“Give me, ye powers, the wondrous scenes to show,
Concealed in darkness in the caves below.”
Among the various subterranean wonders of the world, which are worthy of
special notice, we would first mention the “Grotta del Cane.” This name
has been given to a small cavern between Naples and Pozzuoli, on this
account, that if a dog be brought into it, and his nose held to the
ground, a difficulty of respiration instantly ensues, and he loses all
sensation and even life, if he be not speedily removed into purer air.
There are other grottos endowed with the same deleterious quality,
especially in volcanic countries; and the pestiferous vapors they
exhale, are quickly fatal both to animals and man, though they do not
offer to the eye the slightest indication of their presence. These
vapors are, however, for the greater part temporary; while that of the
Grotta del Cane is perpetual, and seems to have produced its deadly
effects even in the time of Pliny. A man standing erect within, does not
suffer from it, the mephitic vapor rising to a small hight only from the
ground. It may, therefore, be entered without danger.
The smoke of a torch extinguished in this vapor, or gas, sinks downward,
assumes a whitish color, and passes out at the bottom of the door. The
reason of this is, that the fumes which proceed from the torch mix more
readily with the gas than with the atmospherical air. It has been
supposed, that the mischievous effects of the vapor were the result of
the air being deprived of its elasticity; but it has been clearly
demonstrated by M. Adolphus Murray, that they are solely to be
attributed to the existence of carbonic acid gas.
The person who is the keeper, or guide, at the grotto, and who shows to
strangers the experiment of the dog for a gratuity, takes the animal,
when he is half dead and panting, into the open air, and then proceeds
to throw him into the neighboring lake of Agnano, thus insinuating that
this short immersion in the water is necessary to his complete
restoration. This, however, is a mere trick, to render the experiment
more specious, and to obtain a handsome present from the credulous, the
atmospherical air alone sufficing for that purpose.
The celebrated naturalist, the Abbe Spallanzani, projected a regular
series of experiments on the mephitic vapor of this grotto, from a
persuasion that they would tend to throw a new light on physiology and
natural philosophy. Being, however, prevented from undertaking this, by
his duties as a professor, his friend, the Abbe Breislak, who resided
near the spot, engaged in the task; and the following is an abstract of
his learned memoir on this subject.
It is well known, the abbe observes, that the mephitic vapor occupies
the floor of a small grotto near the lake Agnano, a place highly
interesting to naturalists from the phenomena its environs present, and
the hills within which it is included. This grotto is situated on the
south-east side of the lake, at a little distance from it. Its length is
about twelve feet, and its breadth from four to five. It appears to have
been originally a small excavation, made for the purpose of obtaining
pozzuolana, an earth which, being applied as mortar, becomes a powerful
cement. In the sides of the grotto, among the earthy volcanic matters,
are found pieces of lava, of the same kind with those which are met with
scattered near the lake.
The abbe is persuaded that, if new excavations were to be made in the
vicinity of the grotto, at a level with its floor, or a little lower,
the same mephitic vapor would be found; and thinks it would be curious
to ascertain the limits of its extent. It would also be advantageous to
physical observations, if the grotto were to be somewhat enlarged, and
its floor reduced to a level horizontal plane, by sinking it two or
three feet, and surrounding it by a low wall, with steps at the
entrance. In its present state it is extremely inconvenient for
experiments, and the inclination of the ground toward the door causes a
great part of the vapor, from the effect of its specific gravity, to
make its way out close to the ground.
When the narrow limits of this place are considered, and the small
quantity of the vapor which has rendered it so celebrated, there can not
be any doubt but that it has undergone considerable changes; since it
does not appear probable that Pliny refers to the present confined vapor
only, when, in enumerating many places from which a deadly air exhaled,
he mentions the territory of Pozzuoli. The internal fermentations by
which it is caused, are certainly much diminished in the vicinity of the
lake Agnano. The water near its banks is no longer seen to bubble up,
from the disengagement of a gas, as it appears from accounts, not of
very remote antiquity, to have done. The borders of the lake were
attentively examined by the abbe, when its waters were at the highest,
and after heavy rains; but he could never discover a single bubble of
air. A number of aquatic insects which sport on the surface, may at
first sight occasion some deception; but a slight observation soon
detects the error. If, therefore, we do not suppose those authors who
have described the ebullition of the water near the banks of the lake
Agnano to have been deceived, it must at least be confessed, that this
phenomenon has now ceased. The quantity of the sulphureous vapors which
rise in the contiguous stoves, called the stoves of St. Germano, must
likewise be greatly diminished from what it anciently was: for,
adjoining to the present stoves, we still find the remains of a spacious
ancient fabric, with tubes of _terra cotta_ inserted in the walls,
which, by their direction, show for what purpose they were intended. It
appears certain, that this was a building in which, by the means of
pipes properly disposed, the vapors of the place were introduced into
different rooms for the use of patients. To these ruins, however, the
vapors no longer extend; so that, if this edifice had remained entire,
it could not have been employed for the purpose for which it was
intended. The veins of pyrites which produced the more ancient
conflagrations of the Phlegrean fields, between Naples and Cuma, and
which, in some places, are entirely consumed, approach their total
extinction. We will now proceed to the experiments within the grotto.
The object of the first was to determine the hight of the mephitic vapor
at the center of the grotto, that is, at the intersection of the line of
its greatest length with that of its greatest breadth. The hight varies
according to the different dispositions and temperatures of the
atmosphere, the diversity of winds, and the accidental variations which
take place in the internal fermentations by which the vapor is produced.
It may, however, be estimated at a mean, at nearly nine English inches.
The second set of experiments regarded the degree of heat on entering
into the mephitis: it was slightly sensible in the feet and lower part
of the legs; notwithstanding which, on taking out of the vapor several
substances which had remained in it for a long time, such as stones,
leaves, the carcasses of animals, &c., the abbe found that these were of
the same temperature with the atmospheric air. Feeling in his body a
slight degree of heat, which he could not perceive in the substances
removed from the mephitic vapor, he was led by comparison to conclude,
that the temperature of the latter was the same with the atmospherical
air, agreeably to the principles of Dr. Crauford. He was, however,
mistaken; for in subsequent experiments, he found a very distinct degree
of heat. He was now provided with a thermometer, his former one having
been broken, and, having suspended it at the aperture of the grotto,
three feet above the surface of the vapor, found the mercury to stand at
from sixty-two to sixty-four degrees of Fahrenheit; but, on placing the
ball on the ground so as to immerse it in the vapor, the mercury rose to
eighty, and even eighty-two degrees. That the substances taken out of
the mephitis did not exhibit this diversity of temperature, was, he
thinks, owing to the quantity of humidity with which they are always
loaded, and which produces on their surface a constant evaporation. He
was the more particular in repeating these experiments, because the
naturalists who had, before him, made similar ones in the Grotta del
Cane, had not observed the vapor to produce any effect on the mercury in
the thermometer.
Thirdly. He repeated for his own satisfaction, the usual experiments
made by naturalists, with the tincture of turnsole, lime-water, the
crystallizations of alkalies, the absorption of water, and the acidulous
taste communicated to it; which prove, beyond all doubt, the existence
of fixed air, or carbonic acid gas, in the vapor of the grotto. He
ascertained that it was not formed of fixed air alone, as might have
been conjectured; but that the relative quantities of the different
gases which compose its mephitic air, are as follows: in one hundred
parts there are ten of vital air, or oxygen gas; forty of fixed air, or
carbonic acid gas; and fifty of phlogisticated air, or azotic gas.
Fourthly. The phenomena of magnetism and electricity were investigated
by the abbe in this grotto. With respect to the former, there was not
any new appearance: the magnetic needle, being placed on the ground, and
consequently immersed in the mephitis, rested in the direction of its
meridian, and, at the approach of a magnetized bar, exhibited the usual
effects of attraction and repulsion, in proportion as either pole was
presented. As to the latter, electricity, it was impossible to make the
experiments within the mephitis, not because this kind of air is a
conductor of the electric fluid, as has been imagined, but because the
humidity by which it is constantly accompanied, disperses the electric
matter; and this, not being collected in a conductor, can not be
rendered sensible. He attempted several times to fire inflammable gas,
with electric sparks, in the mephitic vapor, by means of the conductor
of the electrophus; but, notwithstanding his utmost endeavors to animate
the electricity, he could never obtain a single spark, the non-conductor
becoming a conductor the moment it entered into the mephitis, on account
of the humidity which adhered to its surface.
Fifthly. His latest experiments were directed to the theory of the
combustion of bodies. He first endeavored to ascertain whether those
spontaneous inflammations that result from the mixture of concentrated
acids with essential oils, could be obtained within the grotto. He
placed on the ground a small vessel, in such a situation that the
mephitis rose six inches above its edges, employing oil of turpentine,
and the vitriolic and nitrous acids: the same inflammation, accompanied
by a lively flame, followed, as would have taken place in the open
atmospheric air. The dense smoke which always accompanies these
inflammations, being attracted by the humidity of the mephitis,
presented its undulations to the eye, and formed a very pleasing object.
As he had put a considerable quantity of acid in the vessel, he
repeatedly poured in a little of the oil, and the flame appeared in the
mouth of the vessel fifteen times successively. The oxygenous principle
contained in the acids, and with which the nitrous acid principally
abounds, undoubtedly contributed to the production and duration of this
flame, though enveloped in an atmosphere inimical to inflammation.
The abbe had, in the district of Latera, observed that in a mephitis of
hydrogenous sulphurated or hepatic gas a slow combustion of phosphorus
took place, with the same resplendence as in the atmospheric air. On the
present occasion, his first experiment, in the mephitis of Agnano, was
made with common phosphoric matches, five of which he broke, holding
them to the ground, and consequently immersed in the mephitis. They
produced a short and transient flame, which became extinguished the
moment it was communicated to the wick of a candle. His second
experiment was as follows: he placed on the ground, within the grotto, a
long table, in such a manner that one extremity was without the
mephitis, while the other, and four-fifths of its length, were immersed
in it. Along this table he laid a train of gunpowder, beginning from the
end without the mephitis; and, at the other end, which was immersed in
it to the depth of seven inches, he placed, adjoining to the gunpowder,
a cylinder of phosphorus, eight lines in length. The gunpowder, without
the mephitis, being fired, the combustion was soon communicated to the
other extremity of the train, and to the phosphorus, which took fire
with decrepitation, burned rapidly with a bright flame, slightly colored
with yellow and green, and left on the wood a black mark, as of
charcoal. The combustion lasted nearly two minutes, when the whole
phosphoric matter was consumed.
In succeeding experiments not any alteration was perceptible in the
flame, or manner of burning, of the lighted phosphorus, either at the
moment of its entrance into the mephitis, or during its continuance in
it. When suddenly withdrawn, it ignited gunpowder equally well. Hence
the abbe deduces, that the mephitic gas of Grotta del Cane, however it
may be utterly unfit for the respiration of animals, and for the
inflammation of common combustible substances, readily allows that of
phosphorus, which not only burns in it, but emits, as usual, luminous
sparks.
[Illustration: GROTTO OF ANTIPAROS.]