The bombs ejected from the craters are like those carried down by the
lavas, but of smaller size, and they seldomer contain a nucleus similar
to those found in the latter. With the bombs properly so called, many
pieces of incandescent lava were thrown up, and in their fall went
beyond the base of the cone. A quantity of small scoriæ varying in size
accompanied these projectiles, and those fragments, which we call
_lapilli_, fell at a greater distance. With the lapilli, and sometimes
without them, the smoke carried a very minute dust or sand, which is
generally called ashes. These ashes, when washed with water, lose
soluble constituents which they have collected in the smoke--such as
chloride of sodium and other chlorides and often free acids. The
insoluble part originates in the detritus of lava, and with the
microscope we can detect abundant fragments of those crystals which most
frequently occur in the lava of the same eruption.
The lavas of 1871, which were eminently leucitic, and almost entirely
deprived of pyroxene, resembled the ashes, which appeared to be
fragments of crystals of leucite, more or less enveloped in the paste of
the lava, so that having triturated the scoriæ of the lava, and looked
at the powder through the microscope, it was apparently quite the same
as the ashes.
But at the beginning of the eruption of the 26th April, a white sand
fell in the Atria del Cavallo, close to the Crocella[5], which on the
dark scoriæ of 1871 looked like snow. Its fall had a limit so well
defined that one passed without any gradation from white to black.
Having collected some of this sand that very morning, I put it up in
white paper, for at that moment it was impossible for me to examine it.
Taking it out some days after, I found it had become reddish, and having
put it under the microscope, I observed that it was exclusively formed
of little pebbles more or less round, of a transparent vitreous matter,
partly covered with a red substance. Fragments of green crystals
occurred in this sand, upon which no red was perceptible. I consulted
our eminent crystallographer, Arcangelo Scacchi, whether these little
pebbles were leucite, as I suspected, and whether the green particles
were pyroxene: he confirmed my suspicion, and remarked that the red
colour was superficial only. We then washed a little of the sand in hot
water, and saw the pebbles become whitish; but having heated some on
platinum, we observed that they first turned black and then became
perfectly white, proving that the red was a deposit of organic matter.
To see these leucites, rounded like small pebbles transported by a
torrent, deprived of the soluble chlorides which generally accompany
Vesuvian ashes, is a matter worthy of attention. Whilst heating this
sand upon platinum, decrepitation was audible, which indicated the
cracking of some of the little pebbles. It is evident, therefore, that
crystals of leucite raised to a certain temperature may break, and thus
we can understand how almost all Vesuvian ashes contain fragments of
the said crystals enveloped in the paste of the lava. It is evident that
the soluble part of the ashes is obtained from the smoke through which
it passes. On this occasion the smoke from the craters did not
apparently contain much acids, for no bad smell was perceptible, and the
water in which I washed the ashes scarcely reddened litmus paper. Even
chloride of iron, which was so abundant in the lavas, was scarcely
perceptible in the smoke, which almost exclusively deposited sea-salt on
the surrounding rocks; I say sea-salt advisedly, and not chloride of
sodium, to show that I include all that sea-salt contains. The slight
disturbance it manifested with chloride of barium, and the small
precipitate with oxalate of ammonia, reveal sulphate of lime, without
excluding the possibility of the chloride.
But how can these ashes do so much injury to the vegetation of the
ground they cover, especially at the first fall of rain? I think that
the damage is due partly to the sea-salt, and partly to the acids
contained either in the ashes or in the rain-water itself. Upon watering
the tender tops of some plants with a saturated solution of the salt
from Vesuvius itself, I noticed that they withered away after a few
hours. But very often the rain alone which traverses the smoke of
Vesuvius, or is produced by condensation from it, gives manifest acid
reactions, and destroys the grass and the tops of the trees. The
peasants believe that the rain is warm or of boiling water, from
observing that the tender parts of the plants are, by its deposit, all
burnt up. Vegetation is now recovering, but without flowers, and
consequently without fruit.