_d_. Some fresh dark coagulum of venous blood, was exposed to nitrous oxide. A very slight alteration of color took place at the surface of the blood, perceptible only in a strong light, and a minute quantity of gas was absorbed. A taper burnt in the remaining gas as brilliantly as before, hence, it had apparently suffered no alteration. _e_. To compare the physical changes effected in the venous blood by nitrous oxide, with those produced by other gases, I made the following experiments.—I filled a large phial, containing near 14 cubic inches, with blood from the vein of the arm of a man, and immediately transferred it to the mercurial apparatus. Different portions of it were thrown into small graduated cylinders, filled with the following gases: nitrogene, nitrous gas, common air, oxygene, nitrous oxide, carbonic acid, and hydrocarbonate. The blood in each of them was successively agitated till it began to coagulate; and making allowances for the different periods of agitation, there was no marked difference in the times of coagulation. The color of the coagulum in every part of the cylinder, containing nitrogene, was the same very dark red. When it was agitated so as to tinge the sides of the jar, it appeared exactly of the color of venous blood received between two surfaces of glass; no perceptible absorption of the gas had taken place. The blood in nitrous gas was dark, and much more purple on the top than that in nitrogene. When agitated so as to adhere to the jar as a thin surface, this purple was evidently deep and bright. An absorption of rather more than ⅛ of the volume of gas had taken place. The blood in oxygene and atmospheric air, were of a much brighter tinge than that in any of the other gases. On the top, the color was vermilion, but no perceptible absorption had taken place. The coagulum in nitrous oxide, when examined in the mass was dark, and hardly distinguishable in its color from venous blood; but when minutely noticed at the surface where it was covered with serum, and in its diffusion over the sides of the jar, it appeared of a fine purple red, a tinge brighter than the blood in nitrous gas. An absorption had taken place in this cylinder, more considerable than in any of the others. In carbonic acid, the coagulum was of a brown red, much darker than the venous blood, and a slight diminution of gas had taken place. In the hydrocarbonate,[190] the blood was red, a shade darker than the oxygenated blood, and a very slight diminution of the gas[191] was perceptible. [190] The hydrocarbonate employed, was procured from alcohol, by means of sulphuric acid. This gas contains more carbon, than hydrocarbonate from water and charcoal. [191] The curious fact of the reddening of venous blood by hydrocarbonate, was discovered by Dr. Beddoes.