Priestley, Joseph (1733-1804), an English chemist and clergyman. In 1774 Priestley discovered dephlogisticated air (oxygen). He found that the gas, which he obtained by heating mercuric oxide, supported burning and respiration better than did ordinary air. (Karl W. Scheele of Sweden had discovered oxygen earlier, but Priestley is usually credited with the discovery because his findings were published first.) Priestley's discoveries led Antoine-Laurent Lavoisier to disprove the phlogiston theory, although Priestley himself believed in the theory until his death. Priestley also discovered that oxygen is given off by plants.

Priestley improved the techniques for the study of gases, isolating and describing—in addition to oxygen—nitrous oxide, nitric oxide, nitrogen peroxide, ammonia, silicon fluoride, sulfur dioxide, hydrogen sulfide, and carbon monoxide. In his book History and Present State of Electricity (1767), he suggested but did not prove the inverse-square law of electrostatics (Coulomb's Law) and explained the rings (called Priestley rings) formed on metallic surfaces by electrical discharges.

While conducting scientific research, Priestley wrote extensively on theology and became a leader of British and—after 1794—American Unitarianism. He wrote several political articles, including his Essay on the First Principles of Government (1768), which influenced Jeremy Bentham.

Priestley was born near Leeds, the son of a clothmaker. He was ordained a Congregational minister in 1762, but gradually turned Unitarian. Priestley served as a pastor for most of his life, leaving the pulpit briefly in the 1760's to teach school and in the 1770's to work as librarian to Lord Shelburne. He was admitted to the Royal Society in 1766.

Priestley's sympathy for the French Revolution caused a mob to burn his house in 1791. Three years later he moved to the United States, where he was well received and became a friend of Thomas Jefferson. Priestley founded two Unitarian churches, one in his new home town of Northumberland, Pennsylvania (1794), and one in Philadelphia (1796). His home has been preserved as a museum.