82 M. Gay-Lussac on Vulcanos. 



diiced; an equilibrium must have established itself^ the agitated 

 mass must have subsided into a state of repose which could no 

 longer be troubled by intestine causes, and which can only 

 now be disturbed by fresh contact between bodies accidentally 

 brought together, and which were, pei'haps, only added to the 

 mass of the globe subsequeiitly to the solidification of its sur- 

 face. 



Now the possibility of contact between bodies in the interior 

 of the earth, the ascent of lava to a considerable height 

 above its surface, ejections by explosion, and earthquakes, 

 necessarily imply that those extraneous substances which pe- 

 netrate into volcanic furnaces must be elastic fluids, or rather 

 liquids capable of producing elastic fluids, either by means of 

 heat which converts them into vapour, or by afluiity which 

 sets at liberty some gaseous elements. According to analogy, 

 the only two substances capable of penetrating into the vol- 

 canic furnaces in volumes sufficiently large to feed them, are 

 air, and water, or the two together. Many geologists have 

 assigned to the air an important office in volcanos ; its oxygen, 

 according to them, sustains their combustion : but a very sim- 

 ple observation will suffice to overthrow this opinion entirely. 



How, indeed, is it possible for the air to penetrate into the 

 volcanic furnaces when there exists a pressure acting from 

 within towards the extei'ior, capable of raising liquid, lava, a 

 body three times as heavy as water, to the heig-ht of more than 

 1000 metres, as at Vesuvius, or even of more than 3000, as is 

 the case in a great number of volcanos? A pressure of 1000 

 metres of lava, equivalent to a pressure of 3000 metres of water, 

 or to that of about three hundred atmospheres, necessarily ex- 

 cludes the introduction of any air whatever into volcanos; and 

 as this pi'essure subsists for a long series of years, during which 

 the volcanic phaenomena continue in the utmost activity, it fol- 

 lows that the air can have no share whatever in their production. 



It is moreover evident, that if the air had a free communi- 

 cation with the volcanic furnaces, the ascent of lava, and earth- 

 quakes, would be impossible. 



If the air cannot be the cause of volcanic phaenomena, it is 

 probable, on the contrary, that water is a very miportant agent 

 in them. 



It can hardly be doubted that water does penetrate into 

 volcanic furnaces. A great eruption is invariably followed by 

 the escape of an enormous quantity of aqueous vapour, which, 

 being condensed by the cold which prevails above the sum- 

 mits of volcanos, falls again in abundant rains accompanied 

 by terrific thunder, as was the case at the famous eruption of 

 Vesuvius in 1794, which destroyed Torre del Greco. Aqueous 

 vapours and hydrochloric gas have also frequently been ob- 

 served 



