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loofe and porous. It cannot, therefore, be faid that It retains 

 the fame chymical conftitution as before ; the cafe is quite 

 different with refpedl to glaffes formed of earthy fubflances 

 without any fait, as I know from my own experience, when 

 once they are perfectly vitrified, a fecond fufion makes no 

 alteration whatfoever in them, though ever fo flowly cooled. 

 Thus, feltfpars, garnets, fh&rls and bafalts, being converted into- 

 glafs by the heat of a furnace, remain glafs even when ex- 

 pofed to the higheft heat producible by art, namely, that arifing 

 from the a(5lion of pure air ; nor will any retardation of their 

 cooling produce the fmalleft change. As to the flagg of iron 

 furnaces, it is a compound in which the metallic particles, 

 being by far the mofl abundant, feparate themfelves, during 

 fufion, from the earthy. Thefe laft then vitrify, vitrification 

 being the effedl of the heat to which they are then expofed, 

 and not in confequence of their rapid refrigeration ; the metallic 

 particles, on the contrary, aflume the grain that is peculiar to 

 them, being incapable of vitrification ; hence all analogy with 

 bafalt fails. 



The Do(flor, however fenfible of the difficulty of fuppofing 

 that a fubftance once uniformly fufed, as he imagines granite 

 to have been, fliould prefent us 2, 3, 4, 5 and 6 fepai'ate 

 fubftances, as granites frequently do, further adds, " That 

 " this difficulty does not prefs the igneous more than the oppofite 

 " hypothefis, fince the conflituent parts of granite are cryflals,. 

 " the. whole mafs mufl have once exifted in that flate of entire 

 *' difunlon of Its particles which Is nccefTary to cryftalllzation. 



Now 



