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PRIESTLEY, prēst'lĭ, Joseph, English physicist and Unitarian divine: b. Fieldhead, Yorkshire, 13 March 1733; d. Northumberland, Pa., 6 Feb. 1804. After a secondary education he accepted an invitation to become Presbyterian minister of Needham Market, Suffolk, on an average salary of £30 a year. After three years he took charge of a congregation at Nantwich, Cheshire, to which he joined a school. Here his reputation increased, and in 1761 he was invited by the trustees of the Dissenting academy at Warrington to occupy the post of tutor of languages and belles-lettres under Dr. Aiken. A visit to London having introduced him to Drs. Franklin, Watson and Price, he was encouraged to compose a ‘History of Electricity,’ which appeared in l767. In the same year he became minister, at a salary of £100 a year, of the Mill Hill chapel at Leeds, where his religious opinions grew decidedly Unitarian. At Leeds his attention was first drawn to the properties of “fixed air” (carbonic acid gas); and about this time he completed his ‘History and Present State of Discoveries relating to Vision, Light and Colors.’ In 1771 he agreed to accompany Captain Cook on his second voyage in the capacity of astronomer, but his appointment was canceled, as the Board of Longitude objected to his theology.

After six years at Leeds he accepted an invitation from the Earl of Shelburne, afterward Marquis of Lansdowne, to reside with him at a companion in the nominal capacity of librarian. While forming a part of the establishment of this nobleman, he occupied himself in scientific pursuits, and in 1773 gave to the ‘Philosophical Transactions’ a paper on the different kinds of air, which obtained the prize of Copley's gold medal. On 1 Aug. 1774 he discovered oxygen or “dephlogisticated air,” as he called it, a result quickly followed by other important discoveries, such as nitrous gas, nitrous oxide gas, sulphurous oxide gas, fluoric acid gas, muriatic gas and ammoniacal gas, etc. By these exploits he may be said to have been almost the first render chemistry a precise science. In 1775 he published ‘Examination of the Doctrine of Common Sense, as held by Drs. Reid, Beattie, and Oswald,’ and soon after the treatise of Hartley. He had already declared himself a believer in the doctrine of philosophical necessity, and in a dissertation annexed to his edition of Hartley expressed some doubts of the immateriality of the sentient principle in man. This doctrine he still more forcibly supported in his ‘Disquisitions on Matter and Spirit’ (1777}. Probably because of his “materialistic” views, the connection with Shelburne was dissolved in 1780, Priestley retaining an annuity by original agreement. He next removed to Birmingham, where he became once more minister of a Non-Conformist congregation, and occupied himself on his ‘History of the Corruptions of Christianity’ (1782), and ‘History of the Early Opinions concerning Jesus Christ’ (1786). With Erasmus Darwin, Watt, Boulton, Wedgewood and other famous men be founded the now celebrated Lunar Society, which rendered Birmingham for the time of its existence almost the intellectual centre of England. His ‘Familiar Letter to the Inhabitants of Birmingham’ was written in support of the claims of Dissenters for a repeal of the test acts. The era of the French Revolution, whose principles Priestley vindicated, added to the usual animosity of theological dispute. The anniversary of the capture of the Bastile being celebrated at Birmingham, a mob assembled, and although Priestley was not present, burned his house, and nearly all his library, manuscripts and apparatus. The outrage was countenanced by too many exercising both lay and clerical influence, and the legal compensation which he obtained fell short of his real losses by about £2,000. On 1 Aug. 1874, Birmingham made some amends for the wrong done to her greatest citizen, by the erection of a marble statue of him in front of the town-hall. On quitting Birmingham (1791) he succeeded his friend Price as preacher in the Gravel Pit Chapel, Hackney, where he remained some time in the cultivation of his scientific pursuits, until finally goaded by party enmity to seek an asylum in the United States, which he reached in 1794, and took up his residence at Northumberland, Pa. In America he dedicated his whole time to his accustomed pursuits. Some discourses of his led to the formation in Philadelphia of a Unitarian congregation and he held private meetings in his own house. He was never naturalized; and, curiously enough, never learned to make change in United States coinage. Priestley was an ardent controversialist, chiefly in consequence of extreme simplicity and openness of character; but no man felt less animosity toward his opponents than he did, and many who entertained the strongest antipathy to his opinions were converted into friends by his gentleness and urbanity in personal intercourse. As a man of science he stands high in the field of invention and discovery, and to few has pneumatic chemistry been so much indebted. On his own admission his researches in his favorite science were superficial, and his great discoveries to a considerable extent accidental. But this should detract nothing from the honor due to the earnest pioneer. As a metaphysician his elucidation of Hartley's theory of association, his works upon philosophical necessity and upon materialism will always insure attention. As a theologian Priestley, who followed his convictions wherever they led him, passed through all changes, from Calvinism to a Unitarian system in some measure his own; but to the last remained a zealous opposer of infidelity. Of his theological and controversial productions, those most generally esteemed are his ‘Institutes of Natural and Revealed Religion’ (1772-74), and ‘Letters to a Philosophical Unbeliever’ (1780-82). He also wrote many works of practical divinity. There is an edition of his ‘Theological and Miscellaneous Works, with Memoirs and Correspondence’ (1817-32).