for upwards of a year by the constant flow of lava from the summit of
the cone.

From the month of November, 1868, until the end of December, 1870, the
mountain remained quiet, except that the fumaroles at the head of the
fissure showed a degree of activity by which chlorides and sulphides of
copper, sulphide of potash and other products, were engendered.

But in the beginning of 1871 the seismograph was disturbed,[1] and the
crater discharged, with a slight detonation, a few incandescent
projectiles. Then I announced that _a new eruption had commenced, which
might be of long duration, but with phases that could not possibly be
foreseen_; and on the 13th January, on the northern edge of the upper
plain of the Vesuvian cone, an aperture appeared, from which at first a
little lava issued, and then a small cone arose and threw out
incandescent projectiles, with much smoke of a reddish colour, whilst
the central crater continued to detonate more loudly and frequently. The
lava-flow continued to increase until the beginning of March, without
extending much beyond the base of the cone, although it had great
mobility. In March, this little cone appeared not only to subside, but
even partly to give way, as almost happens with eccentric cones when
their activity is at an end. Upon visiting it, I observed that four
prismatic or pillar-like masses remained standing, three of which were
formed of scoriæ which had fallen back again in a pasty condition, and
had become soldered together, the fourth consisting of a pyramidal block
of compact and lithoidal lava, which appeared to have been forced up by
impetus from the ground beneath. A little smoke issued from the small
crater, and a loud hissing from the interior was audible. By lying
along the edge, I could see a cavity of cylindrical form about ten
metres in depth, tapestried with stalactitic scoriæ covered with
sublimations of various colours. The bottom of this crater was level,
but in the centre a small cone of about two metres had formed, pointed
in such a manner that it possessed but a very narrow opening at the
apex, from which smoke issued with a hissing sound, and from which were
spurted a few very small incandescent scoriæ. This little cone increased
in size as well as activity until it filled the crater, and rose four or
five metres above the brim.[A] New and more abundant lavas appeared near
the base of this cone, and, pouring continually into the Atria del
Cavallo, rushed into the Fossa della Vetrana in the direction of the
Observatory and towards the Crocella, where they accumulated to such an
extent as to cover the hill-side for a distance of about 300 metres;
then turning below the Canteroni, they formed a hillock there without
spreading much farther. These very leucitic lavas are capable of great
extension, the pieces which are ejected forming for the most part very
fine filiform masses, which may be collected on the mountain in great
quantities, and specimens of which I presented to the Academy under the
name of _filiform lapilli_. These threads were often of a clear
yellowish colour, and, when observed under the microscope, were found to
consist of very minute crystals of leucite embedded in a homogeneous
paste. The crystals were still smaller as the diameter of the threads
was less, and never formed knots or swellings even in the most hair-like
threads. These observations led me to reject the opinion of those who
hold that crystals of leucite are pre-existent in the lava. The viscous
nature of these lavas prevented their being covered with fragmentary
scoriæ, but caused the formation at first of a skin, which, thickening,
became at last a more or less pliable shell, that, when more solidified,
allowed the still fluid part to run as in a tube formed of this solid
shell. For many months the lava descended thus from the cone and
traversed the Atria del Cavallo, always covered, appearing below the
Canteroni of a lively fluidity, until it could no longer be enveloped in
its skin, which was stretched by the addition of new lava, and finally
rent asunder to give room to the current until, owing to diminished
liquidity, it was constrained to stop. When the lava, having traversed
the covered channel it had made for itself from the top of the mountain
to below the Canteroni, made its appearance still running, it frequently
formed large bubbles on the surface, which mostly burst to give vent to
smoke, and then disappeared.

In October, 1871, near the edge of the central crater, another small
crater was formed by falling in, which, after a few days, gave vent to
smoke and several jets of lava. The principal cone frequently opened in
some point of the slope to give egress to small currents of lava, which
quickly ceased. But towards the end of October the detonations
increased, the smoke from the central crater issued more densely and
mixed with ashes, and the seismograph and accompanying apparatus were
disturbed: for all these reasons, I said in one of my bulletins, _we
have either reached a new phase or the end of the eruption_, not knowing
whether the new phase would be the last. On the 3rd and 4th November
copious and splendid lava streams coursed down the principal cone on its
western side, but were soon exhausted. The cone of 1871 appeared again
at rest, and partly even fell in, but did not cease to emit smoke and to
show fire in the interior.

In the beginning of January, 1872, the little cone again became active,
the crater of the preceding October resumed strength, with frequent
bellowings and projectiles, and soon after lavas of the same kind as
before reappeared. The cone of 1871, formed again by the lava ejected,
became so full that the lava poured from its summit in the most singular
and enchanting manner. So far only an eccentric or ephemeral cone had
risen close to the central crater, which, after exhaustion, regained
vigour and discharged lava from the apex instead of the base, as usually
happens.

In the month of February matters were somewhat moderated; but in March,
with the full moon, the cone opened on the north-west side--the cleavage
being manifest by a line of fumaroles--and a lava stream issued from the
lowest part without any noise and with very little smoke, and poured
down into the Atria del Cavallo as far as the precipices of Monte di
Somma. This lava ceased flowing after a week, but the fumaroles pointed
out the cleft of the cone; and between the small re-made cone, which had
risen to the height of 35 metres, and the central crater, a new crater
of small dimensions and interrupted activity opened.

On the 23rd April (another full moon) the Observatory instruments became
agitated, the activity of the craters increased, and on the evening of
the 24th splendid lavas descended the cone in various directions,
attracting on the same night the visits of a great many strangers. All
these lava streams were nearly exhausted on the morning of the 25th;
only one remained, which issued from the base of the cone, not far from
the spot whence that of the preceding month had issued. Numbers of
visitors, attracted by the splendour of the lava streams of the
preceding night, which they supposed still continued, soon arrived, but,
finding them exhausted, were for the most part conducted by their guides
to see the one still flowing. It was almost inaccessible, and to reach
it one had to walk over the rough inequalities of the scoriæ. It took me
two hours to get there from the Observatory, when I visited it that
morning, and therefore I endeavoured to dissuade those who wished to
visit it at night from the attempt, but set out myself from the
Observatory at 7 p.m., leaving my only assistant there. The instruments
were agitated. After midnight the Observatory was closed, and my
assistant retired to rest. Late and unlucky visitors passed unobserved
with an escort of inexperienced guides; at half-past 3 o'clock in the
morning of the 26th they were in the Atria del Cavallo, when the
Vesuvian cone became rent in a north-westerly direction, the fissure
commencing at the little cone which disappeared, and extending to the
Atria del Cavallo, whence a copious torrent of lava issued. Two large
craters formed at the summit of the mountain, discharging numerous
incandescent projectiles with white ashes, and glittering with particles
of mica, which frequently recurred.

A cloud of smoke enveloped these unfortunates, who were under a hail of
burning projectiles and close to the lava torrent. Some were buried
beneath it[B] and disappeared for ever; two dead bodies were picked up,
and eleven grievously injured, one of whom died close to the
Observatory. He alone revealed his name, Antonio Giannone. I learned
afterwards that he was a fine young fellow, and Assistant-Professor in
one of the Universities.

Assistant-Professors Signor Franco, who is a priest, and Signor
Francesco Cozzolino, a priest also, entrusted with the festive mass for
the Observatory, hastened to assist the dying. On my own return thither,
the sad spectacle of the dead and dying awaited me; the former were
conveyed, through the assistance of the municipal officer of Resina, to
the Cemetery, and the latter to the Hospital. But we must leave this
scene of grief and sorrow, and return to the eruption.

The fissure of the cone on the north-west side was large and deep, and
extended into the Atria del Cavallo, about 300 metres. No mouth opened
along the cleft of the cone itself; all the lava issued from that part
which extended into the Atria. From previous experience I should have
expected to have seen the formation of adventitious cones along the
widest part of the fissure, which is never that most elevated, and these
discharging from their summits æriform matter frequently mixed with
projectiles, and from their base lava; but on this occasion no cone
appeared at the widest part of the fissure, but a long hillock was
formed like a little chain of mountains, one point of which was elevated
about fifty metres above the plain beneath, and bearing no resemblance
to a cone.

Another fissure opened in the cone on the south side, which did not
extend to the base, and lava issued from this and flowed in the
direction of the Camaldoli. Streams of less importance furrowed the cone
in other directions, but the largest quantity of lava proceeded from the
fissure in the Atria del Cavallo, below the hillock or miniature chain
of collines just described. This lava stream was for some time
restrained within the Atria del Cavallo, among the holes and
inequalities of the lavas of 1871, but these being filled up and
overcome, it divided into two branches--the smaller one flowing through
a hollow which separated the lavas of 1867 from those of 1871, and made
its way over the lavas of 1858, threatening Resina, but stopped as soon
as it reached the first cultivated ground; the larger branch
precipitated itself into the Fossa della Vetrana, occupying the whole
width, about 800 metres; and traversing the entire length of 1,300
metres in three hours. It dashed into the Fossa di Faraone; here it
again divided into two streams, one overlying the lava of 1868, on the
Plain of the Novelle, partially covering the cultivated ground and
country-houses; the other flowing on through the Fossa di Faraglione,
over the lava of 1855, reached the villages of Massa and St. Sebastiano,
covering a portion of the houses, and thence continued its course
through the bed of a foss or trench which, contrary to my advice, had
been excavated after the eruption of 1855, in the expectation of
diverting the course of that lava. I did not fail to observe that the
rains which previously descended through these steep channels, would in
future be kept back to filtrate through the scoriæ, without ever
reaching the new channel.

The lava of this eruption, meeting with this said excavation, flowed
into it, instead of pursuing its road over the lava of 1855, and thus
invaded highly cultivated ground and towns of considerable value,
extending to the very walls of a country-house belonging to the
celebrated painter, Luca Giordano. This lava stream, having surmounted
the obstacles which the heaps of scoriæ in the Atria del Cavallo
presented to it, ran with great velocity (notwithstanding its being
greatly widened out in the Fossa del Vetrano), so that between 10 a.m.
and 11 p.m. it traversed about five kilometres of road, occupying a
surface of five to six square kilometres. If it had not greatly
slackened after midnight, from the failure of supply at its source, in
twenty-four hours more, by occupying Ponticelli, it would have reached
Naples, and flowed into the sea.

Although I had often visited the two villages of Massa and St.
Sebastiano, previously greatly injured by the lava of 1855, yet I could
not well estimate, upon now seeing them again, the number of houses
which had disappeared. Massa seemed to me diminished by about one-third,
and St. Sebastiano by somewhat less than a fourth. But the way of escape
was open to the inhabitants of Massa; whilst a great river of lava
occupying the road leading to St. Giorgio a Cremano would have hindered
the flight of the inhabitants of St. Sebastiano, if they had been
dilatory. The lava stream now separating the two villages is little less
than a kilometre in width, and is about six metres in height.

On the night of the 26th April, the Observatory lay between two torrents
of fire, which emitted an insufferable heat. The glass in the
window-frames, especially on the Vetrana side, was hot and cracking, and
a smell of scorching was perceptible in the rooms. The cone, besides
being furrowed by the lava streams just described, was traversed by
several others, which appeared and disappeared. It seemed completely
perforated, and the lava oozed as it were through its whole surface. I
cannot better express this phenomenon, than by saying that _Vesuvius
sweated fire_. In the day-time, the cone appeared momentarily covered
with white steam jets (fumaroles), which looked like flakes of cotton
against the dark mountain-side, appearing and disappearing at brief
intervals.

Simultaneously with the grand fissure of the cone, two large craters
opened at the summit, discharging with a dreadful noise, audible at a
great distance, an immense cloud of smoke and ashes with bombs and
flakes, rising to the height of 1300 metres[C] above the brim of lava
(_sull' orlo de essi_). The white ashes, before described, although they
did not fall beyond the Crocella, were carried by the wind as far as
Cosenza, from whence they were sent to me by Dr. Conti. These ejections
were followed by dark sand, with lapilli and small fragments of scoriæ
of the same colour. The smoke, driven up with violence, assumed the
usual aspect of a pine tree, of so sad a colour that it reminded us of
the shadowy elm of Virgil's dreams ("_ulmus opaca ingens_"). From the
trunk and branches of the pine-tree cloud fell a rain of incandescent
material, which frequently covered all the cone. The lapilli and the
ashes were carried to greater distances.

The victims of the morning of the 26th, the torrents of fire which
threatened Resina, Bosco and Torre Annunziata, and which devastated the
fertile country of the Novelle, of Massa, St. Sebastiano and Cercola,
the two partially buried villages, the continual and threatening
growlings of the craters, caused such terror that numbers fled from
their dwellings near the mountain into Naples, and several in Naples
went to Rome or to other places. Very many delayed from the knowledge
that I was in the Observatory, and held themselves in readiness for
flight whenever I should abandon it.

The rapidity with which the vast torrent of fire assailed the houses
(_i.e._, in these villages), and the great heat which spread to a
distance, scarcely allowed the fugitives to carry away any of their
belongings; many were completely destitute. The authorities vied with
each other in zealous efforts to relieve the distress, and the
municipality of Naples sheltered and fed the wretched beings for many
days.

The igneous period of the eruption was short, for on the morning of the
27th the lava stream, bearing down upon Resina, having covered a few
cultivated fields, stopped; the lava descending from the summit of the
mountain towards the Camaldoli also stopped; and the great lava torrent,
which passed the shoulders of the Observatory through the Fossa della
Vetrana, lowered the level of its surface below those of its two sides,
which appeared like two parallel ramparts above it.

If these streams had continued on the 27th, flowing in the same manner
as they did on the night of the 26th, they would have reached the sea,
bringing destruction to the very walls of Naples.

But before leaving the subject of these lavas I must narrate an
important fact to which I was witness, and which was thrice repeated,
near the banks of the great river of fire that ran close to the
Observatory. At three several points, and at different times, I observed
great balls of black smoke issue from the lava, driven up with continued
violence, as if from a crater; through the smoke I frequently observed
numerous projectiles thrown up into the air, but I could not say whether
with noise or in silence, for the noise of the central crater was
deafening. Each of these little eruptions, which I may call _external
eruptions_, lasted from fifteen to twenty minutes. The first took place
at the most elevated point of the Fossa della Vetrana, on the right bank
of the torrent; the second, under the hill of Apicella, where the lava
divided into the two branches, before described; and the third near to
the Observatory on the left bank of the lava stream. These singular
explosions terminated without leaving little cones or craters, the lava
in its impetuosity carrying every trace away. These eruptions were seen
from Naples, and the Observatory was justly believed to be in danger.
One has been clearly photographed, the one which was the best seen from
Naples, being the nearest and the least darkened by the smoke of the
lava. (Plate 4.) Is this the first time that the phenomenon has been
remarked? I believe that it is at least the first time it has been
authenticated. The authority of Julius Schmidt, quoted by Scrope, has no
weight with me, for I was also a witness of what happened at Vesuvius in
1855; and, although these cones were in the midst of the lava in the
Atria del Cavallo, they originated, according to the opinion of
everyone, from the fissure from which the other and much larger cones
proceeded. The same phenomenon was observed in the Atria del Cavallo in
1858, when I caused two of the little cones to be brought to the
Observatory; but these also might belong to the fissure along which the
other cones were arranged. The same may be said of the little craters
observed, after they had been exhausted, by Professor Scacchi in 1850.
But the discharging mouths now observed in the Fossa della Vetrana,
which existed for twenty minutes and then disappeared, and which were
not at all in a continuous line, and could not be supposed to correspond
with any fissure beneath, constitute a circumstance which, if not new,
is evident for the first time, and cause the recognition of a power in
the lava itself to form eruptive fumaroles.[2]

The igneous period of the eruption having terminated on the evening of
the 27th, the ashes, lapilli, and projectiles became a little more
abundant, whilst the roaring noises of the craters apparently became
greater. The pine-tree cloud was of a darker colour, and was furrowed by
continual lightning, visible by daylight from the Observatory. Many
writers on the subject of Vesuvius affirm that the flashes which appear
through the smoke cloud were lightning unaccompanied with thunder, but
they studied the phenomena from Naples, or some place more or less
distant from the crater, where the report of the thunder was inaudible,
or could not be distinguished from the bellowing and detonation of the
mountain. The fact is that these flashes were constantly followed by
thunder, after an interval of about seven seconds.[D] When the flash was
very short, a simple noise like the report of a gun was heard, but if it
were long, a protracted sound like that from torn paper ensued.

On the 28th the ashes and lapilli, continuing to fall abundantly,
darkened the air, yet without diminishing the terrible noise; at Resina,
Portici, St. Giorgio a Cremano, Naples, etc., terror was universal.

On the 29th, with a strong wind blowing from the east, scoriæ of such a
size fell at the Observatory, that the glass of the windows unprotected
by external blinds was broken. The noise from the crater continued, but
the projectiles rose to a less height, indicating a diminution in the
dynamic power of the eruption. Towards midnight the noise of the craters
was no longer continuous, and recurred with less force and for shorter
intervals. Almost at the same hour a tempest burst over the Campania
with loud thunder and a little rain. The grass, the seeds, the vine
tendrils, the leaves and tops of the trees dried up immediately, and the
country was changed from spring to winter. The storm, although repeated
on the following days, passed away by degrees, and thus the floods,
which I strongly feared, did not occur. Almost always after great
eruptions of Vesuvius, storms of heavy rain have followed, and the
ground being covered with ashes, the water could not filtrate through
into the soil, but descended in muddy torrents over the adjacent
country, occasioning as much damage as the fire itself.

On the 30th, the detonations were very few, and the smoke issued only at
intervals, and by the 1st May the eruption was completely over.

When the smoke had cleared off the figure of the cone was seen to be
changed. (_Vide_ Plate 5a.)

The ground was perpetually disturbed whilst the Volcano raged, so that
the Observatory oscillated continually. Some shocks were felt not only
in the adjacent territory, but at a greater distance, at Montovi and
elsewhere. The oscillations at the Observatory were chiefly undulatory,
from N.E. to S.W. They were observed for some days after the termination
of the eruption, but not continuously, although they maintained some
intensity.

If we refer to January, 1871, we shall find that that eruption was
preceded by several earthquakes, among which were those of the months of
October, November and December, in the previous year, that wrought such
destruction in Calabria, and especially in the province of Cosenza; if
we consider that as only the last phase, we shall find that it was
preceded by great shocks of earthquake that devastated some regions of
Greece.[3]

The great quantity of lapilli which fell buried the scoriæ with which
the Vesuvius cone was covered, so that it became somewhat more difficult
to ascend to the summit, and much less difficult to descend. Having
reached the top of the mountain, I found a large crater divided into two
parts by what seemed a cyclopean wall. The two abysses had vertical
sides, and revealed the internal structure of the cone. Their vertical
depth was 250 metres; and beyond that I observed a sort of tunnel
perforated in the rock, with a covering arch raised above the bottom of
the eastern abyss about 12 metres, judging by the eye. The interior
walls of the crater showed neither the usual stalactitic scoriæ nor
sublimations, nor fumaroles, but alternate beds of scoriæ and of compact
lava. The fumaroles and sublimations abounded, only about the brims of
the craters. Hydrochloric and sulphuric acid and sometimes sulphuretted
hydrogen affected respiration, and the temperature rose sometimes to 150
degrees. Various fissures about the brim of the double crater indicated
prolongations downwards, which allowed me to descend with a rope, in
order to examine the interior of the tunnel to which I have just
alluded. The highest brim of the crater was fissured for a distance of
80 metres, and the greatest depth of fissure was at that place.

By measurement with the barometer, we ascertained approximately (for
only one barometer was used) that the height of the Vesuvian cone was
somewhat diminished.

Not only the Vesuvian cone, but the whole adjacent country appeared
white for many days, as if covered with snow, when exposed to sunlight.
This was due to the sea-salt contained in the ashes with which the
surface was strewn.

A great quantity of coleoptera assembled on the flat roof of the
Observatory, where the ashes and lapilli were heaped up two decimetres
in height. I found the same species on the cone, where many insects were
observed on other occasions, such as the _Cuccinella septempunctata_;
the crysomela populi, etc., were wanting. This phenomenon of the
extraordinary concourse of insects on the top of Vesuvius, in order to
die in some of the fumaroles, especially noted previous to and after
great eruptions, is a circumstance for which I cannot account.[4] The
whole of the lava emitted in this eruption occupies a surface of about
five square kilometres; allowing an average thickness of four metres, we
obtain a mass of twenty millions of cubic metres. About three-fifths of
this lava did no injury, being deposited upon other pre-existing lava.
However, the lava in the Novelle, which was deposited upon the lava of
1858, covered quarries of the best stone which had been worked at the
time, covered many paths that had been cleared, and buried the new
Church of St. Michele, with some houses that surrounded it, which had
been rebuilt on the site of the former church, which was covered by the
lava of 1868. The destruction of land in occupation, of buildings and of
crops, exceeded three million francs in value. Many proposals for
relieving the sufferers have been received. Wishing to aid in this
benevolent work, I gave a public lecture, admission for each person
being one franc; and this lecture, from notes badly taken, was printed
by private speculation, and I was compelled to repudiate the report of
it through the public papers.

The evolutions of carbonic acid (_mofette_), which usually appear at the
end of great Vesuvian eruptions at low-situated spots or hollows, with
very rare exceptions, were observed on this occasion a few days after
the eruption had completely ceased. They appeared in the direction of
Resina. I found the most elevated at Tironi, and the most numerous
between La Favorita and the Bosco Reale di Portici.

The water in wells was on this occasion neither deficient nor scarce
previous to the eruption, but was very acid after the appearance of the
carbonic acid evolutions in those neighbourhoods in which they
abounded. Having stated that the disastrous conflagration of the 26th
April ought, in my opinion, to be regarded as the last phase of a long
period of eruption, which commenced at the beginning of 1871, I consider
it right to discuss the question at somewhat greater length.

Not only from twenty years' personal observations, but from the
attentive study of accounts of previous eruptions, I have found that
when the central crater awakens with small eruptions after a certain
time of previous repose, these almost always have a long duration, and,
after various phases of increase and decrease, terminate in a great
eccentric eruption, that is to say, with the production of an aperture
from which a copious lava stream issues. The eruptions of 1858, 1861,
1868 and 1872, furnish the most recent examples of what I affirm. I
might cite many others of earlier date, but I shall content myself with
recording the greatest conflagration of this century, that of October,
1822.

Before the erection of the Vesuvian Observatory, it was impossible to
obtain a consecutive account of all the phases which the Volcano
presented; but we generally obtained the description of the more
splendid phases of the eruption which arrested the attention of
everyone. Hence, notices of the small phenomena which preceded a great
eruption are frequently wanting. We cannot always ascertain whether the
fumaroles of the craters became active and at what periods, what was
their temperature and what the diverse nature of their emanations, etc.:
whether and when any change in the crater with slight eruptive
manifestations occurred; discharges which sometimes commenced in the
bottom of a crater becoming active, and so are invisible at Naples.

But it may be asked whether the inverse proposition be equally true,
that is, whether all the great eruptions of our Volcano were preceded by
small fiery manifestations of long duration? There have undoubtedly been
great eruptions not preceded by small central eruptions, but these also
had their period of preparation or precursory signs. After the great
eruption of 1850, Vesuvius remained in apparent repose until the end of
May, 1855, when there was an eccentric eruption and a great flow of lava
lasting twenty-seven days. But for a year before the fumaroles on the
top of the mountain had acquired great activity, their temperature
increased, and hydrochloric and sulphuric acid became more abundant, and
generated the usual coloured products on the adjacent scoriæ. Finally,
in the month of January, a crater was formed by falling in of the
ground, and although it did not discharge fire, yet it poured forth
dense smoke. This was the beginning of the fissure manifested four
months afterwards.

Ignazio Sorrentino, who spent a long life in the study of Vesuvius, and
frequently ascended it, considered the increase of those yellow
products--which are chiefly chlorides of iron, but were, at that time,
mistaken for sulphur--as the sign of an approaching eruption.

The only grave objection that can be alleged is that of the memorable
eruption of 1631, which surprised the neighbouring population so
suddenly that many perished miserably, surrounded or covered with lava.
But that terrible conflagration occurred after centuries of repose, so
that trees had grown in the interior of the crater. No one suspected the
possibility of danger. It took place, too, at the end of autumn, when
the cone is usually covered with clouds, and, therefore, no one had an
opportunity of observing any precursory phenomena.

When the Observatory was established, I was able--in the first instance,
at my own expense, and afterwards with some slight assistance from
Government--to undertake studies more assiduous than any previously
made. I had two instruments adjusted to indicate the internal efforts of
the Volcano, viz., M. Lamond's apparatus of variations, which, by means
of finely-balanced needles and methods of amplification proposed by
Gauss, indicates the slightest trepidation of the ground, and my own
electro-magnetic seismograph, a self-registering instrument of exquisite
delicacy. These instruments, when attentively observed, give the most
valuable information with respect to the activity of the adjacent
Volcano.

If the very slightest eruption occurs, these instruments manifest slight
perturbation, increasing with the activity of the mountain. When the
Volcano attains a certain degree of activity, and the instruments are
proportionately disturbed, it is impossible to foresee a new phase of
increase without constantly watching the changes in the intensity of the
perturbations; and to effect this it is requisite to have upon the spot
a staff of assistants sufficiently numerous, scientific and intelligent.
If, therefore, on the night preceding the 26th of April the instruments
had been properly watched, they would have undoubtedly indicated the
great increase in the activity of the Volcano. The perturbations on the
23rd were steadily increasing, and on the evening of the 25th they were
much stronger than on the 24th, but on the morning of the 26th they had
become extraordinarily strong; they must, therefore, have increased
considerably during the night.

II.

NATURE OF THE LAVAS.

When the observer is near the source of the lava, he sees matter in a
state of fusion, which, like a torrent of liquid fire, runs along, with
more or less impetuosity, between two banks formed by itself. But as
soon as the surface of the torrent cools to the point of congelation, it
loses the splendour of its first incandescence. The part which begins to
harden breaks readily in some lavas into fragments which float on the
viscous fluid beneath; these, increasing in number with distance from
the source, conceal the molten matter beneath and retard its progress,
and at last nothing is seen but the more or less red-hot scoriæ moving
along. These lavas I shall call "_Lavas with fragmentary scoriæ_."

On other occasions, a skin forms on the surface of the lava, which,
gradually thickening, keeps flexible for some time, and then wrinkles or
swells or extends and breaks to give egress to the hot fluid within,
which, in its turn, skins over and repeats the same phenomena. This I
shall call "_Lavas with a united surface_."

These, in their course, discharge less smoke than the first, draw out
more easily into threads, and, when cold, have a dark colour, something
like bitumen or pitch. _The lava with fragmentary scoriæ_, when
stretched, breaks easily, discharges smoke copiously, and, when
hardened, has a more bluish tint, like clods of upturned earth (_formato
di zolle_). It is noisy in its course, because the incoherent scoriæ
that it carries along strike and crunch against each other; the other
lava flows silently, except for a sort of crackling arising from the
actual fracturing up of the solid skin by distension from the liquid
matter within. If required to give the mineralogical characteristics of
this lava, I would say that it was rich in leucite and contained little
or no pyroxene; the fragmentary lava, on the contrary, is poor in
leucite and rich in pyroxene. The lavas of 1871 were of the "united
surface" character; those of 1872 were "fragmentary," with some
characteristics which I shall describe:

     1. They were of the clearest tint I have ever seen, when
     regarded superficially, but, when broken, the fracture was
     darker than any other lava.

     2. They had very little leucite and abounded in pyroxene and
     olivine, and sometimes contained a few crystals of amphibole.

     3. Their specific gravity varied with their porosity; the most
     compact attained 2·75.

     4. These lavas carried along in their course a quantity of
     scoriæ which had long been subjected to the action of the acids
     of the fumaroles close to the craters, and also a great many
     bombs (_bombe_)--that is, round masses similar to those ejected
     from craters. These varied in size, some having a diameter of
     four to five meters. They frequently contained a large nucleus
     of very leucitic lava, like that of 1871, with a larger or
     smaller quantity of feroligiste (peroxide of iron). Others
     contained lavas changed by the action of the acid vapours near
     the craters. These bombs must have flowed out with the lava,
     for they are found through its whole course, and they were
     certainly not ejected from the crater; for not only are they
     found on the lava exclusively, but masses so enormous were not
     thrown up from the craters during the eruption; those lying on
     the cone near the craters seldom exceed a decimetre in
     diameter.

As to the qualitative chemical analysis of the lavas, it always presents
the same elements, with the exception of small quantities of some
metals, lead for example, which have escaped the researches of good
chemists, but which I have constantly found in the sublimations of the
fumaroles of the lava. With respect to the quantitative analysis, two
specimens of the same lava appear indeed to have their constituents in
different proportions. To arrive at any conclusion a long and patient
investigation, requiring means and assistance which the Observatory does
not possess, would be necessary.

Professor Fuchs, of Heidelberg, has devoted himself to this work for
years past, and if he continue it with well-selected and sufficiently
large specimens we may hope some day to obtain satisfactory results.

     5. Every specimen of lava which I examined with a very
     sensitive magnetoscope improved by myself, was invariably
     magneto-polar, not excepting the pieces of the bombs, whether
     rejected from the crater or carried along with the lava.

III.

FUMAROLES OF THE LAVAS.

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.

IV.

BOMBS, LAPILLI AND ASHES.

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.

V.

THE CRATERS AND THEIR FUMAROLES.

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.

VI.

THE ELECTRICITY OF THE SMOKE AND ASHES.

Our ancestors could judge that a great amount of electricity was
occasionally evolved in the smoke, from their observation of the
lightning flashes that darted through the Vesuvian pine tree; but they
had no proper instruments for ascertaining whether this evolution of
electricity was constant or accidental, or what laws regulated its
manifestations. My _apparatus, with movable conductor_, by which
comparative observations of electric meteorology can be made, and the
errors arising from dispersion corrected, supplied me with an easy
method of studying the electricity evolved during eruptions.

I must begin by describing the bifilar electrometer, in order to explain
the apparatus which I have named as above, "_Apparechio a conduttore
mobile_."

_A A_ (Plate VIa, Fig. 1) is a glass cylinder, the lower edge of which
is ground, well varnished with gum lac, and let into a wooden base, B,
furnished with three levelling screws. Through a sufficiently wide glass
tube, _a a_, runs a copper rod covered with insulating mastic, having a
little plate or cylindrical cavity of gilded brass at the top (Figs. 2
and 3), with two arms _d d_, _d' d_. In the plate a disc of aluminium,
_m_, is suspended by means of two silk fibres, and to the disc a very
fine aluminium wire is attached, _f f'_, bent a little at the ends, as
are the arms, _d d_, _d' d_. The disc has about three millimetres less
diameter than the plate. The diameter of the plate may vary within
certain limits, but I have found it convenient to make it eighteen
millimetres. The glass tube, _a a_ (Fig. 1), should descend below the
base as much as it rises above it, that is three to four centimetres.
The length of the index is about one decimetre.

The upper ends of the two silk fibres, by which the disc and index are
suspended, are attached to the top of the glass tube, _C_, by a
contrivance which permits a change in the distance between the two
points of suspension, and a screw, _p_, is provided to raise and lower
the disc with the index. At _n_, at the lower part of the tube, _C_,
there is a kind of torsion micrometer, arranged so as to bring the index
to the zero of the scale engraved on the graduated ring, _B_, which is
formed of a strip of good paper pasted on the rim of a glass disc. The
index must be placed at the zero of the scale, and must be some distance
from the ends of the arms of the plate with which it is parallel. The
plate is about three millimetres deep.

Having levelled the instrument, so as to render the disc concentric with
the plate, and placed the index at zero, it is obvious that if an
electric charge through the wire, _h_, reach the plate with the arms, it
will electrify the disc and index: the disc will have the opposite
electricity, and the extremities of the index will take the same
electricity as the arms, and consequently the index will describe an arc
more or less great. The motion of the index is sufficiently slow to
allow the eye conveniently to follow it. Having traversed the first arc,
which I call the _impulsive_ one, the index returns, and, after only two
oscillations, comes to rest at what I shall call the _definite_ arc.

When the electric charges are of very brief duration, the impulsive arcs
are within certain limits proportional to the tensions, and the ratio
between the impulsive and definite arcs is expressed by the following
equation:

     α(β - α) / β = tang. (1/2) α

In which β is the impulsive arc and α the definite arc, showing that α
comes out nearly equal to 1/2 β. In dry weather all goes perfectly
within the limits of proportion, and I can tell whether, during the time
in which the index traversed the impulsive arc, there were any
_dispersions_ and of what nature; for if the definite arc is not close
to the limit of the impulsive arc, it is a sign of _dispersions_ having
taken place during the motions of the index. Every degree less in the
definite arc denotes two degrees of loss for the impulsive arc; but as
the index employs double the time traversing the definite as it does the
impulsive arc, we may consider the loss of one equal to the loss of the
other.

In excessively damp weather the index gives no definite arc, and it is
necessary to resort to artificial heat in order to dry the insulators.
The most simple means I know of is to hold the instrument over some
hollow vessel, which, for the time, is converted into a stove by the
introduction of a spirit lamp.

From Gauss's formula for the bifilar system of instruments of this
class, we learn that the maximum sensitiveness of such instruments is
given when the length of the suspending fibres is greatest, and the
distance between them is smallest, with the weight of the movable or
rotating member a minimum; and these elements being the same, the
sensitiveness of the instruments is invariable.

To some electrometers, in order to avoid errors of parallax, a small
telescope, with a micrometer wire, has been added; but, with a little
practice, we can read accurately without this refinement. In order to
obtain comparative measurements, it is necessary to select some given
unit of tension. I have observed that by making a galvanic pile of
copper, zinc and distilled water, and insulating it well, each pole has
a tension which remains the same for many days, if the conditions of
temperature and the moisture of the surrounding atmosphere are not very
different. With thirty pairs of this pile, each element having
twenty-five square centimetres of surface, I have on the electrometer a
definite arc of 15°, with the temperature of the atmosphere at 20° C.,
and with the difference of 4° to 5° C. between the thermometers of the
psychrometer of August's construction. The first observation was made
twenty-four hours after mounting the pile. For unit of tension I took
that which corresponded to a single pair, that is, the thirtieth part of
the total tension. Other electrometers may be compared with one already
properly adjusted, without always having recourse to the pile.

This done, let us see the arrangement of all the apparatus:

_H H_ (Plate VIIa, Fig. 1) is the ceiling of a well-situated lofty
room, with an opening, _o o_, at the upper part.

_M M_, a bracket or table fastened against the wall, about a metre
distant from the ceiling, _H H_.

_N N_, a wooden platform for the observer.

_A_, the bifilar electrometer.

_B_, Bohnenberger's electroscope.

_a a_, a movable conductor formed of a brass rod 15 to 18 millimetres in
diameter, insulated below by means of a glass rod, well varnished with
gum lac, having a suspending pulley, _c_, and a wooden guide-rod
underneath it, _l_, within the guiding tube, _k_. At the upper part of
this conductor, _a a_, there is a sliding roof, _b_, which can be
adjusted so as to prevent rain entering at the opening, _o o_. The
conductor terminates in a disc made of a sheet of thin brass, _d_, 24
centimetres in diameter. Upon this disc, or even in place of it, we may
use metallic points.

As a support to the conductor at the upper part, I have made use of a
triangular ring, _x_, drawn at its full size in Fig. 2. The conductor
passes between three springs, and the triangular ring is held in place
by three silk cords, _m m m_. Their material should not be mixed with
any cotton, and it may be advisable to saturate them with an alcoholic
solution of gum lac.

_f f f_ is a hempen cord, which is used to raise and lower the
conductor.

_i_ is a copper wire covered with silk, by means of which the triangular
ring, _x_, and through that and its springs the conductor communicates
with either the electrometer or the electroscope.

Quickly raising the conductor by pulling the cord, _f_, the index of the
electrometer will describe a more or less large impulsive arc, and,
after two oscillations, will stop at the definite arc. Having thus
measured the electric tension of the air, and having lowered the
conductor, I next place the wire, _i_, in communication with the
electroscope, _B_, and by again raising the conductor, I ascertain
whether the electricity be positive or negative. It is scarcely
necessary to say that the conductor, when raised, gives electricity of
the same nature as that prevailing at the moment in the atmosphere; and
when lowered, manifests the opposite. In some conjunctures we must keep
the conductor raised and in communication with the electroscope, in
order to observe certain phenomena which I shall presently describe:
this method I call observation with a _fixed conductor_.

I have also constructed a similar but portable apparatus for use on
eruptive cones, when required.

Having given this description of the apparatus, it remains for me to
relate the results obtained, especially on the occasion of the last
eruption of Vesuvius.

The Observatory is distant, in a direct line from the central crater of
Vesuvius, 2,380 metres, so that, when the smoke is copious, it is
properly situated for the study of electricity, particularly when the
wind inclines the pine-tree cloud in the direction of the Observatory,
as frequently happened on the last occasion.

With smoke alone, without ashes, we obtained strong tensions of positive
electricity; with ashes only, which sometimes fell while the smoke
turned in the other direction, we had strong negative electricity; when
the smoke inclined towards the Observatory, accompanied with ashes and
lapilli, we had sometimes one kind of electricity, and sometimes the
other, just as the smoke or the ashes predominated; and often with a
"fixed conductor" we obtained negative electricity, and with a "movable
conductor" positive electricity. In Naples, too, at the Meteorological
Observatory attached to the University, my colleague, Professor Eugenio
Semmola, observed negative electricity of strong tension whilst ashes
were falling there in abundance. The tensions on this occasion were so
strong as to equal those obtained at changes of weather or during storms
(_temporali_), and, being beyond measure with a delicate electrometer,
we marked them with the symbol ∞: the same phenomena were observed when
lightnings flashed.

When there is but little smoke, it is necessary to approach the eruptive
mouths with a portable apparatus, in order to observe those phenomena
which, in great eruptions, may be studied from the Observatory itself.

The conditions under which (_folgori_) lightning flashes are seen from
the cloud of smoke are, that it is conveying great abundance of ashes.
In 1861, there were small flashes even from the line of eccentric mouths
above Torre del Greco, although the smoke was not very great; and when
these ceased to discharge, and the central crater became somewhat
active, with a moderate amount of smoke but a great deal of ashes, small
and frequent lightning flashes were observed in the twilight darting
through the smoke, which was dark in colour. In 1850 the eruption was
more vigorous, the smoke more abundant, and the ashes scarce, but the
flashes were very rare. In 1855, 1858, and 1868, with a scanty supply of
ashes and at intervals, no flashes were observed, and the electricity
remained constantly positive. But having regard to the facts of
antecedent eruptions, one sees that the flashes are always derived, from
the midst of smoke accompanied with ashes and lapilli, which separate
like rain from the rolling volumes of smoke, in the midst of which they
were ejected.

But how can we account for the positive electricity of the smoke, and
the negative electricity of the falling ashes? Without denying the
probability that a part of the positive electricity depends upon the
elevation of the smoke, as in the case of every other conductor we raise
aloft, or with a jet of water sent from a vessel by compressed air, I
think that the greater part of the electricity proceeds from the rapid
condensation of vapours, which are changed from the gaseous condition
into dense clouds; for even when the smoke issues tranquilly and does
not rise, because carried away horizontally by the wind, it gives signs
of positive electricity. From all my studies of atmospheric electricity,
and from some experiments made specially, it follows that the
condensation of vapours is the origin of this development of positive
electricity.

The negative electricity of the falling ashes certainly arises from the
fact itself of their fall; for if we place a metallic vessel full of
ashes upon an elevated and well-situated terrace, while the atmospheric
electricity is positive, and cause the ashes from the vessel to fall
gradually into an insulated metallic cup, communicating with
Bohnenberger's electroscope placed at three or four metres distance from
the vessel, the electroscope will manifest negative electricity. If the
upper vessel be insulated, and the ashes permitted to fall upon the
ground, we shall obtain, from the vessel, positive electricity. The
intensity of these electric manifestations depends (other things being
equal) upon that predominant at the moment in the air; so that if the
experiment be made while negative electricity prevails, the falling
ashes will manifest positive electricity, the upper vessel then showing
negative electricity. Now, as the ashes separate from the positively
electrified smoke in order to approach the ground, which is negatively
electrified, it follows that they must manifest negative electricity
upon touching the ground, leaving the positive electricity in the smoke
above. For this reason, the electric tension of the smoke is increased
by the descent of the ashes and lapilli, so that discharges between the
upper and lower part of the pine-tree cloud, or the surface of the
crater, are rendered possible. Hence it follows that the flashes of
lightning of Vesuvius play through the smoke, and with difficulty strike
bodies upon the earth; and from this circumstance our ancestors believed
the thunderbolts of Vesuvius to be harmless. However, if the smoke were
very great, and driven by the force of the wind to some distance from
the crater, with an abundant fall of ashes, it would be possible to have
lightning flashes proceed from the smoke to the earth. I possess some
documents which relate that, in 1631, thunderbolts fell upon the Church
of Santa Maria del Arco, and other places on the coast of Sorrento.

After upwards of twenty years' study and observation of meteoric
electricity, I am enabled to prove that atmospheric electricity is never
manifested without rain, hail or snow, and that manifestations of light
are always accompanied by thunder--manifestations of light (_lampi_),
thunder and rain being most closely connected. We may have rain without
manifestations of light, but never the latter without rain or hail. I
cannot here repeat what I have demonstrated in other memoirs; I can only
say that the lightnings of Vesuvius, erroneously believed to be not
accompanied by thunder, are really not accompanied by rain, but are
induced by the descent of ashes and lapilli.[6]

GENERAL CONCLUSIONS.

We may conclude from what I have stated:

1. That by the assiduous study of the central crater, and the
indications afforded by the "Apparatus of Variations" and the
"Electro-Magnetic Seismograph," we can obtain precursory signals of
eruptions; and that the other premonitory signs pointed out by our
ancestors, such as the drying up of wells, either only happen
occasionally or are mere coincidences, such as those of the coincidence
of a dry or a rainy season, the prevalence of certain winds, etc.[F]

2. That the fumaroles of the lavas are communications between the
external surface of the lava, hardened and more or less cooled, and the
interior lava still pasty, or at least incandescent.

3. That from the lava, while flowing, there is no escape of acid
vapours, neither from the fumaroles at the first period of their
existence, but these, if they last long enough, arrive at an acid
period.

4. That hydrochloric is the first acid that appears, combined afterwards
with sulphurous acid, and, still later, with sulphuretted hydrogen.

5. That vigorous lava streams may have eruptive fumaroles. (See
Translator's Note 2 to p. 94.)

6. That the sublimations follow a certain order in their appearance. In
the neutral period we get sea-salt mixed with some metallic oxides, the
first of which is oxide of copper. But in the great lavas, chloride of
iron appears simultaneously with the acid period. Hydrochloric acid
transforms the oxides into chlorides, which, in their turn, change into
sulphurets or sulphates on the appearance of sulphurous acid.

7. That the acids, by attacking the scoriæ, create new chlorides and
sulphates, which are thus not products merely of sublimation.

8. That micaceous peroxide of iron--so common and abundant near the
eruptive mouths--is very scarce and rare on the lavas, unless conveyed
there from the craters.

9. That chloride of iron--so manifest on the fumaroles of the great
lavas--is only found in small eruptions close to the discharging mouths.

10. That the frequency of chloride of iron in the lavas of great
eruptions masks the order of transformation of the other products.

11. The fumaroles at the summit of Vesuvius present even greater
gradations, for they often emit carbonic acid or pure watery vapour.

12. Lead, which I first discovered in the fumaroles of the lavas of
1855, is a constant product of fumaroles which have a certain duration.
It is often obtained as a distinct and crystallized chloride, and often
is found in combination with other products.

13. Oxide of copper is also a constant and primary (_primitivo_) product
of fumaroles. The chloride and sulphate of copper are formed from the
oxide, directly contrary to general belief.

14. I do not think that the chloride of calcium, which I found on this
occasion in almost all the deliquescent sublimations, is a product
peculiar to this eruption only, in which alone, however, I found it. I
was, therefore, induced to look for it in other sublimates, in which I
might possibly have overlooked it, as, without doubt, my predecessors
have done, owing to the deliquescence of the chloride of iron with which
it was constantly combined. I think that this chloride, in accordance
with the general law, is transformed into a sulphate--a transformation
which readily occurs on Vesuvius.

15. Copious and well-crystallized sal ammoniac is only found on the
fumaroles of those lavas which have covered cultivated or wooded ground.

16. The scarcity of oxygen in the gases of fumaroles may possibly arise
from the formation of the oxides which precede the chlorides.

17. Lavas give a continuous spectrum, although covered with smoke, when
looked at with Hoffmann's spectroscope with direct vision.[G]

18. The smoke gives positive electricity, and the falling ashes negative
electricity.

REFERENCE TO THE PLATES.

PLATE

Ia. The Cone of Vesuvius, in 1870, from a Photograph taken
    near the Observatory.

    _a._ The Atria del Cavallo.
  _b b._ Fossa della Vetrana.
    _c._ Punta del Crocella.
    _d._ Lava of 1858 and 1867.
    _e._ Police Barrack near the Observatory.
    _f._ Part of Monte Somma.

IIa. Profile of Vesuvius, taken from a Photograph of the
     Observatory in the month of September, 1871.

    1. The Cone, on the 13th January, 1871.
 2, 2. Lava of 1871.

IIIa. Profile of Vesuvius on the 16th April, 1872, about ten
      days before the last Conflagration.

IVa. Vesuvius, on the 26th April, 1872, from a Photograph taken
     in the neighbourhood of Naples.

    1. The Observatory.
    2. Fossa della Vetrana.
    3. Eruption of Smoke and Ashes, with Stones, from the
         surface of the Lava.
    4. The Novelle, St. Sebastiano, and Massa.
    5. Lava which took the direction of Resina.
    6. Lava which, from the Crater, took the direction of the
         Camaldoli.
    7. The Grain Stores, near Naples.
    8. Resina.
    9. Torre del Greco.
   10. The Camaldoli.

Va. Profile of Vesuvius after the Eruption of the 26th April,
    1872, from a Photograph taken near the Observatory.

   1, 1. The Fissures of the 26th of April.
   2, 3. Small Hill thrown up on the morning of the 26th of
         April, from below which issued the great current of
         Lava.
4, 4, 4. The Mouths out of which the Lava issued.
   5, 5. The larger Lava Stream, which passed near the Observatory
         by the Fossa della Vetrana.
   6, 6. The other Lava Stream, which, after dividing from
         the last, took the direction of Resina.
   7, 7. The Lava which ran down towards the Camaldoli.
  8 & 9. The two Craters on the summit of the Cone.

VIa. The Bifilar Electrometer of Signor Palmieri.
     (_Details._)

VIIa. The assemblage of the Electroscopic Apparatus of Signor
      Palmieri, as arranged at the Vesuvian Observatory.

VIII. Professor Palmieri's Seismographic Apparatus.

[A] This small cone, as it appeared on the 1st April, is described and
drawn in a Memoir of Professor von Rath, of the University of Bonn, on
"Vesuvius on the 1st and 17th of April, 1871."

[B] Eight young medical students perished beneath the lava, with others
unknown by name. They were all youths of good promise; their names will
be recorded on the marble monument to be erected near the Observatory.
They are: Girolamo Pausini, Antonio and Maurizio Fraggiacomo, Francesco
Binetti da Molfettu, Giuseppe Carbone da Bari, Francesco Spezzaferri da
Trani, and Giovanni Busco da Casamassima and Vitangelo Poli.

[C] If this enormous height of projection really means, that above the
brim of the crater, it involves an initial velocity of projection of
above 600 feet (British) per second.

Observations of the height of ascent of volcanic blocks are always
difficult and deceptive, and never free from error.--_Translator._

[D] Assuming these flashes to have emanated from somewhere within the
cloudy volume of steam and dust called "the head of the pine-tree," this
interval would indicate that the mean height of this cloudy volume
itself was not more than about four thousand feet above the top of the
cone; and, if so, that is not very far from the limit in height of
projection of the dust and lapilli.--_Translator._

[E] COTUNUITE, chloride of lead, in white, lustrous, acicular crystals,
of the trimetric system, easily scratched, Sp. gr., 5·238.

TENORITE, peroxide of copper, in thin, hexagonal plates or scales,
translucent when very thin, dark steel gray, of the cubic system; hard
and lustrous. Sp. gr. about 5·950.--_Translator._

[F] Earthquakes, though in distant regions, usually precede eruptions.
The Earthquake of Melfi preceded the great Eruption of Etna in 1852; the
Earthquake of Basilicata of December, 1857, terminated with the Eruption
of 1858, which filled the Fossa Grande with lava; the Earthquakes of
Calabria of 1867 and 1870 were the precursors of the Vesuvian
conflagrations of 1868, 1871, 1872. A Volcano, also, in the Island of
Java had a great eruption in the month of April, some days before the
last conflagration of Vesuvius, as I learnt from a letter addressed to
Signor Herzel, Swiss Consul at Palermo, communicated to me[7] by the
astronomer, Signor Cacciatore.--_Palmieri._

[G] I have made a large collection of sublimates, which I purpose
examining with the spectroscope, and I shall be able to place some at
the disposal of experimentalists who may desire to pursue investigations
of this kind.

NOTES

BY THE TRANSLATOR.

[1] (P. 82, text). Professor Palmieri has not given any description
in this Memoir of his seismograph--the instruments described being those
only which have relation to atmospheric electricity. The following brief
account of his seismograph will, therefore, form a not unsuitable
complement to his Memoir. The instrument, in general terms, is of that
class in which the wave movements are indicated by the displacement,
relative or absolute, of columns of mercury in glass tubes. It is a
self-recording instrument, composed of two distinct portions--one for
record of horizontal, or rather of what are called undulatory shocks;
the other for vertical shocks. In point of general principle, therefore,
it is very similar to that proposed by me ("Transactions, Royal Irish
Academy," in 1846), and in certain respects appears to me less
advantageous than the latter. Some account of the Palmieri instrument,
together with some critical remarks as to its action, may be found in my
"Fourth Report on Earthquakes" ("Reports, British Association, 1858,"
pp. 75-81). The following description of the instrument is derived from
"The Engineer," of 7th June, 1872, and the publishers have to thank the
proprietors of that journal for permission to use the illustration,
Plate 8.

In Fig. 1, _E_ is a helix of brass wire (gauge about one millimetre);
the helix consists of fourteen or fifteen turns, and has a diameter of
from twenty to twenty-five millimetres; it hangs from a fine metal
spring, and can be raised or lowered by a thumb screw. From the lower
end of the helix hangs a copper cone with a platinum point; the latter
is kept close to the surface of mercury in the iron basin, _f_, which
rests on an insulating column of wood or marble, _G_. The distance of
the point from the surface of the mercury remains constant, as the metal
pillar, _T_, is of such a length that its expansion or contraction by
change of temperature compensates that of the helix; the latter is in
connection (by _T_) with one pole of a Daniell's battery of two cells,
and the basin, _f_, is connected with the other pole. Any vertical
movement, however slight, makes the platinum point dip into the mercury,
and thus completes the circuit. In this circuit are included two
electro-magnets, _C_ and _D_; these, during the circulation of a
current, attract their armatures, which are connected with levers. The
action of _C's_ lever is to stop the clock, _A_, which thus records, to
a half-second, the time of the occurrence of the shock, at the same
instant that the clock strikes an alarm bell, which attracts the
attention of an observer. The lever, attached to the armature of _D_, at
the first instant of the current frees the pendulum of the clock, _B_,
which was before kept from swinging, in a position out of the vertical;
the clock then acts as a time-piece, and its motion unrolls a band of
paper, _k k k_, at a rate of three metres an hour. At the same time the
armature of _D_, while attracted, presses a pencil point against the
band of paper which passes over the roller, _m_, marking on it, while
the earthquake lasts, a series of points or strokes which occupy a
length of paper corresponding to its duration, and which record the work
of the shock. After it is over the paper continues to unroll from the
drum, _i_, and passing round the clock, rolls on to the drum, _l_. If a
fresh shock occur the pencil indicates it, as before, on the paper, and
the length of blank paper between the two sets of marks is a measure of
the interval of time between the shocks. By way of additional check,
several helices, _h h h_, are hung from a stand, with small permanent
magnets suspended from their ends; below and close to these latter are
small basins, holding iron filings; into these the points of the magnets
dip, when their helices oscillate vertically, and some filings remain
sticking to the magnets as a record of the shock. One of the magnets has
a shoulder on it which moves an index hand along a graduated arc, as
shown in Fig. 2, thus again registering the amount of the vertical
movement. Such are the arrangements intended for the record of the
undulatory or horizontal elements of the wave of shock.

The following are the arrangements proposed for recording the horizontal
motions: On the stand, to the right of the clock, _A_, are set four
U-shaped glass tubes, open at their ends. One of each pair of vertical
branches must have a diameter at least double that of the other. These
pairs, with their supporting columns, are shown in plan, where one pair
lies N. and S., another E. and W., a third N.E. and S.W., and the other
N.W. and S.E. It will be observed that metallic bars pass from the
pillar, _P_, over the ends of all the long branches, and similar bars
pass from _R_, over the ends of the short branches; the pillars
themselves, as in the case of the other instruments, are each connected
with one pole of a Daniell's battery, the connections including the
electro-magnets, _C_ and _D_. The description of one U tube, _n_, will
apply to all the others; _n_ is partly filled with mercury, and an iron
or platinum wire, _o_, suspended from the bar above the short branch,
dips into the mercury therein, while another platinum wire hung from the
bar over the mouth of the longer branch, has its end very close to the
surface of the mercury in that branch. Any shock which is not
perpendicular in direction to the plane of the branches of the U will
cause the mercury to oscillate in the tubes, and more sensibly in that
with the smaller diameter; when it rises up in the latter, so as to
touch the platinum point, the connection between _P_ and _R_ is made and
the circuit completed, starting the action of the electro-magnets _C_
and _D_, which record the shock, as already described. By having the
planes of the tubes set in the different azimuths, already mentioned,
one or more of the pairs is sure to be acted upon, and by observing in
which the oscillation takes place the direction of the shock is supposed
to be ascertained. Besides this, each long branch of the U, viz., that
of smaller diameter, has a small ivory pulley, _q_, fixed above it, over
which passes a single fibre of silk, with an iron float at one end,
resting on the surface of the mercury; at the other end of the fibre
hangs a counterpoise; fixed to the pulley is a fine index hand, capable
of moving along a graduated arc. When the shock takes place the mercury,
rising in the long branch, raises the float on its surface, the silk
fibre at the same time makes the pulley revolve with its index hand,
which afterwards remains stationary, as the counterpoise prevents the
float from sinking again with the mercury. The reading on the graduated
arc is thus a measure of the movements produced in the instrument by the
horizontal element of the shock, and is supposed to measure that shock.
It is assumed that in all these instruments shocks, however small, can
be recorded with certainty by adjusting the distance between the
platinum points and the mercury.

The arrangement of Daniell's battery used for the seismograph is shown
in Fig. 4, where, for convenience of cleaning, the copper element is
made of wire (about No. 8 Birmingham wire gauge) coiled flat without the
spirals touching. Crystals of sulphate of copper are placed at the
bottom of the outer cell, into which water is poured; and the inner
cell, into which the zinc plate goes, is filled with siliceous sand.

In addition to the above some instruments of a rougher description are
employed as checks. Thus, at the foot of the pillar, _G_, there is a
wooden trough with eight holes, facing as many equidistant points of the
compass (two of them shown in section) round its inner circumference;
mercury is poured into the basin until its level is nearly up to the
lips of the holes. The effect of a shock is to throw some of the mercury
into one or more of these holes, and the greater the oscillation the
more mercury is thrown into the cells through the holes. The screws
shown outside are for drawing off the mercury from the cells, when its
quantity can be measured. The direction of the shock is shown by seeing
which cells are filled with mercury. This is the old Cacciatore
seismometer which has been long employed in Italy. (See 4 "Report of
British Association, 1858," p. 73), and Daubeny's "Volcanoes," Appendix.
The following is another contrivance. From the arm of the pillar, _G_, a
fine metal wire hangs, with a metal ball at its end, which, by its
oscillation, thrusts out one or more light glass tubes, set horizontally
in a stand, as shown in Fig. 3. The two rings are of wood, and the glass
tubes pass through holes in them; small leather washers are placed
outside the outer rings; the displacement of one or more tubes is
assumed to measure the horizontal element of the shock. By means of this
apparatus the time of the first shock is recorded, as well as the
interval between the shocks, and the duration of each; their direction,
whether vertical or horizontal, is given, as also the maximum of
intensity. Professor Palmieri has the instruments examined three times a
day, and an assistant-observer is always at hand to attend to the bell,
and put back the apparatus to its normal position for fresh observation.

It has been stated that this instrument is sensible to most of the
shocks which occur in the Mediterranean basin.

It is not my intention here to offer any criticism as to the
construction or performances of this instrument, the rather as I must
confess I do not quite share the high opinion of its inventor as to the
certainty or exactitude of its indications.

There can be no question as to the extreme importance to science of the
establishment and continued use of a seismographic instrument of
unexceptionable construction at the Observatory upon Vesuvius; and it
would be a valuable gift to science, were the Italian Government to
enable Signor Palmieri to establish such an one. Its great value and the
very first problem to set the instrument to solve should be, by _a rigid
determination of the direction of propagation of the wave of shock_, of
those slight or stronger pulsations which precede or accompany the
Vesuvian like all other eruptions, on arriving at the Observatory, _to
fix the depth, and the position vertically beneath the cone, whence
these pulses are derived_. This would be, in fact, to fix the depth and
position beneath the mountain at which the volcanic focus is situated
for the time, or, at least, where the volcanic activity is at the time
greatest. And the assured knowledge, even within moderate limits of
accuracy, of this depth, and even for this single mountain, would be an
immense accession to our positive knowledge, and a really new stage
gained for future advances. At present, we know but little as to the
actual depth below our globe's surface at which volcanic activity
occurs, or to which it is limited, either upwards or downwards. I have,
myself, established some data upon the flanks of Etna, not yet
published, which may enable me to afford some information on the subject
hereafter. Meanwhile, Professor Palmieri possesses unrivalled
opportunities for such observations; and I trust health, life and means
may be afforded him, to become the first who shall have made this great
addition to our positive knowledge of Vulcanology.

So far, popularly at least, the alleged chief uses and value of these
seismographic instruments, at the Observatory of Vesuvius, have been
made to depend upon their being presumed to afford means for foretelling
eruptions, or affording precursory warnings of their probable progress
and destructive course.

I feel compelled to express my own total disbelief in the possibility of
any such predictions in the present state of science, by the help of any
instruments whatsoever, of such a nature as to be of any _practical
value_, or any certainty beyond that which a certain amount of _mere
experience_ as to the _rôle commonly played_ by Vesuvius or other
Volcanoes in pretty habitual activity affords to the observer for a
lengthened period. And even this affords scarcely any guide as to what
may happen next. Monte Nuovo was thrown up in a night; Vesuvius _might_
double its volume in a night, or might sink into a hollow like that of
the Val del Bove in a not much longer time. A small _fusillade_ may go
on for months, and yet, without an hour's notice, by any premonitory
sign, may waken up to a roar and darken the air with ashes and lapilli
such as those which overwhelmed Pompeii. One eruption may blow forth
little but dust and ashes (so called), another may pour out rivers of
lava and little else.

The _main_ mischief of all eruptions is effected in two ways: by the
deposit of dust and ashes, lapilli, etc., to the injury or destruction
of fertile land, and by the streams of lava which overwhelm it, as well
as buildings, etc. But what information of any value can seismographic
observation afford as to the course that either of these may take in any
eruption? The volume of pulverulent material that may be ejected cannot
be foreseen; its distribution depends mainly upon its nature and upon
the direction and force of the wind at the time; or again, how shall
these warn us as to the course that the lava, if it appear, shall take,
when we cannot possibly foretell when, how, or by what mouth it may
issue. Even in this late eruption of 1872, with Palmieri stoutly at his
post upon the mountain, and the Observatory instruments in full
activity, they gave no forewarning of the sudden and unexpected belch
forth from the base of the cone, of that tremendous gush of liquid lava
which in a few minutes cut off from life the unhappy visitors whose
deaths he has recorded.

[2] (P. 94). It can scarcely be supposed that these small
eruptive-looking belchings forth from the lava stream, _en route_, are
truly of an eruptive nature at all, _i.e._, in any way connected with
forces seated deeply beneath the bed of the lava stream, or in any way
connected with the volcanic ducts of the cone or beneath it. They are
most probably merely the bursting upwards of large bubbles; that is, of
cavities formed in the mass of the more or less liquid lava by intestine
movements, as its mass winds and rolls along, and by the aggregation of
smaller cavities--all being filled with steam and gases--together with
dust and volatile products which are ejected when the cavity opens up,
and its contents escape at the upper surface of the lava stream in
virtue of the continuation of the twistings and convolutions due to the
stream motion itself, and to the unbalanced hydrostatic pressures acting
upon the parietes of the bubble. Very large single bubbles of like
character rise in the fluid lava within craters in vigorous action, and
often so regularly that their recurrence causes a sort of rhythmical
rush and roar in the column of steam, etc., issuing above the mouth.
This was evident in the discharges issuing in 1857 from the highly
instructive minor _bocca_, then existing, examined by me, and referred
to ("Report, Naples, Earthquakes," etc. Vol. II., pp. 313, 314), as
presenting at the time great facilities for determining pyrometrically
the temperature of the lava within, and of the dry superheated steam
issuing with a rhythmic roar from it. M. Le Coq ("Époques Géologiques
d'Auvergne," Tome IV.) has recorded some examples of the formation and
opening-out of large bubble-like cavities in lava already ejected.
Perhaps that able and laborious vulcanologist, whose death a few months
ago science still deplores, attributes too much importance as well as
magnitude to them, when attributing the formation of what he has
denominated "craters of explosion," to the mechanism of the rise and
bursting of such bubbles upon a gigantic scale. Such blowings forth,
sudden or prolonged, from particular spots of lava streams, _en route_,
undoubtedly may also have their origin in damp places, or water or
air-filled cavities in or beneath the bed over which the lava rolls,
which, getting gradually heated, generate steam, or air or gases under
tension by expansion, etc., which thus at length blow through the liquid
or pasty lava flowing above, and which in bursting through delivers much
dust also, and so simulates a little eruptive crater. Examples of this,
upon a great and convincing scale, can be pointed to in the Val di
Calanna and elsewhere on Etna.

[3] (P. 96). There are strong grounds for the gravest doubts that there
exists any real connection of a physical character between Volcanic
Eruptions, and Earthquakes more or less _approximately_ coincident only,
in time of occurrence; the respective sites being widely apart, and the
less the probability as the intervening distance is greater. The
discussions of the large number of records that are to be found of such
coincidences--mostly but partial, and in but _very_ few instances
complete coincidences--by Perrey, von Hoff, and others, as well as by
myself, do not tend to sustain the view that such imperfect
contemporaneity is based upon any causative connection. The seismic
region of Greece appears to have no _direct_ connection with that of
Southern Italy: the band of connection, if any, seems to lie between
Northern Italy, across the Northern Adriatic, by Ragusa, and thence
spreading into Asia Minor.

[4] (P. 97). The abundance of coleoptera and of various other forms of
insect life about lava beds, both recent and old, is a very singular
fact, and one worthy of the careful observation of entomologists. In the
autumn of 1864, at mid-day, when sitting sketching upon the lava about
the middle of the Val del Bove (Etna), I found it almost impossible to
work, or even to remain for an instant still, in consequence of the
continual cloud of insects, large and small, that struck against me in
flight, endangered the eyes, and swarmed upon my clothes. It is quite
possible that this local superabundance of insect life may arise merely
from the general dryness and warmth of such places, and the plentiful
_nidus_ that the innumerable cavities in lava afford for the eggs and
earlier stages of insect life; still, this apparition of one form of
life may also be connected with other circumstances not unimportant to
discover.

[5] (P. 120). The _Crocella_ is a small wooden cross, erected several
years ago, and which one passes to the right hand at the upper end of
the path along the ridge of tufa and volcanic conglomerate upon which
the Observatory stands, in ascending thence to the Atria del Cavallo.

[6] (P. 134). That the causes assigned by Professor Palmieri for the
potent developments of electricity (positive or negative) which
characterise the ascent of the issuing columns of (chiefly if not
always) _dry_ steam, with a relatively small volume of various gases,
and throwing up, in their blast, volumes of small solid particles in
ashes and lapilli, etc., and the subsequent fall as a mineral or stony
hail-shower of the latter, through the partially condensing vapours and
the circumambient air, are the main causes of electrical development
evidencing itself in lightning flashes, is no doubt true. We must not,
however, lose sight of the many other and very effective agencies at
work here to produce electric excitement. The actual _bocca_ of the
volcanic vent whence the steam roars off constitute the cone a veritable
hydro-electric machine. Mechanical energy in various forms is
transformed into electric energy. Chemical action is going on both in
the solid and in the vapourous and gaseous emanations as they rush into
and remain in the air or descend from it, and chemical action is
transformed in part into electric energy. Percussion between ascending
and descending particles and fragments, fractures and breaking up of
more or less of these, thus and by sudden changes of temperature in
cooling, are likewise operative. In addition, great and violent
movements in the atmosphere itself result from the large local
accessions of temperature by the heated volume driven up into it, and
which in turn give rise to electric disturbance of the same character as
those produced in wind storms and whirlwinds, brought about by the
natural causes which every day effect disturbances in our atmosphere all
over the globe.

[7] (P. 135). The views stated in note 3 (to page 96) may here again be
referred to as in point. How is it possible, in the present state of
science at least, to establish any physical connection between an
eruption in Java and one of Vesuvius, "with half the world between,"
when not even having the solitary connecting link of complete
contemporaneity, and which, if it existed, yet might be nothing but
accidental? A list of shocks upon record, which have occurred more or
less nearly simultaneously at distant parts of the world, may be found
in my fourth Report, ("Facts of Earthquakes," "British Association
Reports, 1858") and the reasons are there given for rejecting the notion
of any direct physical connection between the origins of the respective
shocks.

Shocks, emanating from the close neighbourhood of volcanic vents, or
simultaneity of eruption, in vents not far distant from each other,
stand upon a different footing.

Transcriber's Notes.

Preserved the unusual, but consistent, spelling of "develope."

Preserved the unusual, and inconsistent, references to the Plates.
Sometimes Arabic numerals are used, but usually Roman numerals. Most
have "A" attached to the name (which was changed to "a" to avoid
confusion in the Roman numerals), but not VIII, and sometimes not in the
references to them.

Equations were converted to linear text, adding spacing and parentheses
as necessary.

Changed "fumarolles" to "fumaroles" on page 3: "fumaroles and
salfatares."

Changed "Lyall" to "Lyell" in footnote originally on page 9: "Daubeny,
Lyell, Phillips and others."

Changed "throught" to "through" on page 35: "passing through it."

Removed duplicated word "the" on page 40: "the great Calabrian
Earthquake."

Page 89 refers to the "Fossa del Vetrano;" elsewhere there are
references to "Fossa della Vetrana," which may be what was intended.
However, I did not change this.

Changed "hydrochloride" to "hydrochloric" on page 109: "the hydrochloric
acid which it discharged."

Changed "disk" to "disc" on page 126: "the disc and index."

Changed "azismuths" to "azimuths" on page 143: "the different azimuths."

Changed "silicious" to "siliceous" on page 143: "filled with siliceous
sand."

End of Project Gutenberg's The Eruption of Vesuvius in 1872, by Luigi Palmieri