[ 438 ]

brown; to the common emerald, an opake black; to thewhite parts of the common granite, a violet unequally deep;to serpentine stone, an olive colour ; while the much softerstates, talks, and amianthus received from it no colour atall. The experiments formerly mentioned, page 350,afford room to suspect, that the solution of silver stainsstones only in virtue of their containing a calcareous earth,or such an earth as the acid is capable of dissolving : ifthis be the cafe, there is little wonder, that some of thehard stones should be stained, and some of the soft unaf-fected by it.

Among the hard stones that have been tried, the agatesseem to be those which are acted upon most readily :they are those also which have oftenest been attemptedto be stained. The solution should be made in strongaquafortis or spirit of nitre, and fully satiated with themetal. The stone, after the fluid is applied, should beexposed to the sun for two days or more ; and if, whendry, it be removed into a moist place, and afterwards ex-posed again to the sun, the production of the colour willbe the more speedy. After the stone has acquired thefull colour which the first quantity of the solution cancommunicate, it may be moistened with more, and thisrepeated two or three times, by which the colour will bedeepened,, and made to penetrate further: Mr. du Fayfound that an agate about a sixth part of an inch in thick-ness, by applying the solution on both sides, may bestained throughout its whole substance., The tincture,however, is rarely uniform, on these or other stones; mostof them having veins, which, though indifcernable in thenatural stone, are in this process made apparent, beingmore easily or more difficultly penetrable than the rest ofthe mass, and sometimes forming not inelegant varietiesin the stained stone..

Mn.