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entirely put to the account of alloy. When nomore lead is ufed than neceflary for the perfectreparation of the alloy, M. Tillet reckons thatit carries down into the cupel as much filver as,when the whole is again reduced, would makethe noble metal T -f ¥ of the mafs, when the naturaladmixture of filver is only about tttt* But ifan excefs of lead is employed for cupellation,this lofs of filver is fomewhat greater, thoughit does not encreafe in the ratio of the excefsof the lead, for ten parts of lead to a givenalloy will not carry down twice as much filveras five parts, though the difference of lofs willbe very fenfible.

It might be fuppofed that as the litharge ofthe firft affay has been able to carry down acertain portion of filver into the cupel in thefirft inftance j the fame when again reduced toreguline lead could not be made to reftore itsexcefs of filver by mere cupellation per fe. Thishowever is not the cafe, for the fecond cupel-lation is found to leave a button of filver fullyequal to the lofs of this metal in the firft affay,the lead only carrying down its natural retentof filver or nearly, that is about TT V T of itsweight, as above mentioned. If the litharge ofthe fecond cupellation is again reduced and thelead cupelled a third time, an extremely final!globule of filver is left, fcarcely vifible to thenaked eye. Again reduced and cupelled, aminute grain of filver, only vifible by a lens, isleft on the cupel, after which the quantity be-comes fo fmall as to elude the fenfes. This isa convincing proof againft the converfion oflead into filver by cupellation, formerly fuppofedby fome chemifts of confiderable name, thefilver being obvioufly only extracted from thelead in thefe proceffes, and not generated fromit. In all thefe reductions the filver appearsequally diftributed through the lead, for M.Tillet found that feparate globules of lead byaccident fpurted out upon an empty cupel inthe muffle, each left a minute atom of filverlying upon the fpot where the globules hadfcorified.

Bifmuth will ferve the fame purpofe as leadin cupellation, but befides that is a dearer metaland not always eafily procurable, it is found tocarry down with it into the cupel fomewhatmore of the filver than the fame quantity oflead does.

As the lead muft be proportioned before cu-pellation to the eftimated quantity of alloy inthe filver, a method muft be found of formingthis eftimate with fufficient exactnefs. Theantient affayers made great ufe of Touch-

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Needles, or fmall flips or bars of metal, madewith pure filver, alloyed with known propor-tions of copper, in a regularly encreafing leriesfrom the leaft to the greateft proportion everrequired. The filver to be allayed was thepexamined in comparifon with the touch-needles,in colour, tenacity, and other external charac-ters, and its alloy was eftimated by that of theneedle to which it fliewed the clofeft refem-blance. Thefe needles are now however almofttotally difufed in filver allaying, an experiencedaflayer being able to judge of the alloy withquite fufficient exaCtncfs, by the eafe or diffi-culty with which it is cut, the colour and grainof a frefh cut furface, the malleability, thechange of furface when made red hot, and thegeneral appearance.

Affay of Gold.

The allay of gold is fomewhat more compli-cated than that of filver. Silver, if not mixedwith gold or platina, requires only the lingleoperation of the cupel to feparate its alloy andafcertain its purity; if mixed with gold, thoughthe latter be in a fmall proportion, it is called agold affay, on account of the fuperior value ofthis metal. Copper, or any other bafe metal,when mixed with gold may indeed be feparatedfrom it by cupellation with lead as from Jilver,but it is found by experience that the affinity ofcopper to gold is fo ftrong as fcarcely to beovercome by this method, unlefs the mixturebe firft combined with a certain quantity offilver before cupellation. This therefore necef-farily requires a fubfequent operation, namelythat of feparating the gold from the filver whenmixed in the button left after cupellation.

Gold is alfo frequently alloyed intentionallywith filver in fome foreign coins, and in fomekinds of manufacture. The procefs of parting ,or feparating gold from filver is performed bya dilute nitric acid (or aqua fortis as it is con-ftantly termed in manufacture) the acid diffolv-ing the filver and leaving the gold untouched.But here another lingular circumftance occurs :it is found that when the gold is in confiderableproportion in the mixture, it fo much protectsthe filver from the acid as to prevent its aCtionmore or lefs completely, and the parting is im-perfect. Therefore, when thefe metals are thusmixed, it becomes neceflary to add fo muchfilver as to give this metal a great excefs overthe gold. About three of filver to one of goldare generally confidered to be requilite, andhence the procefs of parting has alfo been calledquartation, the relative proporlion of the goldbeing reduced to no mere than one-fourth of" thep