S8 Prof. Biscliof on the Natural History of 



verns. Davy is of opinion that when volcanos lie at a distance 

 from the sea, as those of South America, the water may be fur- 

 nished from subterranean lakes ; for Von Humboldt asserts that 

 some of these volcanos cast up fish. 



If we wish to ascribe volcanic phenomena to chemical action, 

 says Davy, the oxidation of the metals of the earths and alka- 

 lies merit our attention in preference to any other process. He 

 himself, however, observes, that the observations in mines and 

 in hot springs seem to indicate, with some degree of probability, 

 that the interior of the earth possesses a very high tempera- 

 ture, and that, if the earth's nucleus be supposed in a state of 

 fusion, the explanation of volcanic phenomena is simpler than 

 according to his own theory. 



Gay Lussac very justly remarks, that it is impossible to con- 

 ceive the admission of atmospheric air into the focus of volca- 

 nos, as there must be a force within them acting outwards, by 

 Avhich the liquid lava, a substance about three times as heavy 

 as water, is raised to a height of above 3000 feet, as at Vesu- 

 vius, and more than 9000 feet in many other volcanos. A 

 pressure of 3000 feet of lava, equal to that of a column of wa- 

 ter of 9000 feet high, or to about 300 atmospheres, necessarily 

 prevents the entrance of air into the interior of the volcanos ; 

 and as this pressure continues for many years, during which 

 time the phenomena by no means abate in activity, it is im- 

 possible that air should in any way contribute to it. 



The presence of water in volcanos during the various stages 

 of their activity is, on the other hand, a circumstance repeatedly 

 witnessed by all observers.* Even the smoking during their 

 intervals of repose is, for the most part, nothing but a disen- 

 gagement of aqueous vapour. Violent eruptions are not un- 

 frequently followed by such enormous quantities of steam, that 

 it condenses in the atmosphere, and falls in heavy showers, as 

 was the case after the memorable eruption of Vesuvius, which 

 destroyed Torre del Greco in 1794.-(* Among the elastic fluids 



* See, among others, Monticelli and Covelli, der Vesuv. Deutsch bear- 

 beitet von Noggerath and Pauls. Elberfeld, 1824, p. 167. 



t See von Buch's geognostisclie Beobaolitungen, torn. ji. 152. There is, 

 however, still another cause, which occasions these heavy showers, as we 

 shall shew afterwards. 



