Such are the _mephitical_ Exhalations in a little Cavern in _Italy_, call’d _Bacca Venosa_, the poisonous Mouth, not far from _Naples_, but more generally known by _Grotto de Cani_, the _Dog’s Cave_; because the Experiment of its poisonous Nature is frequently made upon Dogs; tho’ it be fatal as to any other Creatures that come within the Reach of its venemous Fumes. This wonderful Cave is (situated at the Bottom of a Hill) in Dimension, about eight Foot high, twelve long, and six broad. From the Ground within it, arises _a thin warm Fume_ (visible to the Eye) which is one continued Steam covering the whole Surface of the Bottom of the Cave; it does not disperse itself into the Air like Smoke, but quickly after its Rise, falls back again into the Earth. The Fumes rise about a Foot high and never higher, and hurt no Creature whose Head is above that height; but when a little Dog, or the Head of any other Creature is forcibly held in the Steam, it falls down dead, the Limbs convuls’d, and trembling; and if left there a little while, it dies, but if taken out soon, and laid in the open Air, comes to Life again, and sooner, if thrown into the adjacent Lake. _CHARLES_ the Great, King of _France_, and Emperor of the _West_ (a Title of Honour other _Gallick Monarchs_ have had in full view for some time) made the Experiment upon an _Ass_, whose Head was held in the Fume, and was soon destroy’d. _Two Slaves_ put in with their Heads kept down to the Earth, were both soon kill’d. To this, I shall add some Experiments made by the ingenious Mr. _Addison_, who says—— —“A Dog that has his Nose held in the Vapour (within the Cave) loses all the Signs of Life in a very little time.” Then he observes, how long a Dog was expiring the first time, and after his Recovery, and found no sensible Difference. “A _Viper_ put in, adds he, bore it nine Minutes the first time we put it in, and ten the second. When we brought it out after the first Trial, it took such a vast quantity of Air into its Lungs, that it swell’d almost twice as big as before, and it was perhaps, on this Stock of Air, that it lived a Minute longer. “A Torch, Snuff and all, goes out in a moment, when dipt into the Vapours or Steams of that Cave——A Pistol can’t fire in it. I split a Reed, and laid in the Channel of it a Train of Gun-powder, so that one end of the Reed was above the Vapour, and the other at the bottom of it; and I found, tho’ the Steam was strong enough to hinder a Pistol from taking fire in it—that it could not intercept the Train of Fire, when it once begun flashing, nor prevent it from running to the very end—fire will live in it no longer than in Water, because it wraps itself in the same manner about the Flame, and by its Continuity hinders any quantity of Air, or Nitre from coming to its succour[44].” Footnote 44: _His Works_, vol. iii. p. 8, 97.