mineralogy; 75 



manufactory is eftablifhed in a country, it is 

 necefTary to keep it up with much induftry and 

 prudence, if we would wifh it to furmount the 

 caprice of fafhions ; 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 grindflones at Oberflein 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. 



Moft 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 refpect to the alcali, there is no 

 danger of the glafs being eafily attacked by the 

 acids -, as has fometimes happened with glafs 

 made of other fubftances : for inftance, of bot- 

 tles filled with Rhenifh 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-ftones, 

 (Sect, cclxii.) and alfo in crucibles, and fuch vef- 

 fels, contributes moft 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. 



SECT. 



