0[Q ACTION OF HEAT MODIFIED 



eruptions as water woiTld infallibly do, to flow through the same aper- 



paSd*"^^*^ ^'^^^ ^"^■^- ^*^^ ^^'^^^ ^^ ^ material difference in the two cases. As 

 soon as the lava has ceased to flow, and the heat has begun to 

 abate, the crevice through which, the lava had been passings 

 remains filled with a suUtancc,, which soon agglutinates into a 

 mass, far harder and firmer than the mountain itself. This 

 mass, lying in a crooked bed, and being firmly welded to the 

 sides of the crevice, must oppose a most powerful resistance tot 

 any stream tending to pursue, the same course. The injury 

 done to the mountain by the formation of the rent, will thus 

 be much more than repaired ; and in a subsequent eruption, 

 the lava must force its way through another part of the moun- 

 Eventheper- tajn or through sonoe part, of the adjoining country. The 

 vtntmav^be 'Action of heat from below, seems in most cases to have kept a 

 -■rloserl, as hap- channel open through the axis of the mountain, as appears by 

 vRiTfor^Iraira" *^^ smokc and flame which is habitually discharged at the sum- 

 ceatury. mit during intervals of calm. On many occasions, however,, 



this spiracle seems to have been entirely closed by the conso- 

 lidation o\' the lava, so as to suppress all emission. This hap- 

 pened to Vesuvius during the middle ages. All appearance of 

 fire had ceased for five hundred years, and the crater was co- 

 vered with a forest of ancient oaks, when the volcano opened 

 Avith fresh vigour in the sixteenth century. 

 TIic opening of The eruptive force, capable of overcoming such an obsta- 

 maiiVc-asesis ^^^^ must, be tremendous indeed, and seems in some cases to 

 fficoted with have blown the volcano itself almost to pieces. It is irapossi- 



prodigious ex- ,', ' , ,, • re i ■ i • i r r 



plosion and dis- "'^ ^"^ ^^^ ^"<^ -^J"'-'"^^"'' <^^ Somnia, which, m the form pi a, 



luption, crescent, embraces JMovnrt Vesuvius, without being convinced 



that it is a fragment of a large volcano, nearly concentric with 



The present inner cone, which, in some great eruption, had 



been destroyed all but this fragment. In our own tiines, 



an event of no small magnitude has taken place on the same 



spot ; the inner cone of Ves).ivius having undergone so great a 



change during the eruption in 1794, that ft now bears no re- 



.scrablance to what it wa.s when I saw it in 17S5. 



liri.ce the The general or partial stagnation of the internal lavas at tliQ 



earthquakes ' gj^ge of each piuption seems, then, to render it necessary, that 



preceding com- , •' 



jnon eruptions, in every n<nv discharge, the lava should begiji by making a vio- 

 lent l.ncciiition. And this is probably the cause of those tre- 

 n">endous curthcjuakcs which precede all great eruptions, 



and 



