Vulcanos and Earthquakes. 48 



heavy blows with a hammer on the tenacious surface. * If the 

 aqueous vapours of ordinary elasticity and temperature are 

 able to effect this, what effect, it may be asked, may not steam 

 of such extraordinary 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, in- 

 deed, difficult to conceive a state of which even Papin's diges- 

 tor can give us but a slight idea. 



If the channels become obstructed after a considerable quan- 

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

 aqueous vapour 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 

 velocity 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 forma- 

 tion 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,f 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 



* Loco cit. p. 10. It may perhaps be allowed here to mention an obser- 

 vation of my own, though on a somewhat limited scale. I foimd that the 

 stones by which the Kaisersquelle at Aix l<i Chapelle is closed, and that the ca- 

 nals of the Scliwerdtbad at Btu'tscheid, whicli consist of black marble, were con- 

 vei'ted on the inner side into a doughy mass by the continued action of the 

 steam. But the temperature of this steam is only 133° to 167° F. There 

 occur innumerable instances of decompositions and alterations which rocks 

 suffer when exposed to the continued eiFects of heat and acid watery va- 

 pours. Sec among others Krug von Kidda, p. 274. Burkart, loco cit. t. i. 

 p. 194. 



t Necker, Memoires de la Societe de Physique et d'Histoire Naturello 

 de Geneve. Geneve, t. ii. part i. p. 155. 



Z Observations made on Vesurius and the PeaJc of Teneriffe shew, that the 

 greater part of the ashes is thrown out last, so that their appearance may bt* 

 considered as a sign of the approaching termination of the eruption. In pro- 



