_l._ To ascertain whether impregnation with nitrous oxide accelerated or retarded the putrefaction of the blood; I exposed venous blood in four phials, the first filled with hydrocarbonate, the second with hydrogene; the third with atmospheric air, and the fourth with nitrous oxide. Examined after a fortnight, the blood in hydrogene and common air were both black, and stunk very much; that in hydrocarbonate was red, and perfectly sweet; that in nitrous oxide appeared purple and had no disagreeable smell. In a second experiment, when blood was exposed for three weeks to hydrocarbonate and nitrous oxide, that in nitrous oxide was darker than before and stunk a little; that in hydrocarbonate was still perfectly sweet. The power of hydrocarbonate to prevent the putrefaction of animal matters, was long ago noticed by Mr. Watt. _m._ Having accidentally cut one of my fingers so as to lay bare a little muscular fibre, I introduced it whilst bleeding into a bottle of nitrous oxide; the blood that trickled from the wound evidently became much more purple; but the pain was neither alleviated or increased. When however, the finger was taken out of the nitrous oxide and exposed to the atmosphere, the wound smarted more than it had done before. After it had ceased to bleed, I inserted it through water into a vessel of nitrous gas; but it did not become more painful than before. From all these observations, we may conclude, 1st. That when nitrous oxide is agitated in fluid venous blood, a certain portion of the gas is absorbed; whilst the color of the blood changes from dark red to red purple. 2dly. That during the absorption of nitrous oxide by the venous blood, minute portions of nitrogene and carbonic acid are produced, either by evolution from the blood, or from a decomposition of part of the nitrous oxide. 3dly. That venous blood impregnated with nitrous oxide is capable of oxygenation; and vice versa; that oxygenated blood may be combined with nitrous oxide. When blood separated into coagulum and serum, is exposed to nitrous oxide, it is most probable that the gas is chiefly absorbed by the serum. That nitrous oxide however is capable of acting upon the coagulum, is evident from _d._ In the fluid blood, as we shall see hereafter, nitrous oxide is absorbed by the attractions of the whole compound. III. _Of the changes effected in Nitrous Oxide by Respiration._ To ascertain whether the changes effected in nitrous oxide by the circulating blood acting through the moist coats of the pulmonary veins of living animals, were highly analogous to those produced in it by fluid venous blood removed from the vessels, I found extremely difficult.