Oxygen

Webster's Dictionary

(1): (n.) A colorless, tasteless, odorless, gaseous element occurring in the free state 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. Symbol O. Atomic weight 15.96.

(2): (n.) Chlorine used in bleaching.

The Nuttall Encyclopedia

A colourless, inodorous gas which constitutes one-fifth in volume of the atmosphere, and which, in combination with hydrogen, forms water. It is the most widely diffused of all the elementary bodies, and an essential support to everything possessed of life.