C «8S ) 



wliy fome volcanos produce pumices, but 

 never glafs ; as may be remarked of Santo- 

 rine. Their fires are only fufficiently 

 powerful to generate pumices, but not to 

 produce glafs ; an obfervation applicable to 

 the prodigious eruption from the fea de- 

 fcribed by Thevenot. On the contrary, 

 at Rotaro in Ifchia, at Vefuvius, and other 

 parts of the Phlegrsan Fields, as alfo at 

 Vulcano, Lipari, Felicuda, and Alicuda, 

 glafTes are found mixed with lavas ; which 

 fhews that the fire has aded unequally ia 

 thofe places. It is to be remarked, at the 

 fame time, that the heat requifite for vitrifi- 

 cation is not of the moft violent kind, as I 

 have {hewn in Chap. XVI. 



We muft however obferve, that as the 

 produdlion of ccmpa£l: lavas is a fecret 

 which Nature has hitherto referved to her- 

 felf ; fmce v/e are unable to imitate them 

 with our common fires ; fo alfo are we 

 ignorant of the preciie caufes of the forma- 

 tion of pumices. Among iome thoufands 

 of fufions which I have made in the fur- 

 nace. 



