The greater part of the lava issued from the base of the great fissure
in the cone which I have described; and although two other lava streams
descended from the top of the mountain, neither proceeded from the
crater, but from apertures near it. The great crater, divided in two as
already described, opened wide on the morning of the 26th April,
destroying the brim of the antecedent crater, and remaking it in another
shape with ejected matter, except on the south-west side, where the brim
was split. (See Plate 5.)
From this double crater, copious smoke, bombs and incandescent scoriæ,
with ashes and lapilli, issued with violence, and from the depths below
came dreadful detonations and bellowings, producing great terror. And
yet the lava poured out into the Atria del Cavallo without any noise,
and not even a column of smoke marked its origin of issue--namely, from
the fissure.
When the eruption was over, the sight of the vertical walls of these
deep craters, of almost horizontal strata of scoriæ and lithoidal
masses, with a fracture fresh, and as if they had never undergone the
action of fire or of acid vapours, without recent scoriæ and without
fumaroles, was to me a marvellous spectacle. The fumaroles were almost
all on the brims of the craters, with emanations of hydrochloric and
sulphurous acid. In a few that were more removed from the brim,
sulphuretted hydrogen was perceptible. In the sublimations, chloride of
iron was most abundant, in combination with other chlorides, for
example, of sodium, magnesium and calcium. This last chloride was
frequent even among the sublimations of the fumaroles of the lavas, and
it was the first time it was ever remarked, but I do not think it was
the first time that it was ever produced: being in combination with
chloride of iron, and very deliquescent, it did not attract attention
from anyone. In a hollow fragment of scoriæ I observed a yellowish
substance, which looked like sulphur in a viscid state, and which boiled
at a temperature of 120°, and evolved hydrochloric acid. Having
collected this substance and poured it into a glass phial, it quickly
coagulated into an amorphous mass of the same colour; but before I
reached the Observatory, I found that it had become liquid by
deliquescence. It consisted of a mixture of the aforesaid chlorides,
according to an analysis made by Professor Silvestro Zinno and myself.
In some fumaroles, where I perceived the smell of sulphuretted hydrogen,
I found sublimed sulphur under the scoriæ.
At the source of the lava stream that flowed towards the Camaldoli, on
the seaward flank of Vesuvius, I observed large fumaroles of steam only,
pure aqueous vapour.
There was no trace of carbonic acid in these fumaroles, but that fact
does not imply that there was none at a later period, for, since the
first investigations of Deville, it is known that carbonic acid is found
under certain conditions on the very summit of Vesuvius.