246 Natural History of Volcanos and Earthquakes. 



tion, or much more the access of new lava from remote places, 

 might require a long period before actual volcanic eruptions could 

 again take place. Of Vesuvius we know that the periods, when 

 it is entirely free from evolutions of aqueous vapor, are not of 

 long duration. On Lancerote some of the cones, which were 

 erupted eighty years ago, still continue to emit steam. The 

 cones of Jorullo emitted boiling hot vapors, and boiling springs 

 rose in the neighborhood at the time when Von Humboldt visited 

 them, that is, forty-four years after the last eruption. Burkart, 

 on visiting Jorullo twenty-four years afterwards, saw scarcely 

 any evolution of watery vapor from these cones ; but vapor of 

 the temperature of between 113° and 129° F. v/as still rising 

 from fissures in the neighborhood of the principal crater.* Yery 

 hot vapor continues to, the present day to issue in all directions 

 from the sides of the rocks on Pantellaria, and yet there seem to 

 have been no eruptions on this island since the commencement 

 of the historical era.f 



But it is very probable that the channels by which the water 

 enters become obstructed from time to time. This may be ef- 

 fected by the lava itself, which is the more likely, as the chan- 

 nels may perhaps be very narrow. It may, however, also be 

 caused by the hot steam. Indeed, Monticelli and Covelli ob- 

 served, during the eruption of Vesuvius in October of 1821, that 

 the fragments of lava, when no longer possessed of a great inter- 

 nal heat, remained separate ; but that when they were them- 

 selves very hot, or traversed by the hot vapors, they united so 

 firmly together, that they could be separated only by heavy 

 blows with a hammer on the tenacious surface.| If the aqueous 

 vapors of ordinary elasticity and temperature are able to effect 



* Aufenthalt and Reisen in Mexico in den Jahren, 1825, bis 1834 Von Burkart. 

 Stuttgart, 1836, t. i, p. 227 and 228. 



t Hoffmann, loco cit. p. 69. 



t Loco cit. p. 10. It may perhaps be allowed here to mention an observation of 

 my own, though on a somewhat limited scale. I found that the stones by which 

 the KaisersqueUe at Mx la Chapelle is closed, and that the canals of the Schicerdtbad 

 at Bwrtec/ieirf, which consist of black marble, were converted 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 decomposi- 

 tions and alterations which rocks suffer when exposed to the continued effects of 

 heat and acid watery vapors. See among others Krug von Nidda, p. 274. Burkart, 

 loco cit. t. i, p. 194. 



