It seems therefore reasonable to suppose, that the whole compound atmospheric air passing through the moist coats of the vessels is first dissolved by the serum of the venous blood, and in its condensed state, decomposed by the affinity of the red particles for its oxygene; the greater part of the nitrogene being liberated unaltered; but a minute portion of it possibly remaining condensed in the serum and coagulable lymph, and passing with them into the left chamber of the heart. From the experiments on the respiration of nitrous oxide and hydrogene, it appears that a certain portion of the carbonic acid produced in respiration, is evolved from the venous blood; but as a much greater quantity is generated during the respiration of common air and oxygene, than during that of hydrogene in equal times, it is not impossible but that some portion of it may be formed by the combination of charcoal in the red particles with the oxygene dissolved in the serum; but this can only be determined by farther experiments. Supposing that no part of the water evolved in solution by the expired gas of common air is formed immediately in respiration, it will follow that a very considerable quantity of oxygene must be constantly _combined_ with the red particles, even allowing the consumption of a certain portion of it to form carbonic acid; for the carbonic acid evolved, rarely amounts to more than three fourths of the volume of the oxygene consumed. Perhaps the serum of the blood is capable of dissolving a larger quantity of atmospheric air than of pure oxygene. On this supposition, it would be easy to explain the smaller consumption of oxygene in the experiments in the last section. IX. _Observations on the respiration of Nitrous Oxide._ The experiments in the first Division of this Research, prove that nitrous oxide when respired by animals, produces peculiar changes in their blood and in their organs, first connected with increased living action; but terminating in death. From the experiments in this Division, it appears, that nitrous oxide is rapidly absorbed by the circulating venous blood, and of course its condensed oxygene and nitrogene distributed in the blood over the whole of the system. Concerning the changes effected in the principles of the impregnated blood during circulation and its action upon the nervous and muscular fibre; it is useless to reason in the present state of our knowledge. It would be easy to form theories referring the action of blood impregnated with nitrous oxide, to its power of supplying the nervous and muscular fibre with such proportions of condensed nitrogene, oxygene and light or etherial fluid, as enabled them more rapidly to pass through those changes which constitute their life: but such theories would be only collections of terms derived from known phænomena and applied by loose analogies of language to unknown things.