OXYGEN. This important element was dis covered in 1774 by Dr. Priestley. There are sev eral 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 gener ally resort to black oxide of manganese, which at a dull-red lmat, gives out a considerable quantity of tolerably pure oxygen gas. Oxygen gas is colorless,. tasteless, and inodorous, it is electro negative, and therefore, when compounds con taining it are electrically decomposed, it always appears at the positive surface. It is a little heavier than atmospheric air, in the proportion of eleven to ten; 100 cubic inches weighing 34.18 grains. It is absorbed by water to the extent of less than one per cent., and is neither acid nor alkaline. It has a powerful attraction for most of the simple substances, especially for the electropositive bodies. The act of combining with it is called oxidation. The compounds thus formed are divided into acids and oxides; among the latter are the alkalies, and almost all salifiahle bases. Oxidation is often attended with the evolution of heat and light, as in all processes of combustion in atmospheric • air; sometimes it is slow, and unattended with such phenomena, as in the gradual rusting of metals.
Oxygen is a most powerful supporter of combus tion; 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 converted into carbonic acid; or it is liquid, as hydrogen, by combustion, produces water; or it is solid, as when iron, by burning, produces oxide of iron. Oxygen gas is also essential to respiration; that is, to the evolution of carbonic acid from the blood ; but requires to be diluted with nitrogen, as in the air, other wise it destroys life by producing over-activity. Seeds can not germinate without oxygen, and are, therefore not to be buried too deep in a compact soil. The leaves of trees, also, can not perform their functions without its presence, although they are always exhaling a large quan tity of this gas. Oxygen is very extensively diffused in nature; iu the compound state it forms eight-ninths of all waters, and at least one-half of all structures, whether mineral or organic, except a few oils and resins.