Sir H. Davy on the Phenomena of Volcanoes. 93 



occasionally to be found in the lava, or the combustion ought 

 to be increased at the moment the materials passed into the 

 atmosphere. But the reply to this objection is, that it is evi- 

 dent that the changes which occasion volcanic fires, take place 

 in immense subterranean cavities ; and that the access of air 

 to the acting substances occurs long before they reach the ex- 

 terior surface. 



There is no question but that the ground under the solfa- 

 terra is hollow, and there is scarcely any reason to doubt of a 

 subterraneous communication between this crater and that of 

 Vesuvius : whenever Vesuvius is in an active state, the sol- 

 faterra is comparatively tranquil. I examined the bocca of the 

 solfaterra on the 21st of February 1820, two days before the 

 activity of Vesuvius was at its height: the columns of steam 

 which usually arise in large quantities when Vesuvius is tran- 

 quil, were now scarcely visible, and a piece of paper thrown 

 into the aperture did not rise again ; so that there was every 

 reason to suppose the existence of a descending current of air *. 

 The subterraneous thunder heard at such great distances un- 

 der Vesuvius, is almost a demonstration of the existence of 

 great cavities below filled with aeriform matter : and the same 

 excavations which in the active state of the volcano throw out 

 during so great a length of time immense volumes of steam, 

 must, there is every reason to believe, in its quiet state be- 

 come filled with atmospheric airf. 



To what extent subterraneous cavities may exist even in 

 common rocks, is shown in the limestone caverns of Carniola, 

 some of which contain many hundred thousand cubical feet of 

 air ; and in proportion as the depth of an excavation is greater, 

 so is the air more fit for combustion. 



The same circumstance which would give alloys of the me- 

 tals of the earths the power of producing volcanic phenomena, 

 namely, their extreme facility of oxidation, must likewise pre- 

 vent them from ever being found in a pure combustible state 

 in the products of volcanic eruptions ; for before they reach 

 the external surface, they must not only be exposed to the air 

 in the subterranean cavities, but be propelled by steam ; which 

 must possess, under the circumstances, at least the same faci- 

 lity of oxidating them as air. Assuming the hypothesis of the 



* In 1814, in 1815, and in January 1819, when Vesuvius was compara- 

 tively tranquil, I observed the solfaterra in a very active state, throwing up 

 large quantities of steam and some sulphuretted hydrogen. 



f Vesuvius is a mountain admirably fitted, from its form and situation, for 

 experiments on the effect of its attraction on the pendulum : and it would 

 be easy in this way to determine the problem of its cavities. On Etna, the 

 problem might be solved on a larger scale. 



existence 



