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that of most rivers, though the preference is always tobe given to such as has been distilled. Those waters,which turn milky with solution of stiver, may be madefit for this use, in the same manner as the impure aquafortis, by dropping in a little of the solution, till all thematter, that is capable of precipitating the silver, is sepa-rated : in this cafe, great care must be taken, not to usemore than is necessary of the solution; for so much ofthe dissolved stiver, as is added after the marine and vitri-olic acids have satiated themselves, will continue dissolvedin the water; and as the gold is at last to be washed inthe water, the moisture, that hence adheres to the gold,containing a proportionable part of the dissolved silver,will on drying leave it in the gold.

Besides the purity of the aqua fortis, a good deal ofcaution is requisite in regard to its strength. The onlysure mark of its due strength, for the parting assay, is itseffect in the process itself; and the manner of adjusting itwill be the more intelligible after the process has beendescribed.

The little bead of gold and silver, remaining after thecupellation, is carefully hammered a little, and passedseveral times between polished steel rollers, screwed gra-dually closer and closer, till it is extended into a verythin plate, which is coiled up into a spiral form, so asthat the several circumvolutions may not touch one ano-ther : by this means it lies in a small compass, so as tobe covered by a quantity of aqua fortis sufficient for dis-solving the silver, and yet exposes a large surface to theaction of the dissolvent. The metal is now and thennealed during the flatting ,* and after this part of theprocess is finished, it is again made red hot, both toburn off any unctuous matter that may have adhered toit, and to soften the silver, which in this state is supposed

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