MODIFIED by COMPRESSION. 159 



paper, prove, that the feebleft exertions of volcanic fire, are of 

 fufficient intenfity to perform the agglutination, and even the 

 entire fufion, of the carbonate of lime, when its carbonic acid 

 is effectually confined by preflure ; for though lava, after its 

 fufion, may be made, in our experiments, to congeal into a 

 glafs, in a temperature of 16 ° or 18 ° of Wedgwood, in which 

 temperature the carbonate would fcarcely be affected ; it mull 

 be obferved, that a fimilar congelation is not to be looked for 

 in nature ; for the mafs, even of the fmalleft ftream of lava, is 

 too great to admit of fuch rapid cooling. And, in fact, the 

 external part of a lava is not vitreous, but confifts of a fub- 

 ftance which, as my experiments have proved, muft have been 

 congealed in a heat of melting filver, that is, in 22 of Wedg- 

 wood ; while its internal parts bear a character indicating that 

 they congealed in 27 ° or 28 ° of the fame fcale. It follows, 

 that no part of the lava, while it remained liquid, can have 

 been lefs hot than 22 ° of Wedgwood. Now, this happens to 

 be a heat, in which I have accomplished the entire fufion of 

 the carbonate of lime, under preflure. We muft therefore 

 conclude, that the heat of a running lava is always of fiiffi- 

 cient intenfity to perform the fufion of limeftone. 



In every active volcano, a communication muft exift between, 

 the fummit of the mountain and the unexplored region, far 

 below its bafe, where the lava has been melted, and whence it 

 has been propelled upwards ; the liquid lava rifing through this 

 internal channel, fo as to fill the crater to the brim, and flow 

 over it. On this occafion, the fides of the mountain muft un- 

 dergo a violent hydroftatical preflure outwards, to which they 

 often yield by the formation of a vaft rent, through which the 

 lava is difcharged in a lateral eruption, and flows in a continued 

 ftream fometimes during months. On ^Etna moft of the erup- 

 tions are fo performed ; few lavas flowing from the fummit, 

 hut generally breaking out laterally, at very elevated ftations. 



At. 



