206 ACTION OP HEAT MODIFIED 



in great number on the vast sides of Mount iEtna, eack indi- 

 cating the discharge of a particular eruption. At the same 

 time, a jet of flame and smoke issues from the main crater, 

 proving the internal communication between it and the lava; 

 this discharge from the summit generally continuing, in a 

 greater or a less degree, during the intervals between eruptions. 

 (Fig. 41. represents an ideal section of IMountiEtna; a b ib 

 the direct channel, and i c is a lateral branch.) 

 The pressure of Let US now attend to the state of the lava within the moun-- 



tain, during the course of the eruption: and let us suppose, 

 quid lava is re- ^ ^ j i ' 



600 feet of li- 

 quid lava is rt 

 quired to con- that a fragment of limestone, torn from some stratum below, 



strain the acid i^^s been included in the fluid lava, and carried up with it. By 

 «f carbjonate. , ^ ^ , ., . , . ^ , \, . , . 



the laws of nydrostatics, as each portion of this fluid sustains 



pressure in proportion to its perpendicular distance below the 



point of discharge, that pressure must increase with the depth. 



The specific gravity of solid and compact lava is nearly 2.8 ; 



and its weight, when in a liquid stcite, is prol ably little dif-r 



ferent. The- table shews, that the carbonic acid of Ijnicstone 



cannot be constrained in heat b)'^ a pressure less than t;. t of 



1708 feet of sea, which corresponds nearly io 6C0 feet of Iquid 



lava. As soon, then, as our calcareous mass rose to withiii 



600 feet of the surface, its carbonic acid would quit the lime^ 



^nd, assuming a gaseous form, would add to the eruptive effer-* 



vescence. And this change would commonly begin in much 



greater depths, in consequence of the bubbles of carbonic acid, 



and other substances in a gaseous form, which, rising with the 



lava, and through it, would greatly diminish the weight of the 



column, and would render its pressure on any particular spot 



extremely variable. With all these irregularities, however, 



and interruptions, the pressure would in all cases, especially 



■where the depth was considerable, far surpass what it would 



Calcareoiiis have been under an equal depth of water. Where the depth 



v^i'ca^tkfusio'if °^ *'^^ stream, below its point of delivery, amounted, then, to 



in the win- 1/08 feet, the pressure, if the heat was not of excessive inten- 



' °"*' sity, would be- more than sufficient to constrain the carbonic 



acid, and our limestone would suffer no calcination, but would 



enter into fusion ; and if the eruption ceased at that moment, 



would crystallize in cooling along with the fevii, and become 



a nodule of calcareous spar. The mass of lava, containing 



this nodule, would then constitute a real whinstone, and would 



belons 



