S I L
SIL
( 324 )
half-whitened copper, or degraded Giver. Beingon the whole an injudicious compound it ismoftly now laid afide. When Giver and copperare in equal parts, the alloy is ftill very yellow,but may be united with about 4 or 5 per cent, ofarfenic, and ftill remain very malleable, with theadvantage of being much whitened by the mix-ture. Vauquelin" has alfo found that a largerdofe of arfenic, does not increafe the whitenefs,but rapidly impairs the malleability, and whenthe arfenic amounts to 10 per cent, the alloy isfo harfh and brittle as not to bear laminating.
Silver unites perfectly with lead apparentlyin every proportion, into an uniform malleablemixture, harder and whiter, and lefs fufiblethan pure lead, in proportion to the quantity offilver. This mixture is formed in the procefsof Eliqudtion (already deferibed in this article)whence it appears that the affinity of filver forlead is fuperior to that of filver for copper.The neceffity of this alloy to the procefs ofcupellation in ajfaying and in refining filver hasbeen already mentioned.
Silver and iron are generally reckoned en-tirely to refufe to unite in any proportions, andit is certain that when they are melted together,they form, on cooling, two perfectly diftineband feparable buttons, the iron at top, and thefilver beneath it. Yet a certain degree of alloy-ing takes place as is proved by the followingcurious experiment of Guyton on this fubjedb. 8In a crucible lined with charcoal, 15 parts ofpure filver, reduced from luna cornea, weremixed with as hruch fine iron filings, and aflux compofed of glafs, charcoal dull, and alittle borax. This was kept for an hour in aheat of 150° to 155 0 Wedgedvood, and on cool-ing, a button was found with fome traces ofcryftallization on the furface, but which fepa-rated molt accurately into two buttons of verydifferent compofition. A part of the lower but-ton, diffolved in nitric acid, gave only a whiteprecipitate, and not a blue one, with pruffiatof potafh, and appeared to be perfectly purefilver, but the metal was found to be prettyftrongly magnetical, and therefore contained avery fmall portion of iron, which was efti-mated by Coulomb to be about T 4 o- The up-per button, which appeared to be pure iron,was exceffively hard, very difficultly frangi-ble, and when diffolved in nitric acid gave acopious precipitate of luna cornea with fait,the quantity of which indicated an alloy ofabout T ' 5 . of filver.
Silver unites with bifmuth, zinc, antimony,
» An. Chim. tom. 39, p. 36 $,
and fome other of the brittle metals, forming,brittle alloys which have not been much exa-amined, and are of little importance.
The affinities of the oxyd of filver for thedifferent acids are in the following order: themuriatic, oxalic, fulphuric, phofphoric, nitric,,arfenic, fluoric, tartareous, citric, acetic, andcarbonic.
This art confifts in covering the furface offubftances with a thin coating of filver. Thereare two motives for this; in the firft place,the fuperior beauty of filver to that of thecheaper metals; and in the fecond place itsfuperior wholefomenefs to copper, brafs, orlead, for culinary purpofes on account of itsnot being added upon by vinegar and otherweak acids.
As an ornament, filver is far inferior to gold,from its great liability to be blackened by ful-phureous vapours, which renders frequentcleaning abfolutely neceflary; hence it is inap-plicable to the purpofe of architectural decora-tion, and is fcarcely ever applied except toutenfils and ornaments of metal. From thefrequent neceffity of friction for the purpofeof removing the tarnifh of filver it is neceflarythat it fhould be much thicker than the moftfolid gilding, otherwife, after a fhort time, thefilver will be worn off the moft prominentparts, difeovering to view the copper or brafsbeneath.
The only metals that are filvered are copper,brafs, and very rarely iron.
There are three modes of filvering, namely,by filver amalgam, by muriated filver, and byfilver in fubftance.
r. Silvering by amalgamation is thus per-formed t
To a folution of nitrated filver add fomeplates of copper which will throw down thefilver in its metallic ftate, and very finely di-vided; ferape it from the furface of the copper,wafh it well and dry it. Of this powder takehalf an ounce, of common fait and fal ammo-niac 2 ounces, and of corrofive fublimate onedrachm, rub them well together, and makethem into a pafte with a little water. Thentake the veffel to be filvered, and clean it bymeans of a little very dilute aquafortis, or byfcouring it with a mixture of common fait andtartar. When it is perfectly clean, rub it withthe above-mentioned pafte till it is entirelycovered with a white metallic coating; thiscoating is an amalgam produced by the de—
• Ibid, tom, 43.