Most of the animals destroyed in these experiments were examined after death; the appearances in their organs were peculiar. To prevent unnecessary repetitions, an account of them will be given in the fourth section. III. _Effects of the respiration of Nitrous Oxide upon animals, as compared with those produced by their immersion in Hydrogene and Water._ Before the following experiments were made, a number of circumstances had convinced me that nitrous oxide acted on animals by producing some positive change in their blood, connected with new living action of the irritable and sensitive organs, and terminating in their death. To ascertain however, the difference between the effects of this gas and those of hydrogene and non-respirable gases, I proceeded in the following way. _a._ Of two healthy rabbits of about two months old, of the same breed, and nearly of the same size. One was introduced into nitrous oxide. In a half a minute, it had fallen down apparently senseless. On applying my hand to the thorax, the action of the heart appeared at first, very quick and strong, it gradually became weaker, and in two minutes and half, the animal was taken out quite dead. The other was introduced into a jar of pure hydrogene through water. He immediately began to struggle very much, and in a quarter of a minute fell on his side. On feeling the thorax, the pulsations of the heart appeared very quick and feeble, they gradually diminished; his breathing became momentarily shorter, and in rather more than three quarters of a minute, he was taken out dead. Dr. Kinglake was present at this experiment, and afterwards dissected both of the animals. _b._ Of two similar rabbits of the same breed, nearly three months old. One was introduced into nitrous oxide, and after being rendered senseless by the respiration of it for nearly a minute and half, was exposed to the atmosphere, before a warm fire. He recovered gradually, but was occasionally convulsed, and had a paralysis of one of his hinder legs for some minutes: in an hour he was able to walk. The other, after being immerged in hydrogene for near half a minute, was restored to the atmosphere apparently inanimate. In less than a minute he began to breathe, and to utter a feeble noise; in two minutes was able to walk, and in less than three minutes appeared perfectly recovered.