S I I

S IL

( su )

the refidual copper, by retaining mcft of thefilver that was at firlt diffufed through thewhole mafs, becomes rich enough to be palledimmediately to the refinery. 1 he matt is tobe ground to powder, and mixed with commonfait and quicklime, in the proportion of 12 pertent, of each, and is then to be roafted for 1 ohours. This being finifhed, it is to be amalga-mated in the manner above defcribed, and bythree fucceffive roaftings and amalgamationsthe whole of the filver may be extradited.

§. 4. Physical and Chemical Properties .

Silver is a metal of a beautiful white colour,perfectly free from tafte and fmell. The colouris very diftinguilhable from that of every othermetal, being a pure brilliant white, free fromany other admixture of hue. Its fpecific gravitywhen fimply fufed in mafs is about 10.4 whichis fomewhat'increafed by hammering or la-mination, but lefs fo than molt other metals.It is a foft metal, being eafily fcratched bycopper. It is confiderabiy elaftic, and whenhardened by alloying, is highly fonorous, andeven in fmall quantity much increafes thefonoroufnefs of the Alloys of Copper, as men-tioned under that article. Silver is one of themoll extenfible metals that we are acquaintedwith, its dudlility is only lefs than that of gold,in confequence of which it may be beaten outinto extremely finefleaves, and drawn out intowire thinner than the fineft human hair. Forthis latter purpofe however a fmall alloy of cop-per is found neceffary, though lefs than that offtandard filver, which is one twelfth. Hence thefilver procured from the filver and filver-giltwire ufed for laces, embroidery, and otherornamental purpofes, bears a higher price thanany other ufually met with. This metal is alfovery tenacious, fo that a wire a tenth of an inchdiameter will fupport about 240 lbs. withoutbreaking.

Silver, when quite free from alloy, melts ina moderately intenfe red-white heat, fo that aplate about the dimenfions of the thinneft palte-board, will fcarcely fupport the fulleft heat of avery brilk fire in a common grate. When infufion, if pure, the furface is moll ftrikinglybrilliant and beautiful, and like a while poliihedmirror, but as it congeals, it becomes of a cleandead white. If cooled haftily, the furface as itfixes moots up into fmall irregular projectionswith feme little force, fo as to difperfe a fewparticles of ttiat part of the metal Hill in fufion.Hence m the delicate bufinefs of the affay arifestile piecautioil of cooling melted filver very

gradually while it is fixing. Pure filver is notfenfibly volatilized by being kept in a heatnot much above its melting point for anylength of time, though when mixed withanother metal which is itfelf volatile, either inthe metallic or oxidated flate, a little of thefilver is then diffipated. Silver is a perfBmetal, that is, it does not oxidate by being keptin fufion whillt expofed to air. This pofitionhowever, though fufficiently accurate for allpra£tical purpofes, requires fome limitation;for when this metal is intenfeiy heated withaccefs of air, the fupport on which it Handsgradually becomes yellow, owing to the form-ation of an oxyd of filver.

The fame change takes place, as was ob-ferved by Darcet, Macquer , and others, oninclofing filver within fmall porcelain balls, andexpofing them to a very long continued andintenfe heat, after wliich a iois of filver isexperienced, and the earthen veflels are foundenamelled with a yeilowifh-white vitreousglazing, refulting from the union of the vitrifiedoxyd with the earth of the porcelain, throughwhofe pores fufficient oxygen had entered tooxidate part of the filver. The pure oxyds offilver are however reducible to metallic filver bymere beat, when not in contact with any earthor other fubllance with which they can vitrefy,fo that the permanence of the oxyd in thisinflance is owing to its intimate union withthe earth. Under the blow-pipe, filver, whenintenfeiy heated, emits copious fumes, whichwill render brown a furface of gold expofed toit, and confift of the filver volatilized by theextreme heat. The pure oxyd cannot beprepared by mere heat, but is readily fur-nilhed by the precipitation of the acid folutionsby an alkali. In this ftate it is infoluble inwater, free from tafie, and cannot be fublimed,but when heated per fe it returns to its metallicftate, except the part immediately in contadlwith any earthy matter which becomes fixed byvitrefying with it.

Silver is nearly unalterable by fimple expofureto air and moifture, fo that it is incapable ofrufting. A poliihed furface of this metal willremain bright for fome time in a pure air freefrom fulpbureous and animal vapours; bydegrees however the metal becomes dull andbrownifli, and after a while a very flight coatingforms on its furface, which rubbing with anyfoft powder will take off, and the brightnelswill be reftored with fcarcely any lofs of weight,the outer coating long protefting the metalwithin from further change. This takes place,