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 
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 Jorudlo 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. was still rising 
from fissures in the neighborhood of the principal crater.* Very 
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.t 
Bat it is very probable that the channels by which the watet 
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 - 
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 $0 
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 
onto mt ae 
* 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. ill 
t Loco cit. p. 10. It may perhaps be allowed here to mention an aboer etn 
my own, though on a somewhat limited scale. I found that the stones by whic 
the Kaisersquelle at Aix la Chapelle is closed, and that the canals of the Schwert 
at Burtscheid, which consist of black snitthle. sete converted on the inser side into 
a doughy mass by the continued action of the steam. But the Kea ce 
suesie of 
274. Burkatt 
heat and acid watery vapors. See among others Krug von Nidda, p- 
loco cit. t. i, p. 194. 
