in, the whole current
resembled nothing so much as a heap of unconnected cinders from an
iron-foundry." In another place he says that "the rivers of lava in the
plain resembled a vast heap of cinders, or the scoriae of an
iron-foundry, rolling slowly along, and falling with a rattling noise
over one another."[523] Von Buch, who was in company with MM. de
Humboldt and Gay-Lussac, describes the lava of 1805 (the most fluid on
record) as shooting suddenly before their eyes from top to bottom of the
cone in one single instant. Professor J. D. Forbes remarks that the
length of the slope of the cone proper being about 1300 feet, this
motion must correspond to a velocity of many hundred feet in a few
seconds, without interpreting Von Buch's expression literally. The same
lava, when it reached the level road at Torre del Greco, moved at the
rate of only eighteen inches per minute, or three-tenths of an inch per
second.[524] "Although common lava," observes Professor Forbes, "is
nearly as liquid as melted iron, when it issues from the orifice of the
crater, its fluidity rapidly diminishes, and as it becomes more and more
burdened by the consolidated slag through which it has to force its way,
its velocity of motion diminishes in an almost inconceivable degree; and
at length, when it ceases to present the slightest external trace of
fluidity, its movement can only be ascertained by careful and repeated
observations, just as in the case of a glacier."[525]

It appears that the intensity of the light and heat of the lava varies
considerably at different periods of the same eruption, as in that of
Vesuvius in 1819 and 1820, when Sir H. Davy remarked different degrees
of vividness in the white heat at the point where the lava
originated.[526]

When the expressions "flame" and "smoke" are used in describing volcanic
appearances, they must generally be understood in a figurative sense. We
are informed, indeed, by M. Abich, that he distinctly saw, in the
eruption of Vesuvius in 1834, the flame of burning hydrogen;[527] but
what is usually mistaken for flame consists of vapor or scoriae, and
impalpable dust illuminated by that vivid light which is emitted from
the crater below, where the lava is said to glow with the splendor of
the sun. The clouds of apparent smoke are formed either of aqueous and
other vapor, or of finely comminuted scoriae.

_Dikes in the recent cone, how formed._--The inclined strata before
mentioned which dip out