This is the
magnificent phenomenon which Pliny the younger, in his celebrated letter to
Cornelius Tacitus, compares, in the case of Vesuvius, to the form of a lofty
and thickly-branched and foliaceous pine. That which is described as flames
in the eruption of scoriae, and the radiance of the glowing red clouds that
hover over the crater, can not be ascribed to the effect of hydrogen gas in
a state of combustion. They are rather reflections of light which issue
from molten masses, projected high in the air, and also reflections from the
burning depths, whence the glowing vapors ascend. We will not, however,
attempt to decide the nature of the flames, which are occasionally seen now,
as in the time of Strabo, to rise from the deep sea during the activity of
littoral volcanoes, or shortly before the elevation of a volcanic island.
When the questions are asked, what is it that burns in the volcano? what
excites the heat, fuses together earths and metals, and imparts to lava
currents of thick layers a degree of heat that lasts for many years? it is
necessarily implied that volcanoes must be connected with the existence of
substances capable of maintaining combustion, like the beds of coal in
subterranean fires.
[footnote] *See the beautiful experiments on the cooling of masses of rock,
in Bischof's 'Warmelehre', s.
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