Other educational works followed in rapid succession: A Theory of Language and Universal Grammar (1762), then A Chart of Biography, Essay on a Course of Liberal Education for Civil and Active Life, and Lectures on History and General Policy, all in 1765, the same year in which he received an honorary LL.D. from Edinburgh University.
At the same time, Priestley was attending Matthew Turner's public lectures on practical chemistry and spending a month of each year in London studying science, meeting Benjamin Franklin and other scientific luminaries. His electrical experiments brought him some attention from other scientists, and in 1766 he was elected to the Royal Society.
His first important scientific work, The History and Present State of Electricity, appeared in 1767: it provided a summary of all earlier electrical knowledge and described his original contributions to the field. From electricity he turned his attention to chemistry, studying the properties of gases. Within just a few years he advanced on the previous knowledge of gas chemistry, which recognized only three "airs" (air, carbon dioxide, and hydrogen), by adding ten new ones, including oxygen, nitric oxide, nitrous oxide, hydrogen chloride, ammonia, sulfur dioxide, silicon tetrafluoride, nitrogen, and carbon monoxide. He summarized his early results in an essay of 1772, "On Different Kinds of Air," published in the Philosophical Transactions. The essay caught the attention of Lavoisier, who provided the theoretical framework with which to make sense of Priestley's discoveries.
In the same year he made important contributions to the study of optics in The History and Present State of Discoveries Relating to Vision, Light, and Colours, on the strength of which Priestley was invited to travel with Captain Cook on his second voyage. He declined the offer, and instead became tutor to the children of William Fitzmaurice-Petty, Second Earl of Shelburne, First Marquis of Lansdowne.
He continued his work on gases, publishing Experiments and Observations on Different Kinds of Air in six volumes from 1774 to 1786. His discovery of oxygen, for which he is best known, came in 1774; he called it "dephlogisticated air." On a trip to Paris with Shelburne he met Lavoisier, who at once recognized the significance of his discovery, although the two differed over how to interpret his results; Priestley remained committed to phlogiston theory until his death. His work, however, earned him considerable recognition: he was named to the French Academie in 1772, won the Royal Society's Copley Medal in 1773, and was named to the St. Petersburg Academy in 1780.
Priestley moved to Birmingham in 1779 where he served as minister of the New Meeting congregation and became an active member of the Birmingham Lunar Society. Even while he was busiest with his scientific discoveries, he continued to publish works of social theory and theology: An Essay on the First Principles of Government, and on the Nature of Political, Civil, and Religious Liberty (1769), for instance, proved influential on social thinkers such as Bentham, and Letters to a Philosophical Unbeliever appeared in 1774. But after his move to Birmingham he became increasingly concerned with matters other than science: his History of the Corruptions of Christianity (1782, for instance, rejected predestination and the Trinity, and The History of Early Opinions concerning Jesus Christ (1786) was similarly heretical and controversial. He was also well known as a supporter of the French Revolution. In 1791, an anti-revolutionary mob ransacked Priestley's house and laboratory, and he was driven from Birmingham to Hackney, near London. In 1794 he left England for America, settling in Northumberland, Pennsylvania, where he corresponded with a number of American revolutionaries, including Thomas Jefferson and John Adams. He was offered but declined the chair of chemistry at the University of Pennsylvania. In America he published his General History of the Christian Church (1790-1803), and wrote The Doctrines of Heathen Philosophy Compared with Those of Revelation (published posthumously). He died in 1804.