I design very shortly, to repeat these experiments, and to make others on the comparative consumption of oxygene and atmospheric air, by the larger quadrupeds. Whatever may be the results, I hope to be able to ascertain from them, why pure oxygene is incapable of supporting life. VIII. _Observations on the changes effected in the blood, by atmospheric air and oxygene._ From the experiments of Mr. Cigna and Dr. Priestley,[202] it appears that the coagulum of the venous blood becomes florid at its surface when exposed to the atmosphere, though covered and defended from the immediate contact of air by a very thick stratum of serum. [202] Dr. Priestley found that it likewise became florid at the surface when covered by milk; but that it underwent little or no alteration of color under water and most other fluids.—Vol. 3. p. 372. Hence it is evident, that serum is capable of dissolving either the whole compound atmospheric air, or the oxygene of it. Supposing what indeed is most probable from numerous analogies, that it dissolves the whole compound; it would follow, that the coloring of the coagulum of blood under serum, depended upon the decomposition of the atmospheric air condensed in the serum, the oxygene[203] of it combining with the red particles, and the nitrogene either remaining dissolved in the fluid, or being liberated through it into the atmosphere. Now the circulating blood consists of red particles, floating in and diffused through serum and coagulable lymph. [203] There are many analogous decompositions. Dr. Priestley noticed (and I have often made the observation) that green oxide of iron, or the precipitate from pale green sulphate of iron by caustic alkali, became red at the surface, when covered by a thick stratum of water. In my experiments on the green muriate and sulphate of iron, I observed that part of some dark oxide of iron which was at the bottom of a trough of water 9 inches deep, became red at the surface nearly in the same time as another portion of the same precipitation that was exposed to the atmosphere. This oxygenation must depend upon the decomposition of atmospheric air constantly dissolved by the water. In natural respiration, the red particles are rendered of a brighter tinge during the passage of the blood through the pulmonary veins. And as we have seen in the last sections, during respiration atmospheric air is decomposed; all the oxygene of it consumed, _apparently_ a small portion of the nitrogene lost, and a considerable quantity of carbonic acid produced.