( 348 ) 



£re, and, confeqiiently, thofe fo extremely 

 fimilar frequently found in lavas, are pro- 

 bably not the effects of permanent gafes, 

 but of the gafification of the lavas them- 

 felves : and this difcovery remains uncon- 

 troverted notwithflanding the above- cited 

 obfervations of Dr. Prieftley, 



I fhall now proceed to enquire what part 

 this aeriform vapour ad:s in the eruptions 

 of volcancs. Where it exifts in the depths 

 of a volcanic crater, abundantly mixed with 

 a liquid lava violently urged by fubterra- 

 nean conflagrations, I can eafily conceive 

 that by its energetic force it may raife the 

 lava to the top of the crater, and compel it 

 to flow over the fides, and form a current. 

 Art can imitate this great operation of na- 

 ture, on an infinitely lefs fcale. I placed 

 in a glafs-furnace a cylindric crucible, one 

 foot high, and two inches and a half in 

 breadth, which I filled half full with one of 

 thofe volcanic products which moft inflate 

 a|id boil in the fire. After fome hours, I 

 obferved that the liquid matter began flowly 

 to rife, and afterwards to rife higher until 



it 



