t *54 ]

tabled is perfectly free from any admixture of the vitri-olic or marine acids, so as not to require the purifica-tion necessary for the common forts : it is generally toostrong for the common purposes of aqua fortis, and istherefore to be diluted with a proper quantity of purewater. The calx of copper may be revived, without muchloss, by melting it, in a suitable furnace, in contact withthe burning charcoal.

III. Purification of gold from silver and base metals by

cementation.

Though the nitrous acid in its liquid state does notextract silver from gold, unless the quantity of silver greatlyexceeds the gold ; yet in cementation, where the acid,resolved into fumes, is applied to the metal at the fametime strongly heated, it attacks and corrodes a part of thesilver though its proportion be very minute.

For this purpose, nitre in substance is mixed with.equal its weight of common green vitriol calcined, ordried as for the making of aqua fortis, and with twiceits weight of powdered bricks; the one to extricate itsacid when sufficiently heated, and the other to preventthe mixture from growing fluid in the fire. The metal isflatted into thin plates, which are surrounded and inter-laid with this powder, in a crucible, or in an earthenvessel made on purpose for this use called a cementingpot: the vessel is closely covered, and the juncture securedwith a mixture of soft clay and sand, or other properclayey compositions ; and being placed in any convenientfurnace, a moderate heat is kept up for twelve or sixteenhours. The silver, and most of the base metals alongwith it, are corroded by the nitrous vapour into a salineconcrete, which partly adheres in the pores of the gold,

and