248 Natural History of Volcanos and Earthquakes. 
through the walls, and thus to reopen the communication.* This 
may even be caused by the expansion of the cooled walls of the 
focus by the heat communicated to them from all sides; in the 
same manner as a small crack in a crucible increases when expo- 
sed to a red heat. 
The more the temperature of the lava is reduced by the water 
and the generation of the steam, the longer will be the time re- 
quired for the refusion of the solidified lava. In this manner a 
long period may elapse, as the lava is so very bad a conductor of 
heat.t The repose and activity of a volcano are, therefore, the 
alternate solidification and liquefaction of the lava, and the inter- 
ruption and renewal of the supply of water to the volcanic focus.f 
If. the store of lava in the volcanic focus should at first become 
exhausted by repeated discharges, the volcano is entirely reduced 
may here notice the well-known phenomenon, that among the ejected 
masses ies a voleano, pieces of rock occur, which neither belong to the pssst 
ani 
Vesuvius particularly, furnishes remarkable instances of this kind. Suc 
es, however, are. now found much more rarely than formerly on this or 
Saag from which it seems to follow, that the channels of the ejections have 
been by degrees widened. 
t Monticelli and Covelli, loco cit. p. 15 and 39. 
} Experiments hitherto made shew, that long spaces of time are oem to pro- 
duce the strongest effects, viz., the elevation of lava to the greatest height. Von 
Humboldt (Reise, &c., t. i, "961, calls our attention to the circumstance, 
long intervals of quiescence seem to characterize the very high volcano. 
smallest of all, Stromboli, is nearly always in activity. The eruptions of 
are less frequent, although they are still more so than those of Etna an 
Teneriffe. During the quiescence of the latter, from 1706 to 1798, ‘sixteen erup- 
tions of the former “in place. From the colossal summits of the @ndes, Cotopart 
and Tungurahua, an eruption is observed scarcely once in a century- We a 8 
venture to state, that the frequency of the eruptions of active voleanos is inversely | 
as their height and mass. After these general remarks, we may mention the cit 
cumstance that large lava streams, namely, such as issue from Etna a and 
never flow from the crater itself. and that the quantity of the melted a 
commonly inversely as the height of the fissures from which the lava issues- 
a ee map: of these two last- mentioned volcanos always pry tes — 
itself This phenomenon has not bees seen on the Peak of Teneriffe 
hundred years. The crater was most inactive during the eruption in pi 
1798. Its basis did not sink, whilst the greater or less depth of the crater of 
tions requisite for producing the greatest effect, viz., for producing the highest. 
gree of the increased elasticity of the watery vapors, are not always preset 
