40 Ox the Effeéts produced on different Stones 
differs from the coloured kinds by this circumftance—that 
by expofure to heat,.the former undergoes no change, while 
the greater part of the latter do. It is equally well known 
that ‘the action of the fluor acid on glafs is increafed by heat. 
In order that I might begin a feries of comparative expe- 
riments in this ré{pe et, I expofed, for thirty-fix hours, at a 
temperature of 40°, a ruby, fapphire, leuco-fapphire, eme+ 
rald, Oriental garnet, amethyft, chryfolite, avanturine, gi- 
rafol, a Brafilian topaz burnt, a Saxon topaz raw, and an 
opal; but after being taken from the apparatus, they feemed 
as little injured as the pure tran{parent rock-cryftal of the 
firft experiment. 
The diamond, which by its combuftibility thews itfelf to 
be a peculiar genus, did not fuffer the leaft change after four 
days expofure to the vapour of the fluor acid; the apparatus 
being placed on a common German ftove. 
Polifhed granite, being expofed to the vapour for three 
days on a ftove as before, neither the quartz nor the mica 
feemed to have been attacked. The feld-fpar, however, at- 
tracted my attention; being opaque and, muddy, and co- 
vered with a white powder. TI therefore repeated the expe- 
riment on a thin fragment of feld-fpar of a reddifh colour, 
noting its weight, which was 38 grains; and found it to be 
2% grains lighter. The ftone had alio hecome whitith and 
friable at the furface, exactly as when in a natural flate of 
efforefcence. 
The different fpecies of flints, mixed with foreign earths, 
are more or lefs fufible. But as this difference is not exactly 
in proportion to the quantity of the earths mixed with them, 
the proper explanation of the phenomenon mutt be fought 
for in the different degrees of affinity which thefe kinds of 
earth have for caloric. 
Figures traced out on the following ftones through a co- 
vering of wax, after being expofed for twenty-four hours to 
the vapour of the fluor acid, the apparatus being placed on 
the ftove, were all found to be etched: Chryfopras, Hunga- 
rian opal, onyx, Perfian cornelian, agate, chalcedony, green 
Siberian jafper, common flint. On the chryfopras the cor- 
rofion was above half a line in depth. In thofe places the 
green. 
