Natural History of Volcanos and Earthquakes. 47 
this, what effect, it may be asked, may not steam of such extra- 
ordinary elasticity, and of a temperature eqnal or even greater 
than the melting point of lava, exert upon fusible rocks, solidified 
masses of lava, &c., which it meets with far above the volcanic 
focus in colder regions? Would not such steam convert the rock 
into melted liquid matter? It is, indeed, difficult to conceive a 
state of which even Papin’s digestor can give us but a slight idea, 
If the channels become obstructed after a considerable quantity 
of water has found its way to the volcanic focus, the aqueous 
Vapor may attain its maximum elasticity, as. the focus will act 
like a steam-boiler closed on all sides, that is to say, it will be 
able, according to the above calculations, to raise a column of lava 
of 88747 feet. 
The filtration of a large quantity of water, which, although it 
| becomes gradually heated as it descends, is prevented by its velo- 
city from assuming the temperature of the strata through which. 
it passes, must tend to cool the volcanic masses. But it will be 
Cooled to a far greater extent by the considerable formation of 
steam. In this manner a gradual solidification of the lava will 
take place not only in the crater, but also in the great volcanic 
focus itself,* whereby the termination of the volcanic eruptions 
18 produced.t The contraction of the walls of the voleanic focus 
during the reduction of their temperature causes fissures in the 
tocks,t by which the waters are admitted in other places. But 
in doing so, it may frequently happen that these fissures do not 
Communicate with the channels by which the water is admitted, 
and that the volcanic action is consequently for a time suspended, 
that on its revival the slightest shock is sufficient to break 
. Necker, Memoires de la Société de Physique et d'Histoire Naturelle de Ge- 
neve. Genéve, t, ii, parti, p. 155. 
t Observations made on Vesuvius and the Peak of Tencriffe shew, that the 
Bteater Part of the ashes is thrown out last, so that their appearance may be con- 
ed as a sign of the approaching termination of the eruption. In prepornce = 
se elasticity of the vapors diminishes, the substances will be thrown to a Jess dis- 
tance, sé that the black rapilli, which are the first ejected after the lava has ceased 
sy will be cast farther than the white ones. Von Humboldt’s Reise, t. i, 
_ tItis well known that considerable fissures are formed in lava during its cool- 
™B; especially when it is on the surface of the earth. The streams of lava in the 
wy surrounding the Laacher-See, offer many instances of this kind. Hamilton 
: also mentions great fissures in the lava-streams of Vesuvius. Gilbert’s Annal. t. vi, 
PBL Bee also Necker, loco cit. 
