A butterfly was introduced into a small jar, filled with pure nitrous oxide, over mercury, He struggled a little during the first two or three seconds; in about seven seconds, his legs became convulsed, and his wings were wrapt round his body; in about half a minute he was senseless; taken out after six minutes, he did not recover. Another butterfly introduced into hydrogene, became convulsed in about a quarter of a minute, was senseless in twenty seconds, and taken out after five minutes, did not revive. A large drone, after being in nitrous oxide for a minute and a quarter, was taken out senseless. After being for some time exposed to the atmosphere, he began to move, and at last rose on his wings. For some time, however, he was unable to fly in a straight line; and often after describing circles in the air, fell to the ground as if giddy. A large fly, became motionless in nitrous oxide after being convulsed, in about half a minute. Another was rendered senseless in hydrogene, in less than a quarter of a minute. A fly introduced into hydrocarbonate, dropt immediately senseless; taken out after about a quarter of a minute, he recovered; but like the fly that had lived after exposure to nitrous oxide, was for some time vertiginous. Flies live much longer under water, alcohol, or oil, than in non-respirable gases, or gases incapable of supporting life. A certain quantity of air always continues attached in the fluid to the fine hairs surrounding their breathing holes, sufficient to support life for a short time. Snails and earth-worms, live in nitrous oxide a long while, they die in it however, much sooner than in water or hydrogene; probably from the same causes as the amphibious animals. DIVISION II. _Of the CHANGES effected in NITROUS OXIDE, and other GASES, by the RESPIRATION of ANIMALS._ I. _Preliminaries._ As soon as I had discovered that nitrous oxide was respirable, and possessed of extraordinary powers of action on living beings, I was anxious to be acquainted with the changes effected in it by the venous blood. To investigate these changes, appeared at first a simple problem; I soon however found that it involved much preliminary knowledge of the chemical properties and affinities of nitrous oxide. After I had ascertained by experiments detailed in the preceding Researches, the composition of this gas its combinations, and the physical changes effected by it in living beings, I began my enquiry relating to the mode of its operation. Finding that the residual gas of nitrous oxide after it had been breathed for some time in silk bags, was chiefly nitrogene, I at first conjectured that nitrous oxide was decomposed in respiration in the same manner as atmospheric air, and its oxygene only combined with the venous blood; the following experiments soon however convinced me of my error. II. _Absorption of Nitrous Oxide by venous blood. Changes effected in the blood by different Gases._