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 trnie 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.f 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.J 

 If the store of lava in the volcanic focus should at first become 

 exhausted by repeated discharges, the volcano is entirely reduced 



* We may here notice the well-known phenomenon, that among the ejected 

 masses from a volcano, pieces of rock occur, which neither belong to the substances 

 composing the edge of the volcanic cone, nor to those found in the vicinity, and 

 therefore must be derived from masses concealed very deep under the volcano. 

 Vesuvius particularly, furnishes remarkable instances of this kind. Such ejected 

 masses, however, are now found much more rarely than formerly on this or other 

 volcanos, 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. 



X Experiments hitherto made shew, that long spaces of time are requisite to pro- 

 duce the strongest effects, viz., the elevation of lava to the greatest height. Von 

 Humboldt (Reise, &c., t. i, p. 261,) calls our attention to the circumstance, that 

 lono- intervals of quiescence seem to characterize the very high volcano. The 

 smallest of all, StromhoU, is nearly always in activity. The eruptions of Vesuvius 

 are less frequent, although they are still more so than those of Etna and the Peak 

 of Teneriffc. During the quiescence of the latter, from 1706 to 1798, sixteen erup- 

 tions of the former took place. From the colossal summits of the Andes, Cotopaxi 

 and Timcrurahua, an eruption is observed scarcely once in a century. We may 

 venture to state, that the frequency of the eruptions of active volcanos is inversely 

 as their height and mass. After these general remarks, we may mention the cir- 

 cumstance that large lava streams, namely, such as issue from Etna and Vesuvius, 

 never flow from the crater itself, and that the quantity of the melted matters is 

 commonly inversely as the height of the fissures from which the lava issues. But 

 a lateral eruption of these two last-mentioned volcanos always terminates by an 

 emission of the ashes from the crater, that is, from the summit of the mountain 

 itself This phenomenon has not been seen on the Peak of Teneriffe these last 

 hundred years. The crater was most inactive during the eruption in the year 

 1798. Its basis did not sink, whilst the greater or less depth of the crater of Vesu- 

 vius, according to the acute remark of Von Buch, is an almost infallible sign of an 

 impending fresh eruption. Von Humboldt, p. 268. All this shews that the condi- 

 tions requisite for producing the greatest effect, viz., for producing the highest de- 

 gree of the increased elasticity of the watery vapors, are not always present. 



