The little dog--he was hardly better than a puppy--looked at me and wagged his tail hopefully. I understood him perfectly. He had detected my nationality; and I resolved to be no less humane than a countrywoman of my own who visited this grotto no great while ago, and who, when asked by the brigand whether he should put the dog in, answered hastily, "Certainly not." "Ah!" said the guide, "you are Englees! If you had been American you would have said, 'Why, certainly.'" I made the same condition. The fellow shrugged his shoulders. He did not care, he knew another way of extorting as many francs from me; and accordingly we all went gaily down the hill, preceded by the happy cur, running on with tail erect, till we reached a gate in the wall through which we passed to the Grotta del Cane. A low entrance, hardly more than a man's height, a long tubular passage of uniform dimensions sloping backwards into the bowels of the hill--such is all one sees on approaching the Dog Grotto. A misty exhalation rises from the floor and maintains its level while the ground slopes downwards. Thus, if a man entered, the whitish vapour would cling at first about his feet. A few steps further would bring it to his knees, then waist high, and in a little more it would rise about his mouth and nostrils and become a shroud indeed; for the gas is carbonic acid, and destroys all human life. King Charles the Eighth of France, who flashed across the sky of Naples as a conqueror, came here in the short space of time before he left it as a fugitive, bringing with him a donkey, on which he tried the effects of the gas. I do not know why he selected that animal; but the poor brute died. So did two slaves, whom Don Pietro di Toledo, one of the early Spanish viceroys, used to decide the question whether any of the virtue had gone out of the gas. That question is settled more humanely now. The guide takes a torch, kindles it to a bright flame, and plunges it into the vapour. It goes out instantly; and when the act has been repeated some half-dozen times the gas, impregnated with smoke, assumes the appearance of a silver sea, flowing in rippling waves against the black walls of the cavern.