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it admits better of being burnished. It is obvious, thatfor uses of this kind, the mercury ought to be pure, aswell as the gold : for the lead or other base metals, withwhich mercury is too frequently impregnated, will be leftbehind, and discolour the gold.

If an amalgam of gold be spread upon copper, andthe mercury evaporated by fire, the gold will remainfixed all over the surface of the metal, and thus afforda firm and durable gilding. The workmen rarely chusepure copper for gilding upon in this manner, but gene-rally mix with it about a seventh part of brass, that is, ofa composition of copper and zinc : they suppose that thisaddition renders the copper less porous, and makes a lessquantity of gold to serve : whatever there may be in thisnotion, the brass is plainly of use on another account, tofacilitate the adhesion of the mercury ; for mercury unitesexceeding difficultly with pure copper, and much moreeasily, as I have often found in experiments of amalgama-tion, with copper divided by zinc.

The piece to be gilt being well cleaned, some mer-cury, shaken with a little aqua fortis, is spread upon it,till the surface appears all over white as silver : beingthen heated, and re-touched in those parts which haveescaped the mixture, the amalgam of gold is laid on :the heat, softening the amalgam, makes it spread themore freely ; and the intervention of the mercury andaqua fortis occasions it to adhere more uniformly. Thepiece, thus covered with the amalgam, is placed on aconvenient support, over a charcoal fire ; and examinedfrom time to time, as the mercury evaporates, that if anydeficiencies appear, they may be supplied with a littlemore of the amalgam before the process is completed..If a thicker gilding is required, than can result from somuch of the amalgam as is applied at once, the piece,

after