VOLCANIC FIRE. 



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the most inflammable substances, carbon and sulphur, when subjected 

 to intense heat in vacuo, undergo no change in their weight or prop- 

 erties. It is only when air or water obtains access to volcanic fire 

 that it can produce effects analogous to those of combustion on the 

 surface. Indeed, it appears probable that volcanic explosions and 

 eruptions are occasioned by the access of water to subterranean fire. 

 A sudden evolution of steam and vapour thus produced, will force a 

 passage to the surface, in those parts where the incumbent rocks offer 

 the least resistance, and the lava and fragments of rock will be ejected 

 with a force, proportionate to the quantity of steam or air suddenly 

 evolved. 



