Natural History of Volcanos and Earthquakes. 247 



this, what, effect, it may be asked, may not steam of such extra- 

 ordinary elasticity, and of a temperature equal or even greater 

 than the melting point of lava, exert upon fusible rocks, solidified 

 masses of lava, &c., which it meets with far above the volcanic 

 focus in colder regions ? Would not such steam convert the rock 

 into melted liquid matter ? It is, indeed, difficult to conceive a 

 state of which even Papin's digestor can give us but a slight idea. 



If the channels become obstructed after a considerable quantity 

 of water has found its way to the volcanic focus, the aqueous 

 vapor may attain its maximum elasticity, as the focus will act 

 like a steam-boiler closed on all sides, that is to say, it will be 

 able, according to the above calculations, to raise a column of lava 

 of 88747 feet. 



The filtration of a large quantity of water, which, although it 

 becomes gradually heated as it descends, is prevented by its velo- 

 city from assuming the temperature of the strata through which 

 it passes, must tend to cool the volcanic masses. But it will be 

 cooled to a far greater extent by the considerable formation of 

 steam. In this manner a gradual solidification of the lava will 

 take place not only in the crater, but also in the great volcanic 

 focus itself,* whereby the termination of the volcanic eruptions 

 is produced.! The contraction of the walls of the volcanic focus 

 during the reduction of their temperature causes fissures in the 

 rocksjj by which the waters are admitted in other places. But 

 in doing so, it may frequently happen that these fissures do not 

 communicate with the channels by which the water is admitted, 

 and that the volcanic action is consequently for a time suspended, 

 but that on its revival the slightest shock is sufficient to break 



* Necker, Memoires de la Societe de Physique et d'Histoire Naturelle de Ge- 

 neve- Geneve, t. ii, part i, p. 155. 



t Observations made on Vesuvius and the Peak of Tencriffe shew, that the 

 greater part of the ashes is thrown out last, so that their appearance may be con- 

 sidered as a sign of the approaching termination of the eruption. In proportion as 

 the elasticity of the vapors diminishes, the substances will be thrown to a less dis- 

 tance, so that the black rapilli, which are the first ejected after the lava has ceased 

 to flow, will be cast farther than the white ones. Von Humboldt's Reise, t. i, 

 p. 245. 



t It is well known that considerable fissures are formed in lava during its cool- 

 ing, especially when it is on the surface of the earth. The streams of lava in the 

 country surrounding the Laacher-See, offer many instances of this kind. Hamilton 

 also mentions great fissures in the lava-streams of Vesuvius. Gilbert's Annal. i. vi, 

 p. 2'S. See also Necker, loco cit. 



