OXYGEN. This important element was discovered in 1774, by Dr. Priestly. It has been termed dephlogisticated air, vital air, and empyreal air. As it forms a component part of many of the acids, it was termed, at the framing of the new nomenclature, oxygen gas. There are several compounds of oxygen, which, when exposed to heat, are decomposed, and yield the gas in a state of purity : of these the best is chlorate of potash • but as that salt is expensive, we generally re sort to black oxide of manganese, which, at a dull-red heat, gives out a consider able quantity of tolerably pure oxygen gas. Oxide of manganese, or bichromate of potass, heated with sulphuric acid, gives out oxygen.
Oxygen gas is colorless, tasteless, and inodorons; it is clectro-negative, and therefore, when compounds containing it are electrically decomposed, it always ap pears at the positive surface. It is a lit tle heavier than atmospheric air, in the proportion, that is, of 11 to 10 ; 100 cub ical inches weighing 34•6 grains. It is not absorbed by water, and is neither acid nor alkaline. It has a powerful at traction for most of the simple substances, especially for the electro-positive bodies : the act of combining with it is called ox idation. The compounds thus formed
are divided into acids and oxides: among the latter are the alkalies, and almost all salifiable bases. Oxidation is often attend ed with the evolution of heat.and light, as in all processes of combustion in at mospheric air : sometimes it is slow, and unattended with such phenomena, as in the gradual rusting of metals. Oxygen is a most powerful supporter of combustion ; it constitutes one fifth of the bulk of the atmosphere, and is the principle which enables combustible bodies to burn in it. The product of combustion, that is, the oxide or acid, is sometimes itself gaseous, as when charcoal, by burning, is convert ed into carbonic acid; or it is liquid, as hydrogen, by combustion, produces wa ter; or it is solid, as when iron, by burn ing, produces oxide of iron. Oxygen gas is also essential to respiration ; that is to the evolution of carbonic acid from the blood.