The oxygen gas in atmospheric air is the principle of combustion, as the vehicle of heat; and is absolutely necessary for the support of animal life. Pure oxygen gas has the property of accelerating the circulation of all the animal fluids, and occasions the most rapid combustion of all combustible substances; so that it is the most energetic and powerful agent that chemists are acquainted with. Oxygen gas is a little heavier than atmospheric air, and 740 times lighter than water. Nitrogen gas is chiefly distinguished by certain _negative_ qualities, such as being incapable of supporting combustion and animal life. It is uninflammable, and somewhat lighter than atmospheric air. Nitrogen gas has the effect of neutralizing, in some measure, the properties of oxygen gas, and rendering it fit for respiration and combustion. By the union of nitrogen gas with the oxygen gas this change is effected: the latter, which would burn every thing within its reach with an unparalleled activity, is, as it were, dissolved and diluted; and the nature of the former is so much enveloped by the latter, that the compound possesses properties different from either of these gasses, so as to be fitted for every purpose for which it was designed. Though nitrogen gas is, by itself, so noxious to animals, it answers an important end when mixed with oxygen gas in atmospheric air. Were it not for this large quantity of nitrogen in the atmosphere, the blood would flow with too great rapidity through the vessels, and all animals would have too great spirits; the consequence of which would be, that the life of man would not be protracted to the length that it now is. “If the proportions of oxygen and nitrogen were reversed in the atmospheric air, says Dr. Lambe, the air taken in by respiration would be more stimulant, the circulation would become accelerated, and all the secretions would be increased: but the tone of the vessels, thus stimulated to increased action, would be destroyed by over-excitement; and, if the supply from the stomach were not equal to the consumption, the body must inevitably waste and decay.” Hence the wisdom of God is remarkably displayed in the constitution of the atmospheric air! See Parke’s Chemical Catechism, chap. ii.