I soon learnt from Mr. D. himself the course of his experiments; many of which were in the highest degree hazardous, when, with friendly earnestness, I warned him against his imminent perils. He seemed to act, as if in case of sacrificing one life, he had two or three others in reserve on which he could fall back in case of necessity. He occasionally so excited my fears that I half despaired of seeing him alive the next morning. He has been known sometimes to breathe a deadly gas, with his finger on his pulse, to determine how much could be borne, before a serious declension occurred in the vital action. The great hazards to which he exposed himself may be estimated by the following slight detail. Dr. Mitchell, as well as Dr. Priestley, had stated the fatal effects on animal life, of the gazeous oxide of azote; Mr. Davy, on the contrary, for reasons which satisfied himself, thought it respirable in its pure state; at least, that a single inspiration of this gas might neither destroy, nor materially injure the powers of life. He tried one inspiration. No particularly injurious effects followed. He now breathed, out of his _green bag_, three quarts of this nitrous oxide (gazeous oxide of azote,) when it was attended with a degree of giddiness, great fulness in the head, and with loss of distinct sensation and voluntary power, analogous to intoxication. Not being able fully to determine whether the gas was "stimulant" or "depressing," he now breathed four quarts of it from his _green bag_, when an irresistible propensity to action followed, with motions, various and violent. Still, not being satisfied, he proceeded in his experiments, and at length found that he could breathe nine quarts for three minutes, and twelve quarts for rather more than four, but never for five minutes, without the danger of fatal consequences, as before five minutes had expired, the mouth-piece generally dropped from his unclosed lips. By breathing from six to seven quarts only, muscular motions were produced, and he manifested the pleasure it excited, by stamping, laughing, dancing, shouting, &c.