This is a little cavern near Poz-zuoli, four leagues from Naples: the air contained in it is of a mephitical or noxious quality; it is in truth carbonic acid gas, whence also it is called Bocca Venenosa, the Poisonous Mouth. "Two miles from Naples, (says Dr. Mead,) just by the Lago de Agnano, is a celebrated mofeta, commonly called La Grotto del Cani, whir h is destructive of all animal life that comes within the reach of its vapours. It is a small grotto, about eight feet high, twelve long, and six broad; from the ground arises a thin, subtile, warm fume, visible enough to a discerning eye, which does not spring up in little parcels here and there, but in one continued stream, covering the whole surface of the bottom of the cave; having this remarkable difference from common vapours, that it does not, like smoke, disperse into the air, but quickly after its rise falls back again, and returns to the earth, or hovers to a certain height, above which it never rises. This fact is ascertained by the colour of the sides of the grotto, which, as high as the vapour ascends, is of a darkish green, but above this it has only the appearance of common earth. As I found no inconvenience from standing in it myself, so I believe no animal, if its head were kept above this mark, would be in the least injured. But when, as is often the case, a dog, or any other creature, is forcibly kept below it, or the animal is so small that it cannot hold its head above this noxious vapour, it presently loses all voluntary motion, falls down as dead, or in a swoon; the limbs at first become convulsed and trembling, till at last no more signs of lite appear, than a very weak and almost insensible beating of the heart and arteries; which, if the animal is left a little longer, quickly ceases also, and then its fate is irrevocable; but if it be snatched out and laid in the air, it soon revives, and, if thrown into the adjacent lake, resuscitation is still more rapid."