Geological Obfervattons. 391 



of which a very fmall quantity is fufficient to 

 produce flame : this flame once produced, may 

 be fupported by dephlogifticated air from 

 other ores, which Dr. Prie/lly has {hewn to 

 afford it, and the phlogifton may be fupplied 

 by pyrites, bituminous fhiftus, bitumen and 

 coal ; marl, fhiftus, horn-ftone, ftioerl, with 

 a further addition of iron from the pyrites, 

 are the true fources of the melted matter or 

 lava. The explolion and eruption of this 

 melted matter proceeds, in all probability, 

 from the accefs of a large quantity of water, 

 which either enters through fome crack in 

 the bottom of the fea, or from fources in the 

 earth ; if the mafs of water fo admitted be 

 fufficiently great, it will extinguifh the fub- 

 terraneous fire; if not, it will fuddenly be 

 converted into vapour, whofe elaftic force is 

 known to be feveral thoufand times greater 

 than that of gunpowder ; if the fuperincum- 

 bent weight be too great, it may caufe earth- 

 quakes, but will propel the melted matter lat- 

 terally towards the mouth of the volcano, 

 where, meeting with lead refiftance, it will 

 expel it, together with all the unmelted 

 ftony maffes it meets in its paflage. It is 

 eafy to conceive that before the denfe melted 

 matter is ejected, the dilated air of the vol- 

 cano will firft be forced out, and carry with 

 it the afhes and loofer ftones adhering to the 

 fides and crater of the volcano, as has been 

 C c 4 obferved, 



