In describing the various shades of color of the blood in the preceding observations on the different dissected animals, the poverty of the language of color, has obliged me to adopt terms, which I fear will hardly convey to the mind of the reader, distinct notions of the differences observable by minute examination in the venous and arterial blood of animals that die of privation of atmospheric air, and of those destroyed by the action of nitrous oxide. This difference can only be observed in the vessels by means of a strong light; it may however be easily noticed in the fluid blood by the introduction of it from the arteries or veins at the moment of their incision, between two polished surfaces of white glass,[186] so closely adapted to each other, as to prevent the blood from coming in contact with the atmosphere. [186] The colour of common venous blood, examined in this way, resembles that of the paint called by colour-men red ochre; that of blood saturated with nitrous oxide, approaches to the tinge of lake. Having four or five times had an opportunity of bleeding people in the arm for trifling complaints, I have always received the blood in phials, filled with various gases, in a mode to be described hereafter. Venous blood agitated in nitrous oxide, compared with similar blood in common air, hydrogene, and nitrogene, was always darker and more purple than the first, and much brighter and more florid than the two last, which were not different in their color from venous blood, received between two surfaces of glass. It will be seen hereafter, that the coagulum of venous blood is rendered more purple when exposed to nitrous oxide, whilst the gas is absorbed; likewise that blood altered by nitrous oxide, is capable of being again rendered vermilion, by exposure to the air. The appearances noticed in the above mentioned experiments, in the lungs of animals destroyed in nitrous oxide, are similar to those observed by Dr. Beddoes, in animals that had been made to breathe oxygene for a great length of time. There were many reasons for supposing that the large purple spots in the lungs of animals destroyed in nitrous oxide, were owing to extravasation of venous blood from the capillary vessels; their coats being broken by the highly increased arterial action. To ascertain whether these phænomena existed at a period of the action of nitrous oxide, when the animal was recoverable by exposure to the atmosphere.