( 155 ) 



ixthiB — // feems to proceed lefs from iht 



• quality of the fones a^ed on by the vol- 



fame fire^ than from the i?teficacy of that 



agent to produce vitrif cation — A fnccef- 



fively fro7iger degree of heat reqnifite for 



a fione to pafs from the fate of lava into 



pumice^ and from that of pumice into per- 



J'eEi glafs — Elucidation of the caufc why 



fome volcanos produce pumices^ but not 



glaffes — The formation , of pumices hitherto 



not attainable in our furnaces — Blacli the 



natural colour of pumices ^ which are ren^ 



dered white by external caufes, 



Enquiries relative to bafaltes — Thefe origi-- 

 nate i?i the hiunid 'way^ vohen the voord 

 bafalt is underfood in the fenfe in which 

 it was tfed by ihe ancients — ^ he columnar 

 fones which^ from their prifnatic conf- 

 guration^ refemble the bafaltes of the an^ 

 (ients^ originate according to circumfances 

 in the humid or the dry way. — F roofs of 

 their origin in the dry way in Vulcano 

 and FcUcuda — Nature^ in the fofil king^ 

 dom^ produces cryfals as well by the dry 



4 'ZC'iTJ 



