English chemist and theologian.

Aged 22, after attending the Dissenting Academy in Daventry, Priestley became a Presbyterian minister. He went on to serve several churches but he was eventually retired as a result of his radical views and outspoken opposition to the established church.

After meeting the scientist Benjamin Franklin, Priestley became increasingly interested in scientific research. Initially he concentrated on physics, proposing an inverse square law for electrostatic charges. He then turned to chemistry, and in 1772 discovered hydrochloric acid and nitrous oxide (laughing gas), and became the first person to isolate gaseous ammonia. He also discovered sulphur dioxide and invented soda water.

Priestley is most famous for his discovery of 'dephlogisticated air' (renamed 'oxygen' by Lavoisier) in 1774 (although Karl Scheele had independently discovered the gas two years earlier). In the same year, as his unpopularity as a sympathiser of the French Revolution grew, Priestley emigrated to America, seeking political and religious liberty.

During his life he produced various works on the history of Christianity. He continued his chemical experimentation in the United States, and his 25-volume Theological and Miscellaneous Works was published between 1817 and 1832.