’Tis a small _Grotta_ at the Foot of a little Hill, about Eight Foot high, Twelve long, and Six broad; from the Ground arises a thin, subtle, warm _Fume_, visible enough to a discerning Eye, which does not spring up in little parcels here and there, but is one continued Steam, covering the whole Surface of the bottom of the Cave; and has this remarkable difference from common Vapours, that it does not, like Smoak, disperse it self into the Air, but quickly after its rise falls back again, and returns to the Earth; the Colour of the sides of the _Grotta_ being the measure of its Ascent; for so far it is of a darkish Green, but higher, only common Earth, and this is about Ten Inches. And therefore as my self found no Inconvenience by standing in it, so no Animal if its Head be kept above this Mark is in the least injured: But when (as the manner is) a Dog, or any other Creature, is forcibly held below it, or by reason of its smalness can’t hold its Head above it, It presently, like one stunn’d, loses all Motion, falls down as Dead, or in a Swoon, the Limbs convuls’d and trembling, till at last no more sign of Life appears than a very weak and almost Insensible beating of the Heart and Arteries, which if the Animal be left there a little longer, quickly ceases too, and then the Case is Irrecoverable; But if snatch’d out, and laid in the open Air, soon comes to Life again, and sooner if thrown into the adjacent _Lake_. In this short, but accurate, History of the _Grotta de Cani_, I have set dow those Particulars which do not only distinguish _Mephitical_ Exhalations from common and innocent Fumes, but also give hints sufficient, I think, Mechanically to determine the Reason and Manner of their surprising Effects. And not to spend time in refuting the Opinions of Others, I shall only take Notice, that here can be no suspicion of any true _Venom_ or real Poison; if there was, it were impossible that Animals taken out of the _Grotta_, should so immediately recover the Effects of it, without any remaining appearance of Faintness and Sickness, or such like Symptoms as those suffer who have been breathing in an Air impregnated with malignant corrosive _Effluvia_. Besides, that the Venomous Corpuscles would certainly, in some Degree at least, infect the Air in the upper Part of the Cave, which continues pure, and fit for Respiration. Neither indeed after what manner soever this Poison be imagin’d to Act, whether by dissolving or coagulating the Blood, could its Efficacy be so sudden and momentaneous, without some Marks of it in the Creatures kill’d, when opened, which yet do discover nothing of this Nature extraordinary, neither in the Fluid, nor in the Solid Parts.