This “Dog Grotto” has been so much cited for its stratum of
carbonic-acid gas covering the floor, that all geological travellers
who visit Naples feel an interest in seeing the wonder.
This cavern was known to Pliny. It is continually exhaling from its
sides and floor volumes of steam mixed with carbonic-acid gas; but the
latter, from its greater specific gravity, accumulates at the bottom,
and flows over the step of the door. The upper part of the cave,
therefore, is free from the gas, while the floor is completely covered
by it. Addison, on his visit, made some interesting experiments. He
found that a pistol could not be fired at the bottom; and that on
laying a train of gunpowder and igniting it on the outside of the
cavern, the carbonic-acid gas “could not intercept the train of fire
when it once began flashing, nor hinder it from running to the very
end.” He found that a viper was nine minutes in dying on the first
trial, and ten minutes on the second; this increased vitality being,
in his opinion, attributable to the stock of air which it had inhaled
after the first trial. Dr. Daubeny found that phosphorus would continue
lighted at about two feet above the bottom; that a sulphur-match went
out in a few minutes above it, and a wax-taper at a still higher level.
The keeper of the cavern has a dog, upon which he shows the effects of
the gas, which, however, are quite as well, if not better, seen in a
torch, a lighted candle, or a pistol.
“Unfortunately,” says Professor Silliman, “like some other grottoes,
the enchantment of the ‘Dog Grotto’ disappears on a near view.” It is a
little hole dug artificially in the side of a hill facing Lake Agnano:
it is scarcely high enough for a person to stand upright in, and the
aperture is closed by a door. Into this narrow cell a poor little dog
is very unwillingly dragged and placed in a depression of the floor,
where he is soon narcotised by the carbonic acid. The earth is warm to
the hand, and the gas given out is very constant.