160 EFFECTS of HEAT 
At the place. of delivery, a quantity of gafeous matter is pro- 
pelled violently upwards, and, along with it, fome liquid lava ; 
which laft, falling back again in a fpongy ftate, produces one 
of thofe conical hills which we fee in great number on the vaft 
fides of Mount A®tna, each indicating the difcharge of a parti- 
cular eruption. At the fame time, a jet of flame and {moke iffues 
from the main crater, proving the internal communication be- 
tween it and the lava; this difcharge from the fummit gene- 
rally continuing, in a greater or a lefs degree, during the in- 
tervals between eruptions. (Fig. 41. reprefents an ideal fection 
of Mount tna; ad is the direct channel, and 4c is a lateral 
branch). 
Ler us now attend to the ftate of the lava within the moun- 
tain, during the courfe of the eruption ; and let us fuppofe, that 
a fragment of limeftone, torn from fome ftratum below, has 
been included in the fluid lava, and carried up with it. By the 
laws of hydroftatics, as each portion of this fluid fuftains pref- 
fure in proportion to its perpendicular diftance below the point 
of difcharge, that preflure muft increafe with the depth. The 
{pecific gravity of folid and compact lava is nearly 2.8 ; and its 
weight, when in a liquid ftate, is probably little different. 
The table fhews, that the carbonic acid of limeftone cannot be 
conftrained in heat by a preflure lefs than that of 1708 feet of 
fea, which correfponds nearly to 600 feet of liquid lava. As 
foon, then, as our calcareous mafs rofe to within 600 feet of the 
furface, its carbonic acid would quit the lime, and, afluming a 
gafeous form, would add to the eruptive effervefcence. And 
this change would commonly begin in much greater depths, in 
confequence of the bubbles of carbonic acid, and other fubftan- 
ces in a gafeous form, which, rifing with the lava, and through 
it, would greatly diminifh the weight of the column, and would 
render its preflure on any particular fpot extremely variable. 
With all thefe irregularities, however, and interruptions, the 
preflure 
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