manufadory is eftablifhed in a country, it is 
neceflary to keep it up with much induftry and 
prudence, if we would wifli it, to furmount the 
caprice of falhions *, fince how much foever the 
natural beauty of thefe ftones feems to plead for 
their pre-eminence, they will at fome periods 
unavoidably fink in the efteem of mankind, but 
they will likewife often recover, and be reftored 
to their former value. 
The grindftones at Oberftelri are of a red co- 
lour, and of fuch particular texture, that they 
neither admit of any polifh, nor are they of too 
loofe a compofition. 
Moll part of the flinty tribe is employed for 
making glafs, as the quartz, the flints and peb- 
bles, and the quartzofe fands. The quartz,, 
however, is the beft ; and, if ufed in due pro- 
portion with refped to the alcali, there is no 
danger of the glafs being eaflly attacked by the 
acids j as has fometimes happened with glafs 
made of other fubftances : for inftance, of bot- 
tles filled with Rhenifli and Mofelle wines, 
during the time of a voyage to China. 
In the fmelting of copper ores, quartz is ufed 
To render the flag glafly, or to vitrify the iron ; 
quartz being more ufeful than any other ftone, 
to regain or revive this metal. 
The prefence of the quartz in the rock-flones, 
(Se(51:. cclxii.) and alfo in crucibles, and fuch vef- 
fels, contributes moil of all to their power of 
refilling the fire : it appears likewife probable, 
that the quartzofe matter makes the^ grind and 
whetftones fit for their intended purpofes. 
5ECT. 
