42 Prof. Bischof oil the Natural History of 



that water and steam cannot be forced through narraw open- 

 ings in the red-hot generator of a steam engine, is apph'cable to 

 the gigantic generator, which formed the volcanic focus ; this 

 might be added to the causes already mentioned, which afford 

 resistance in the channels through which the waters are ad- 

 mitted. 



So long as the communication with the sea remained open, 

 the volcano could never come to a state of rest, although the 

 formation, or much more the access of new lava from remote 

 places, might require a long period before actual volcanic erup- 

 tions could again take place. Of Vesuvius we know that tlie 

 periods, when it is entirely free from evolutions of aqueous va- 

 pour, are not of long duration. On Lancer ate some of the 

 cones, which were erupted eighty years ago, still continue to emit 

 steam. The cones of Jondlo emitted boiling hot vapours, and 

 boiling springs rose in the neighbourhood at the time when 

 von Humboldt visited them, that is forty-four years after the 

 last eruption. Burkart, on visiting Jondlo twenty-four years 

 afterwards, saw scarcely any evolution of watery vapour from 

 these cones ; but vapour of the temperature of between 113" 

 and 129" F. was still rising from fissures in the neighbour- 

 hood of the principal crater.* Very hot vapour 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 erup- 

 tions 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 

 internal heat, remained separate ; but that when they were 

 themselves very hot, or traversed by the hot vapours, they 

 united so firmly together, that they could only be separated by 



* Aufenthalt and Reisen in Mexico in den Jahren, 1825, bis 1834 vou 

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

 t Hoffmann, loco cit. p. C9. 



