Oxygen
This gas was obtained by Dr. Priestley in 1774 from red oxide of mercury exposed to a burning lens, and again in the following year by the Swedish chemist Scheele. With regard to the importance of this great discovery Prof. Liebig observes: "Since the discovery of oxygen the civilized world has undergone a revolution in manners and customs. The knowledge of the composition of the atmosphere, of the solid crust of the earth, of water, and of their influence upon the life of plants and animals, was linked with that discovery. The successful pursuit of innumerable trades and manufactures, the profitable separation of metals from their ores, also stand in the closest connection therewith. It may well be said that the material prosperity of empires has increased manifold since the time oxygen became known, and the fortune of every individual has been augmented in proportion." Eight ninths of water consist of oxygen; it forms one fifth of air and about one half of silica, chalk, and alumina, which three constitute the chief substances of the earth's surface. It is absolutely essential for the support of animal life. In mechanical combination with nitrogen it forms the atmosphere surrounding the globe, and is given off by all growing plants when under the influence of sunlight. Its chemical affinities for other elementary substances are very powerful, combining with all except fluorine. Owing to the intensity with which many of these combinations takes place, this gas has the power of supporting combustion in an eminent degree. It is tasteless, colorless, inodorous, and has hitherto resisted all attempts to liquify it. Combustion is nothing more than a chemical union of the oxygen of the air with the combustible body or some of its elements. We make the fire hotter by bringing more air in contact with the fuel. See COMBUSTION; FLAME, etc.