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and is partly dispersed through the mixture. From thegold, the corroded silver may be boiled out with water,and afterwards recovered from the liquoF in the samemanner as from its solution in aqua fortis : from themixture, it is much more difficultly extracted, by boilingthe matter in melted lead, and afterwards working off thelead, into which the silver has thus transferred itself, upona cupel or test. The quantity of silver however, whichcementation is employed for separating from gold, iscommonly so small as to be entirely disregarded.

The acid of sea salt, applied in the same manner,corrodes all the metallic bodies except gold and platina.Hence either sea salt or nitre may be used in this processindifferently; but they must never be taken together, assome have directed them to be, for the two acids in con-junction would dissolve the gold itself. The mixture ofsea salt with the calcined vitriol and brick dust has beencommonly called the regal cement, from gold, before thediscovery of platina, having been the only known metallicbody that was capable of resisting it.

The gold plates cannot be wholly freed from theiralloy by one operation either with the nitrous or marinecements, the vapours penetrating but a very little way intotheir substance. Hence for the effectual purification ofgold by this method, the metal is to be remelted, flattedinto plates, and again exposed to the fumes. The processindeed appears upon the whole to be incommodious,whether considered as a method of purifying gold or ofascertaining its purity ; and accordingly, though once inmuch esteem, it is now rarely practised. Its principaluse is for extracting silver or base metals from the surfaceof gold, and thus giving superficial purity and high colourto alloyed or pale gold.

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