and then diftilled vinegar poured upon it, to dii- 

 folve the lime of the calcareous fpar adhering to it. 



A given quantity of the ore fo waftied \i now to be finely 

 powdered, and nitric acid poured upon it : this oxy- 

 dates the metals, diflolving the greatcft part, and leaving a 

 yellowifh refiduum. To the part diffolved muriate of foda 

 is to be added, which precipitates the filver. This precipi- 

 tate being waftied and dried, will give 77 per cent, of pure 

 filver. To the liquid from which the laft was precipitated 

 add a folution of potafti ; a lightifti red precipitate is now 

 formed, which, on drying, becomes of a deep brown, and by 

 ignition afTumes the form of powder of a whitilh-grey co- 

 lour : this is the arfeniate of iron. This fiibftance contains 

 iron and arfenic, in the proportions of 50 to 43 or 44. This 

 is ftiewn by treating the arfeniate of iron with charcoal ex- 

 pofed to a red heat ; the arfenic is reduced, and fublimes, 

 while the black oxyd of iron remains. 



The yellowifh refiduum left in the firft folution is to be 

 digefted with muriatic acid : if a white powder remains un- 

 dinolvfed, it will be found to be muriate of filver, and muft 

 be added to that firft obtained. To the muriatic folution 

 add a folution of carbonate of potafti, and a yellowifti-gi een 

 precipitate is formed : to this precipitate, when waftied and 

 dried, add muriatic acid by a little at once, till the powder 

 is diflblved ; add a large quantity of water to this folution, 

 which will produce awhile precipitate ; this being feparated 

 and dried will be pure oxyd of antimony, affording ^^ of 

 pure antimony. What remains in folution, after the laft 

 fubftance is feparated by the water, may be precipitated 

 by pure potafti, and will be found to be oxyd of iron ; 

 which, when treated with charcoal, like the firft obtained, 

 may be added to the fame to make the whole of the iron 

 afforded by the mineral. In this way the arfenical filver- 

 ore afforded, according to Klaproth, the following ana- 

 Jyfis : 



Silver 

 Iron 

 Arfenic 

 Antimony 



The fiilphuretted ores in the large way fometimes merely 

 require to be roafted to drive off the fulphur : the heat being 



urged affords a button of pure filver. This is the cafe with 

 the variety called Silver-glance. 



The brittle filver-ore contains a very fmall portion of an- 

 timony and copper. The metallic button obtained by heat 

 will require to be cupelled with lead, in order to get the 

 filver pure ; it may, however, be made tolerably pure by 

 treating the button with nitre, by which the bafe metals 

 are feparated. 



To effeft the humid analyfis of brittle filver-ore, the 

 powder is dilTolved in dilute nitric acid with a gentle heat. 

 By this treatment a refiduum is left equal to ^% of the 

 whole. The folution is to be treated with muriate of foda, 

 like the laft. 



If the prefence of an alkaline fulphate does not form any- 

 precipitate with the remaining folution, the mineral does not 

 contain lead : add to the folution an excefs of ammonia, and 

 a greyifti-white precipitate will be left, which is the oxyd of 

 iron, often containing a little arfenic. If copper be prefent, 

 ammonia will give to the remaining folution a fine blue co- 

 lour, and that metal may be feparated by a rod of clean 

 iron. 



It now remains to examine the firft refiduum which was 

 unaffcftcd by the nitric acid ; this is to be digefted with 

 nitro-muriatic acid : the refiduum left after this treatment 

 will be found to be pure fulphur. The nitro-muriatic fo- 

 lution is now to be diluted with a large quantity of water ; 

 a white precipitate fall; down, which, when waftied, dried, 

 and ignited, will be found to he oxyd of antimony, of a 

 brown colour. 



Klaproth found 100 grains of this ore to yield as follows ; 



Silver 



Antimony 



Iron - - . 



Sulphur . - . 



Copper and arfenic 



Extraneous matter from the mine 



66.5 

 10 



5 

 12 



95 



The white filver-ores afford nearly the fame ingredients 

 with the laft ; and the mode of analyfis will be fimilar in the 

 dry way. - 



The linhl-iuhile and clark-iuhite filver-ores contain lead 

 and alumine, and ivquire a different treatment. After 

 the filver is precipitated by common fait, a quantity of 

 muriate of lead is formed at the time, which, on concentra- 

 tion, affords the muriate of lead in bright filky cryftals. 

 When thefe are coUeded till the liquid will afford no more, 

 a folution of fulphate of foda is to be added, which precipi- 

 tates the remainder of the lead in the ftate of fulphate of 

 lead. This powder, being waftied and dried, contains iths its 

 weight of metallic lead. The remaining liquid being fuper- 

 faturated with ammonia, as in the analyfis of the brittle 

 filver-ore, a light-brown precipitate is formed : this precipi- 

 tate is oxyd of iron and alumine. To feparate the latter, 

 diflolve the precipitate in nitric acid : feparate the iron by 

 pruffiat of potafti, or pruffiat of lime, and afterwards the 

 alumine with foda. The pruffiat of iron, heated to a red heat, 

 is decompofed, leaving the black oxyd of iron, which con- 

 tains i-J of metallic iron. 



The firft refiduum left by the nitric acid, befides anti- 

 mony and fulphur, which conftituted the refiduum of the 

 brittle filver-ore, alfo contains lead. By the frequent 

 addition of muriatic acid with the application of heat, the 

 lead is feparated in cryftals of muriate of lead ; obtaining, 

 by this means, a folution of the muriates of lead and anti- 

 mony. 



