OXYGEN

(Chemical symbol O) An element occurring free as a colorless, tasteless, odorless diatomic gas, O2 (ordinary oxygen), in the Atmosphere, of which it forms about 23 per cent by weight and about 21 per cent by volume, being slightly heavier than nitrogen. Oxygen combines with most elements, is essential for plant and animal respiration, and is required for nearly all combustion. Oxygen is the most abundant of all the elements on the earth's surface, for, in addition to its occurrence free in air, it forms, in combination with Hydrogen, 88.812 per cent by weight of Water (H2O) and nearly one-half by weight of the rocks composing the earth's crust, being a constituent of silica, the silicates, the carbonates, the sulphates, etc. It is a constituent of all but a very few acids and, in general, the greater the proportion of oxygen with which an element combines, the more acidic does it become. It is also a constituent of a large proportion of organic compounds. Oxygen is also known in an allotropic, more active form, Ozone, O3.