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eompofition of the corrofive fublimate bymeans of the copper, to the furface of which itapplies very clofely and expeditioufly. Thecopper being thus filvered over is to be wafhed,dried, and afterwards heated nearly red, inorder to drive off the mercury ; the filver re-mains behind adhering firmly to the copper, andcapable of being highly polilhed.
2 . Silvering by luna cornea•
Prepare the luna cornea in the ufual mannerby pouring a folution of common fait into ni-trat of filver as long as any precipitation takesplace, and boiling the mixture; the whitecurdy matter thus obtained is to be mixed withthree parts of good pearl-afii, one part ofwafhed whiting, and fomewhat more than onepart of common fait. The furface of the brafsbeing cleared from fcratches is to be rubbedwith a piece of old hat and rotten ftone to re-move any greafe, and then is to be moiflenedwith fait and water, a little of the eompofitionbeing now rubbed on with the finger, the fur-face of the metal will prefently be covered withfilver. Then wafh it well, rub it dry with foftrag, and as the coat of filver is extremely thin,cover it with tranfparent varnifh to preferve itfrom tarnifh. This kind of filvering is veryimperfect, and is only ufed for the faces ofclocks, the fcales of barometers, and fimilarobje£fs.
3. Silvering by Silver in Suljlance.
There are three ways of -performing this.The firft is by mixing together 20 grs. of filverprecipitated by copper,-2 drachms of tartar, 2drachms of common fait, and J a drachm ofalum; this eompofition being rubbed on a per-fectly clean furface of copper or brafs, willcover it with a thin coating of filver, whichmay afterwards be polifhed with a piece of foftleather.
A Hill better way is that which is calledFrench plating, which con lifts in burnifhingdown upon the furface of the copper fucceflivelayers of leaf-filver to any required thicknefs.I11 this the filver has much more folidity thanin any of the former, but the procefs is tedious,and the junftures of the leaves of filver cannotalways be entirely concealed. '
The Englilh method of plating (in thofeworks to which it is applicable) appears to bea ^- It is thus performed: one ofthe furfaces of an ingot of copper is renderedquite fmooth and clean, and is fprinkled overwith glafs of borax 5 upon this is laid a plate offine filver, about ~ £ of the weight of the copperand the two^are carefully bound together by
wire; the mafs is now expofed to a full redheat which melts the borax and caufes the fil-ver to adhere to the copper; the ingot is nowpalled through a rolling prefs, and formed intoa plate; both the filver and copper extendinguniformly during the whole procefs, at theconclufion of which the two metals are inle-parably fixed to each other.
SINOPLE. See Jasper.
SKIN. For the preparation of the ikin ofanimals for the purpofes of mnnufa&ure, feethe article Leather.
SLAG. Is a technical term ufed amongfmelters and workers in minerals, to exprefsany hard vitrefeent generally coloured andopake mafs;- produced by the fufion of anyftony or metallic mixture. It generally confiftsof the gangue or matrix of the ore, togetherwith any faline or earthy flux that may havebeen ufed. Thus the Jlag of iron founderies isfor the moft part compofed of the earthy partof the ore, of the lime ufed as a flux, and thewhole deeply coloured with a part of the oxydof iron which has efcaped reduftion. A Jlagdiffers from a fcoria in being more denfe and •more compleatly vitrefied, whereas the fcoriaor drofs is lighter and porous. When the flagis very opake and heavy, it contains a confider-ble quantity of metallic oxyd fo that in im-proved fmelting works it is often worth whileto work over again with frefli reducing matterthe flags of former operations conduced lefsfkilfully. In fome parts the flag of founderiesis broken into lumps, and ufed for mendingroads, for which it makes an excellent materialwhen a little worn down, being very hard, andimpenetrable by water.
SLATE.. See Thonschieeer.
SMELTING, Schmeltzung, Germ, is theredudfion of metallic ores and fufion of metalsin the large way.
SOAP. Seife, Germ. Savon, Fr.
A foap may perhaps he defined to be a che-mical union of any oily with any faline matterwhereby the oil acquires a folubility in menftruawith which it naturally refufes to unite.
In this extended fenfe many compounds arecalled faponaceous or foapy which are natural orartificial mixtures of the faline and the oily-orrefinous; thus bile is termed a faponaceousfluid becaufe it contains a natural refin and arralkali in intimate union, and inifcible to any