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(b. March 13, 1733, Birstall Fieldhead, near Leeds, Yorkshire, Eng.--d. Feb. 6, 1804, Northumberland, Pa., U.S.), English clergyman, political theorist, and physical scientist whose work contributed to advances in liberal political and religious thought and in experimental science. He is best remembered as one of the discoverers of the element oxygen. (see also IndexUnitarianism) 

Education and early career

Priestley was the oldest of six children of a modestly successful cloth dresser. For several years during his childhood Joseph was often sickly, but, perhaps as compensation, he became an avid learner on his own. Although his parents were Calvinists, they were open to other religious views and wanted him to enter the ministry of the Dissenting church, the diverse congregations of which, such as Presbyterian or Independent, did not conform to the Church of England. To prepare himself, he studied "Hebrew, Chaldee, Syriac, and a little Arabic" with a local Dissenting minister. 

With improving health, in 1752 he entered a new Dissenting Academy at Daventry, Northamptonshire, one of the educational institutions that, in the 18th century, was established independently by Nonconformists. These academies offered high-quality education and attracted the best teachers and students. His unorthodox opinions developed even more as he added to the required curriculum his own intensive study of history, philosophy, and science, and a daily translation of 10 folio pages of Greek. In 1755 he left Daventry Academy to become assistant minister to the independent Presbyterian congregation in Needham Market, Suffolk, but, as he had become a "furious freethinker," his unorthodox and even heretical opinions gradually lost him the confidence of his orthodox congregation. "After much pain and thought," he renounced the doctrine of the Atonement; moreover, his examination of St. Paul's epistles in the New Testament satisfied him that the Apostle's reasoning "was in many places far from being conclusive." Priestley's intellectual development passed from the Calvinism of his family, through Arianism, with its denial of Christ's divinity, to a rational Unitarianism, with its complete denial of the Trinity. At no stage of this development, however, did he ever waver in the sincerity of his religious convictions or in the genuine piety of his life. 

In 1758 Priestley transferred to a more sympathetic congregation in Nantwich, Cheshire, where he opened a day school with 36 students. Becoming interested in science at this time, he provided them with "philosophical instruments," such as an air pump and a static generator for electrical demonstrations. His teaching success led to his appointment in 1761 as a tutor in language and literature at Warrington Academy in Lancashire. Because the universities and learned professions were closed to Dissenters, Priestley developed new courses that were suitable for students preparing for careers in industry and commerce. Textbooks were not available, and these he set himself to write. In 1761 he published the Rudiments of English Grammar, a work that was radical in its reliance on description of actual English usage as opposed to prescription on the basis of dead classical languages; it remained in use for 50 years. Ordained a Dissenting minister in 1762 at Warrington, in the same year he married Mary Wilkinson, aged 18, the only daughter of an ironmaster at Bersham in Wales. They had a daughter and three sons. (see also Indexeducation, history of ) 

Priestley enriched the curriculum with his Theory of Language and Universal Grammar (1762), Chart of Biography (1765), Essay on a Course of Liberal Education for Civil and Active Life (1765), and Lectures on History and General Policy (1765, 1788). By emphasizing history, science, and the arts rather than the classics of university curricula, he sought to prepare students for a practical life. His educational activities made Warrington Academy the most distinguished school of its kind in England. The degree of LL.D. was conferred on him by Edinburgh University in 1765. 

Work in science

Priestley became even more interested in science while attending lectures and demonstrations on practical chemistry given in 1763-65 by the surgeon Matthew Turner. Beginning in 1765, Priestley spent a month of every year in London, where he met the leading men of science, including the American statesman and inventor Benjamin Franklin. On the basis of his electrical experiments, Priestley in 1766 was elected to membership in the Royal Society of London. The next year, with Franklin's encouragement and generous loan of the requisite books, he published The History and Present State of Electricity, an original work in which he summarized the knowledge of electricity to his time and described his own experiments. He anticipated the inverse square law of electrical attraction with his observation that when he electrified a hollow sphere there was no charge inside. He also discovered that charcoal conducts electricity and noted the relationship between electricity and chemical change, whereby one could cause the other. This work "drew him into a large field of original experiments" that were not in electricity. As he became more interested in chemistry, he reflected on the similarity between the processes of burning and respiration. 

In 1767 Priestley was appointed minister of Mill Hill Chapel, in Leeds, Yorkshire, where he had more leisure for writing and experimenting. Although not primarily theoretical in his approach to science, he was able to perceive what research would be most fruitful. He studied gases, or "airs," as they were then called, a subject that led him to perform many significant experiments. The facts he discovered became central in the theoretical development of chemistry. His work on gases began while he was living next to a brewery in Leeds, where he observed "fixed-air" (carbon dioxide) as it effervesced from vats of fermenting liquor. Before his work, only three gases were known: air, carbon dioxide, and hydrogen. With zeal and enthusiasm he discovered 10 new "airs," four in 1767-73 while at Leeds: nitric oxide (nitrous air), nitrogen dioxide (red nitrous vapour), nitrous oxide (diminished nitrous air, later called "laughing gas"), and hydrogen chloride (marine acid air). His success resulted in large part from his ability to design ingenious laboratory apparatus, particularly an improved pneumatic trough, and his skill in its manipulation. Moreover, by collecting gases above mercury in the trough, instead of in water, he was able to isolate, by trial and error, those that were water-soluble. His description of the gas experiments in the Philosophical Transactions, "On Different Kinds of Air" (1772), promptly drew the attention of the French chemist Antoine-Laurent Lavoisier, who provided a theoretical interpretation. 

Concurrently with these experiments, Priestley developed rapidly as a political theorist. Representative of 18th-century liberal thought was his Essay on the First Principles of Government, and on the Nature of Political, Civil, and Religious Liberty (1769), in which he emphasized individualism; he believed that people should have a voice in their government and power over their own actions. The English economist Jeremy Bentham acknowledged that this influential book inspired his phrase "the greatest happiness of the greatest number." Throughout his life Priestley was an ardent believer in human progress and perfectibility. 

In 1772 Priestley's History and Present State of Discoveries Relating to Vision, Light, and Colours was an original contribution to optics. It brought an invitation to join Capt. James Cook's second voyage of exploration (1773-75) as an astronomer, but he was obliged to decline when opposition mounted to his Unitarian views. His technique for "impregnating water with fixed-air" (using sulfur acid and chalk), however, was suggested for use on the voyage to make drinking water palatable--and also later made possible the soda-water industry. 

In December 1772, William Fitzmaurice-Petty, 2nd earl of Shelburne, later 1st marquis of Lansdowne, appointed Priestley as librarian, literary companion, and tutor to his two young sons, on generous terms, with the freedom to preach and write as he wished. He settled at the Shelburne estate at Calne, Wiltshire, in July 1773. The following November he was awarded the Copley Medal by the Royal Society for his experiments on gases, on which he continued to report in a series of volumes entitled Experiments and Observations on Different Kinds of Air, 6 vol. (1774-86). 

Discovery of oxygen

Priestley's most famous discovery occurred on Aug. 1, 1774, when he obtained a colourless gas by heating red mercuric oxide (he called it mercurius calcinatus per se). Finding that a candle would burn in it "with a remarkably vigorous flame," he called it "dephlogisticated air" because he believed, accepting prevailing theory, that ordinary air became saturated with phlogiston when it could no longer support combustion or life. (Phlogiston was thought to be a material that was transferred during burning and respiration; a unifying idea in 18th-century chemistry, it avoided quantitative considerations but was the reverse of the oxidative interpretation of combustion and respiration established by Lavoisier in 1789.) Priestley was not yet sure, however, that he had discovered a "new species of air." (Oxygen was also discovered by the Swedish chemist and apothecary Carl Wilhelm Scheele, probably by 1773.) The following October Priestley accompanied Shelburne on a journey through Belgium, Holland, Germany, and France, where in Paris he informed Lavoisier how he obtained his new "air." The meeting of the two scientists was highly significant for the future of chemistry: Lavoisier required no more than the barest intimation of the success of Priestley's experiments to appreciate their significance. He immediately repeated them and in 1775-80 conducted intensive investigations from which he correctly deduced the elementary nature of oxygen, recognized it as the active "principle" of the atmosphere, interpreted its role in combustion and respiration, and gave it its name (1789). Priestley, however, did not accept all of Lavoisier's views and continued in particular to uphold the phlogiston theory until, in his old age, he was its last champion. 

Other significant scientific discoveries. 

Continuing his studies of the atmosphere, Priestley discovered ammonia (alkaline air), sulfur dioxide (vitriolic acid air), silicon tetrafluoride (fluor acid air), nitrogen (also discovered by Daniel Rutherford in 1772), and a gas later identified as carbon monoxide. Of striking significance was his observation that light was important for plant growth and that green plants gave off "dephlogisticated air" (really oxygen). These observations became fundamental to the systematic work on photosynthesis by the Dutch physician Jan Ingenhousz (begun in 1779) and by the Swiss clergyman-naturalist Jean Senebier during the next two decades. Drawing then on his earlier knowledge of electricity, Priestley decomposed ammonia by sparking with electric sparks and noticed the formation of dew when mixtures of hydrogen and oxygen are exploded. 

In 1779, for reasons not entirely clear, Priestley left Shelburne's employ and settled in the industrial town of Birmingham as minister of the New Meeting congregation. He received a small annuity from Shelburne, and friends raised a subscription to defray the expenses of his experiments. The years at Birmingham were the happiest and busiest of his life. He wrote books on religion and theology and published sermons, tracts, and catechisms. In his History of the Corruptions of Christianity (1782), he rejected most of the fundamental doctrines of Christianity, including the Trinity, predestination, and the divine verbal inspiration of the Bible and traced them to their historical sources of error. This work aroused another storm of protest. By the time of the French Revolution he had acquired a reputation as the antagonist of all establishments, both political and religious. 

Priestley regularly took part in meetings of Birmingham's Lunar Society, which met monthly at the full moon. Flourishing independently of the Royal Society of London, this organization in 1766-91 actively promoted science and its applications to industry and crafts. There, Priestley was the honoured colleague of the naturalist Erasmus Darwin, the pottery manufacturer Josiah Wedgwood, and the inventor of the steam engine, James Watt. Concluding his scientific work, he observed that "calces" (oxides) are changed to the metallic state (reduced) when heated in hydrogen, but he did not notice that water is also produced, and he gave a phlogistic interpretation. 

During these years, Priestley was widely known as the defender of the principles of the French Revolution and an ardent advocate of civil and religious liberty. He angered the antirevolutionary populace by publicly disagreeing with the Reflections on the Revolution in France (1790), written by the British statesman Edmund Burke, who opposed the Revolution. On July 14, 1791, on the second anniversary of the fall of the Bastille prison in Paris, an outbreak of mob violence occurred in Birmingham, in the course of which Priestley's house, library, and laboratory were destroyed. He was driven from Birmingham, never to return. 

For the next two years Priestley resided at Hackney, near London, where he taught at the New College (Hackney College); but the progress of the French Revolution, the execution of Louis XVI in 1793, and the declaration of war against France the same year excited further rancour against him. His three sons had emigrated to the United States in August 1793; Priestley and his wife followed in April 1794, taking up residence in Northumberland, Pa. He was welcomed by various learned societies, including the American Philosophical Society, and offered the chair of chemistry at the University of Pennsylvania, but he refused to accept any public office. 

During his years in America, Priestley continued his literary and religious activities, but he was hampered in his scientific work by the difficulties of communicating with his friends in England. He published in the United States the last four volumes of his six-volume A General History of the Christian Church (vol. 1-2, 1790-1803, and vol. 3-6, 1802-03). He was a friend and correspondent of two leaders of the American Revolution, John Adams and Thomas Jefferson, both of whom held views on religion similar to his own. The Doctrines of Heathen Philosophy Compared with Those of Revelation appeared after his death. It had been written at the suggestion of Jefferson, who felt, however, that "it did not do justice to the undertaking," adding that Priestley "felt himself pressed by the hand of death." He died at Northumberland in 1804. (S.R.) 


The best book-length biography, which does justice to all aspects of Priestley's work, is F.W. Gibbs, Joseph Priestley (1965). A popular description of Priestley's discoveries in chemistry is in Kenneth Davis, The Cautionary Scientists: Priestley, Lavoisier, and the Founding of Modern Chemistry (1966). 

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