y 
By the Vapour of the Fluor Acid. gt 
green colour of the ftone had difappeared, and the cavities 
were filled with a white powder. The ftrokes of the figures 
were expreffed with the greateft fimenefs and regularity on 
the opal; and the lines, in the like manner, were filled 
with a white powder. The onyx exhibited the contours 
very clearly, and the etching was pretty deep: the powder 
with which they were filled was likewife white. On the 
cornelian the figure was in part etched, and filled with white 
powder; in part there was only a white efflorefcence, though 
fill compaé& and entire. The agate and. chalcedony were, 
on the other hand, corroded white, but very unequally: here 
and there cavities were formed, each of which was lined 
with a white compact fubftance. The green jafper-was cor- 
roded very unequally, but almoft to as great depth as the. 
chryfopras. Some parts which remained compaét, and which 
were only as it were eflorefced, had loft their green colour 
and become white. 
Flint. The uncovered part, of a bright brown and fome- 
what tranfparent fpecimen, had become totally white, but 
was neverthelefs compact. As I had covered the ftone with 
wax, leaving a fmall fpot bare, without delineating any re- 
gular figure, I obferved that the efflorefcence had begun at 
the edge of the wax, and proceeded thence to the centre in 
fuch a manner that the white contour thereby produced re- 
fembled an imperfeé figure of a fortification, while the 
inner {pace was only partially efflorefeed, and ftill grey, but 
interfperfed here and there with white points. To free the 
ftone from the wax, I wafhed it in fpirit of wine. The white 
firure then gradually difappeared: in half a minute nothing 
of it was to be feen; and the thin fragment feemed tranf- 
parent as in its native ftate. When it became dry, how-— 
ever, the opaque white figure again made its appearance, 
After moiftening and drying it feveral times in fucceffion 
the revival of the natural colour and opacity became always 
more perfeét, and the white efflorefcence remained vifible 
when wet. As all thefe effects were produced by water and 
other liquids as well as by fpirit of wine, as I afterwards re- 
marked on other {pecimens, thefe phenomena of the flint had 
a great fimilarity to the change produced in the colour of the 
opal, 
