Oxygen: History and Occurrence
Credit for the discovery of oxygen is shared by two men, Joseph Priestley, an English clergyman and amateur scientist, who later moved to the United States to escape religious persecution, and Carl Wilhelm Scheele, a Swedish pharmacist. Working independently, these two men both obtained the gas which we know as oxygen by heating various compounds of the clement, particularly mercuric oxide. They also found evidence that this gas is a component of the atmosphere. Priestley's work was published in 1774, but although Schccle's experiments had probably been performed even earlier, their publication was delayed and no account of them appeared until 1777. Though Priestley recognized that the gas which he had discovered plays an important role in combustion, he remained, along with Schccle, an ardent adherent of the phlogiston theory of combustion; in fact, he called the gas "dcphlogisticated air".
On the basis of the experimental results of Priestley, Scheele, and others, as well as some very fine experimental work of his own, in 1777 the brilliant French chemist Lavoisier established the modern concept that the combustion of a substance consists in its combination with the new gas which Priestley and Scheele had described, and which Lavoisier found an important constituent of the atmosphere. Since the combustion of many substances (now known as non-metals) such as phosphorus and sulphur yields products which react with water and give acidic solutions, Lavoisier named this gas oxygen, derived from Greek words meaning "acid former".
Oxygen occurs in the free state as the second most abundant component of the atmosphere; about one-fifth of the air by volume is oxygen. In the combined state it makes up 88.81% by weight of pure water, and, on the average, 85.79% of sea water. It occurs in the earth's crust, in the form of a multitude of compounds, to the estimated extent of46.43%.
