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.