_c._ A kitten of about two months old, was introduced into a jar of nitrous oxide, at the same time that another of the same breed was plunged under a jar of water. They both struggled very much. The animal in the nitrous oxide fell senseless before that under water had ceased to struggle, and to throw out air from its lungs. In two minutes and three quarters, the animal under water was quite dead: it was taken out and exposed to heat and air, but did not shew the slightest signs of life. At the end of three minutes and half, the animal in nitrous oxide began to gasp, breathing very slowly; at four minutes and three quarters it was yet alive; at the end of five minutes and quarter it appeared perfectly dead. It was taken out, and did not recover. From these experiments it was evident, that animals lived at least twice as long in nitrous oxide as in hydrogene or water. Consequently from this circumstance alone, there was every reason to suppose that their death in nitrous oxide could not depend on the simple privation of atmospheric air; but that it was owing to some peculiar changes effected in the blood by the gas. IV. _Of the changes effected in the Organisation of warm-blooded Animals, by the respiration of Nitrous Oxide._ The external appearance of animals that have been destroyed in nitrous oxide, is very little different from that of those killed by privation of atmospheric air. The fauces and tongue appear of a dark red, and the eyes are dull, and a little protruded. Their internal organs, however, exhibit a very peculiar change. The lungs are pale brown red, and covered here and there with purple spots; the liver is of a very bright red, and the muscular fibre in general dark. Both the auricles and ventricles of the heart are filled with blood. The auricles contract for minutes after the death of the animal. The blood in the left ventricle, and the aorta, is of a tinge between purple and red, whilst that in the right ventricle is of a dark color, rather more purple than the venous blood. But these appearances, and their causes, will be better understood after the following comparative observations are read. _a._ Of two similar rabbits, about eight months old, one A, was killed by exposure for near six minutes to nitrous oxide, the other, B, was destroyed by a blow on the head.