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*9® Sllicecus Genus. Its frafture perfedly conchoidal. Frag, mcnts, 3. & Its hardnefs, from 6 to 8. It is faid to have lometimes occurred in a foft ftate, and to have hardened by expofure to tbe air. Its fp. gr. from 1,700102,144. ^ It frequently contains a drop of water. A fpe- cimen of this fort occurs in Lefke’s Colledion O.841. ' _ It is infufible^r fe at 150°; it yields moft rea- dily to mineral alkali, next to borax, but fcarce at all to microcofmic falt. By Mr. Bergman’s analyfis it contains 0,85 filex, 0,13 argill, contaminated with iron. By that of Klaproth, the white opal of Kofemitz contains 98,75 filex, 0,01 argill, and 0,01 of iron. The green derives its colour from calx of nickel *. Ir is often decompofed by expofure to the at- mofphere. I ^found the opal 844. Lelke O. Infufiblc at 151 0 ; and fo are all opals found to bc. 2d Family. Semi Opal -f~. Its colours are, bluifh, pear), reddifh, greenifh or yellowifh grey ; greyifh, reddifh, or yellowifh white, or honey, or wax yellow; or flefh, or hya- cinth red ; or reddifh brown, or yellowifh, or liver brown ; or grafs, leek, or verdegris gieen, or rnany of thefe in the fame mafs. * 2 Berl. Beob. p. 45. t VVerner’s Cronlt. p. ^ Amorphous