Smoke generally issues from all lava when it cools down to a certain
degree, hence it is more abundant at the edges of the fiery torrent, or
is liberated from the scoriæ that form on its surface. But when the lava
stops, the smoke issues only from certain vent-holes, through which we
can still see the fire, and at the edge of which different amorphous or
crystallized matters collect by sublimation. These centres of heat, of
more or less duration, are the fumaroles of the lavas. I believe I have
on other occasions shown that a fumarole is nothing but a communication
between the more or less cooled and hardened surface of the lava and the
interior, which is still incandescent. Some fumaroles last but a day,
others preserve their activity for weeks, months or years, according to
the depth of lava through which they penetrate; and when they cease to
be active, that is, when the sublimations are formed, or smoke or other
æriform matters issue from them, they still retain a rather elevated
temperature. In the lavas of 1858, in a place where they had a
transverse width of 150 metres, a vent-hole may still be found where the
thermometer registers 60° and the scoriæ are warm. Sometimes, while the
lava is in process of cooling, new fumaroles appear, in which the fire
is visible. This phenomenon, which appeared marvellous and inexplicable
when I first observed it in 1855, is now very easily understood; the
cooled and hardened crust of the lava fractures with noise and suddenly,
and so a new communication is opened with the incandescent lava below,
thus creating a new fumarole.
As the smoke of the fluid lava is perfectly neutral, that is, neither
acid nor alkaline, so the fumaroles at the first period of their
existence with sublimations of sea-salt, mixed frequently with oxide of
copper either in black powder or in shining laminæ, ought also to be
neutral. But if the fumarole continues active, hydrochloric acid issues
with the smoke, and often some time after sulphuric acid. Then the
sublimations turn first yellow, then green, and more rarely azure. The
chemical reactions show that these sublimations are chlorides or
sulpho-chlorides, and sometimes sulphides, and they afford reactions,
indicative of soda, magnesia, copper, lead, and traces of other
substances, not excluding ammonia, which I must speak of separately.
This, I have observed, is the general law with the fumaroles of the
tranquil lavas, which occur with long and moderate eruptions--for
instance, the lavas of 1871, and even those of 1872, preceding the 26th
April.
But in the great lavas of the great conflagrations of Vesuvius, chloride
of iron more or less in combination with all the other substances above
mentioned changes the appearance of the sublimations. The fumaroles in
the lava of the 26th April frequently indicated chloride of iron.
Sulphuretted hydrogen, by reaction of sulphurous acid, is decomposed,
and sulphur sublimed, having a particular aspect, collects on the
scoriæ. This is never found but in fumaroles of the smaller lavas; it
was therefore absent in those of 1871, but frequently occurred in those
of 1872.
Although the sublimations are generally mixtures, yet sometimes distinct
and crystallized chemical or mineral species are found, such as sulphur,
sal ammoniac, _tenorite_, _cotunuite_, etc. Micaceous peroxide of iron
(feroligiste), so common near eruptive cones, is very scarce on lava;
any found in it has been carried down from the craters, and proofs of
this transport are very abundant and striking in the lavas of this last
eruption. Even the iron found in the bombs is evidently transported;
there is a fumarole on the ridge of the lava in the Fossa di Faraone
which contains micaceous peroxide of iron, and this, at first sight,
appears to oppose what I have affirmed; nevertheless, it gives
additional force to my statement. This fumarole is only a bomb or
rounded mass of enormous size, four or five metres in diameter. Smoke
and hydrochloric acid issued from the aperture in its envelope, and
being partly broken it was seen to contain lapilli and pieces of
antecedent lava, covered with micaceous peroxide of iron. The internal
temperature of this mass was very high; the hydrochloric acid which it
discharged had, in some places, covered the micaceous iron with a yellow
coating of chloride of iron. From small apertures, on the lower side of
the mass, white and green stalactites of chloride of calcium were
visible. In one spot only of lava I found a fumarole, with a small
quantity of micaceous peroxide of iron, evidently in a state of
formation; but this was the very spot where the lava became eruptive,
and whence issued the column of smoke which was so well
photographed--the place under the hill of Apicella. (See Plate 4a.)
I have enumerated the products which are constantly collected in
fumaroles, although they are not all found at the same time or place, in
order to show that the sublimations follow a certain law in their
appearance. _Tenorite_, for instance, was formerly considered an
accidental product of certain eruptions, and I have always found it; but
if you visit the fumarole when the acids have had time to transform it,
you will no longer see it. I found the crystallized chloride of lead, or
"cotunuite," as it is called, for the first time in the lavas of 1855,
and thought it a singular circumstance; but from that time I recognised
it in all the lavas, though not always so beautiful and abundant; and
even when not found as a distinct substance, I observed it in
combination with chloride of copper. In the lavas of the 26th April
_cotunuite_ and _tenorite_[E] were not very abundant, because the
chloride of iron disturbed the greater number of the sublimations. I
found sal ammoniac very abundantly on the fumaroles of the lavas that
invaded the cultivated ground. Although chloride of ammonia, contrary to
opinion, was not wanting in the sublimations of the fumaroles of the
lavas deposited on other lavas, yet it was neither abundant nor
crystallized, but combined in small quantities with other substances. It
appeared in great abundance in all the fumaroles of lavas which covered
cultivated or woody ground. At first it was scarce enough, and mixed
with chloride of sodium; but when the rains came the sea-salt was washed
away, and sal ammoniac formed beautiful crystals, nearly free from
adventitious matters, as was the case with the fumaroles of the last
lava. Afterwards, when chloride of iron was produced, ferro-chloride of
ammonia was found. Crystals of sal ammoniac were sometimes found of a
beautiful amber yellow. This colour was, in the opinion of my colleague,
Professor Scacchi, produced by such small traces of chloride of iron
that neither Professor Guiscardi nor I, nor indeed any other chemists to
whom I submitted specimens for examination, could detect any. What I can
affirm with certainty is, that these limpid crystals of a yellow colour
were almost always attached to an amorphous substance, soluble in water,
composed of various chlorides, in which iron was often detected.
From these remarks, it is evident that in the tranquil lavas the
sublimations appear with a certain order of succession, and in the
violent lavas, and those which flow most copiously, they are more
complicated, and render both chemical analysis and spectroscopic
researches more difficult. Notwithstanding, I observed traces of lithium
and thallium, which I had previously perceived in some sublimations of
1871. I purpose submitting many sublimations which I have collected to
more complete spectroscopic investigation, although I am persuaded that
the discovery of traces of certain bodies in the sublimations or in the
lavas is a matter of small importance to the science of volcanoes. I
must say, however, that calcium was discovered on this occasion in great
abundance, not only by the spectroscope, but also by chemical analysis.
Sulphate of lime has often been found in larger or smaller proportions,
but this was the first time I had observed chloride of calcium both
close to the craters, and also in the sublimations of the fumaroles upon
the lavas. The white stalactites which I collected beneath the great
mass or bomb above described were almost exclusively composed of
chloride of calcium, and only a few green drops manifested, with the
usual re-agents, the presence of iron.
I did not fail to look often at the spectrum of the flowing lavas
covered with the smoke which issued from them, but I always had a
continuous spectrum. The spectroscope employed was Hoffmann's
construction, with direct vision; but I think it would be better on
other occasions to use a spectroscope combined with a telescope, like
those used by astronomers.
But avoiding minute particulars of these sublimates, let us see what is
the general direction and the order of their appearance. Sublimations
are generally oxides, chlorides and sulphates, sometimes sulphides.
Among the oxides, we must enumerate in the first place "tenorite" and
_feroligiste_ or micaceous peroxide of iron. The first is almost always
found at the commencement of activity in the fumaroles, simultaneously
with the sublimation of chloride of sodium; the second--which is,
perhaps, never wanting in eruptive cones that are often found lined with
it inside--is seldom generated in the fumaroles of the lava, and
therefore it is not easy to define the moment of its appearance.
Sometimes one collects micaceous peroxide of iron on the lava, but it is
often transported there from the mouths of eruption, as happened on this
occasion.
Trustworthy writers are of opinion that all the oxides are derived from
the decomposition of the chlorides, but I think I have clearly
demonstrated that, with regard to copper and lead, the opposite
statement may be affirmed; for the oxides are changed into chlorides,
and hydrochloric acid liberated. Oxide of copper forms sublimates at the
beginning, at the same time as the sea-salt; and if the fumarole be
anhydrous or, as Deville would say, _dry_, this oxide does not change
into either a chloride or a sulphate; but if the fumarole gives watery
vapour, after a little hydrochloric acid is formed, which changes the
oxide into a chloride, and if whilst this is going on oxide of lead be
developed, it is changed into the chloride of lead, so frequently found
in combination with chloride of copper. Then the sublimations change
from white to red or yellow, and specimens when carried away gradually
turn light blue, but when heated on platinum over a spirit lamp they
resume their yellow tint. Sometimes the yellow colour remains longer,
and in time changes to green; this also happens on the fumarole itself,
the green commencing at the zones furthest removed from the centre,
where the temperature is highest. When these sublimations are greenish,
they become far less soluble than at first. The yellow, so common at a
certain period on the fumaroles of the tranquil lavas, never attracted
attention before I first examined it, doubtless, because it was
considered chloride of iron, and yet in small eruptions this is only
found close to the discharging mouths, and never in the sublimations of
the fumaroles of the lava; but, on the other hand, it is the most
copious and common product on the lavas of the great eruptions. This
probably also accounts for the fact that lead, which is so obvious in
the fumaroles of the lavas, had never previously been observed. In 1855,
I noticed the crystallized chloride of lead in a fumarole in the Fossa
della Vetrana, and this induced me always to look for it on the
fumaroles of the later lavas; and I ascertained that, if it did not
always appear as a distinct mineral, it was easily discovered in
combination with other chlorides. The specimens which I have collected
are not the most beautiful, but the presence of lead in the sublimations
is not less common.
Micaceous peroxide of iron, when found on the lava, has been mostly
conveyed from the eruptive mouths, as I have already stated, and perhaps
never so abundantly and evidently as on this occasion. The lava of the
26th of April carried along a large quantity of round masses or bombs,
varying in size, among which were found antecedent lava more or less
covered with micaceous iron, either collected in the cavities of the
lava, or incorporated with its mass. Sometimes the micaceous iron
appears like little veins in the paste of new lava enveloping the
exterior of these rounded masses, an exterior compact and lithoidal, and
not resembling scoriæ. Among these spherical masses I found one of
enormous size, four to five metres in diameter, which, having broken up
where the exterior envelope was thinnest, I found filled with a great
mass of lapilli and fragments of other lavas covered with micaceous
iron. This bomb still preserves (June 5th) an elevated temperature
within, and emits smoke and hydrochloric acid, which, meeting the
micaceous iron discovered by breaking the envelope with blows of a
hammer, transforms it superficially into chloride of iron, showing most
clearly how, on some occasions at least, chloride of iron is formed from
the oxide which precedes it. That those lapilli and the pieces of lava
were solid when enveloped in the paste of the new lava, we infer from
seeing the impressions on the inside of the said envelope. The chloride
of calcium, which I found in this spherical mass almost pure, caused me
to suspect that the sulphate of lime which is so often found on Vesuvius
is a transformation of the chloride produced by the contact of
sulphurous acid, which easily becomes transformed into sulphuric acid.
The hydrochloric acid which escapes from a fumarole coming into contact
with the scoriæ near its mouth, produces chloride of iron, which is,
therefore, not always obtained by sublimation, although, when the
temperature is very high, chloride of iron is conveyed from the interior
of the lava, and sublimes on the exterior and colder parts; for
instance, the chloride of iron which issues from the eruptive cones is
sometimes found sublimed on the rocks of Monte di Somma. When chloride
of iron has been produced by sublimation, we may collect it inside a
glass bell placed over the fumarole, or upon a piece of brick; but when
it is produced by the action of hydrochloric acid on the scoriæ, it will
only be found on the scoriæ themselves.
If, therefore, the origin of micaceous peroxide of iron were due to the
decomposition of the sesqui-chloride of iron requiring a more elevated
temperature for its decomposition, it would follow that its genesis
would be easier near the discharging mouths, and more difficult on the
lavas, but there the fact was verified: for example, in the great bomb
on the fumarole, where we observed micaceous iron transformed into
chloride of iron. We may therefore consider it _proved_ that some
chlorides--for instance, chloride of sodium--issue from the lava itself,
either being there pre-existent, or being formed there; and that others
are derived from the oxides which precede them, as undoubtedly is the
case with chloride of copper; hence, the theory that derives the oxides
always from the chlorides cannot be considered true. Granting that this
theory might be applicable to the origin of micaceous iron, we should
still want to know how it is found with the paste of the new lava
itself, which forms the exterior coating of the bombs above described.
Many of these rounded masses, which have been rolled along by the lava,
contain scoriæ partly decomposed by the long action of the acids found
on the fumaroles of the craters. They disintegrate easily, and have a
more or less yellowish tint. In the greater number of cases the interior
of these masses is formed of leucitic lava, with cavities lined with
micaceous iron. In short, their contents appeared to me quite similar to
the material of the cone of 1871 and 1872, which in all probability was
engulfed in the large crevasse or fissure that opened below it; and the
fragments having thus fallen down into the lava, were enveloped by it
and carried out by it after having been more or less rounded. The
external envelope of these spheres is not at all scoriaceous, but
compact and lithoidal, and sometimes composed of concentric folds or
plaits.
As to the gaseous emanations of fumaroles, watery vapour with few
exceptions comes first; this conveys the material which first appears in
the sublimations, viz., sea-salt, and for the most part oxide of copper.
If the fumarole continue active, it passes from the neutral period to
the acid period, and first hydrochloric acid is produced, which, in
small lava streams, never conveys chloride of iron, and rarely attacks
the scoriæ to form that salt, but expends its force in changing the
sublimations already there. For this reason chloride of iron, though
completely absent in the lavas of 1871, was abundantly found in those of
the 26th April, 1872. Sulphurous acid follows hydrochloric at a later
period, and sulphuretted hydrogen occasionally succeeds.
Having examined the gases of fumaroles by means of a graduated tube, and
the pyrogallate of potash, I always found that it contained less oxygen
than the surrounding atmosphere.
For several years I wished to see whether the fumaroles of the lavas had
a period of evolution of carbonic acid, as sometimes happens with
fumaroles near the craters, but I have always obtained negative
results. I often found that the atmosphere on the lavas contained an
excess of carbonic acid, but as these lavas had burnt many trees, and it
was probable that carbonic acid springs had formed under the lava, I
never considered it safe to form any conclusion on the subject.