Joseph Priestley

Joseph Priestley (; 24 March 1733 – 6 February 1804) was an English chemist, Unitarian, natural philosopher, separatist theologian, grammarian, multi-subject educator and classical liberal political theorist. He published over 150 works, and conducted experiments in several areas of science.

Priestley is credited with his independent discovery of oxygen by the thermal decomposition of mercuric oxide, having isolated it in 1774. During his lifetime, Priestley's considerable scientific reputation rested on his invention of carbonated water, his writings on electricity, and his discovery of several "airs" (gases), the most famous being what Priestley dubbed "dephlogisticated air" (oxygen). Priestley's determination to defend phlogiston theory and to reject what would become the chemical revolution eventually left him isolated within the scientific community.

Priestley's science was integral to his theology, and he consistently tried to fuse Enlightenment rationalism with Christian theism. In his metaphysical texts, Priestley attempted to combine theism, materialism, and determinism, a project that has been called "audacious and original". He believed that a proper understanding of the natural world would promote human progress and eventually bring about the Christian millennium.Tapper, 314. Priestley, who strongly believed in the free and open exchange of ideas, advocated toleration and equal rights for religious Dissenters, which also led him to help found Unitarianism in England. The controversial nature of Priestley's publications, combined with his outspoken support of the American Revolution and later the French Revolution, aroused public and governmental contempt; eventually forcing him to flee in 1791, first to London and then to the United States, after a mob burned down his Birmingham home and church. He spent his last ten years in Northumberland County, Pennsylvania.

A scholar and teacher throughout his life, Priestley made significant contributions to pedagogy, including the publication of a seminal work on English grammar and books on history; he prepared some of the most influential early timelines. The educational writings were among Priestley's most popular works. Arguably his metaphysical works, however, had the most lasting influence, as now considered primary sources for utilitarianism by philosophers such as Jeremy Bentham, John Stuart Mill, and Herbert Spencer.

Early life and education (1733–1755)

Priestley was born in Birstall (near Batley) in the West Riding of Yorkshire, to an established English Dissenting family who did not conform to the Church of England. He was the oldest of six children born to Mary Swift and Jonas Priestley, a finisher of cloth. Priestley was sent to live with his grandfather around the age of one. He returned home five years later, after his mother died. When his father remarried in 1741, Priestley went to live with his aunt and uncle, the wealthy and childless Sarah (d. 1764) and John Keighley, from Fieldhead. Gray & Harrison: Oxford Dictionary of National Biography, pp. 351–352

Priestley was a precocious child—at the age of four, he could flawlessly recite all 107 questions and answers of the Westminster Shorter Catechism—and his aunt sought the best education for him, intending him to enter ministry. During his youth, Priestley attended local schools, where he learned Greek, Latin, and Hebrew.

Around 1749, Priestley became seriously ill and believed he was dying. Raised as a devout Calvinist, he believed a conversion experience was necessary for salvation, but doubted he had had one. This emotional distress eventually led him to question his theological upbringing, causing him to reject election and to accept universal salvation. As a result, the elders of his home church, the Independent Upper Chapel of Heckmondwike, near Leeds, refused him admission as a full member.

Priestley's illness left him with a permanent stutter and he gave up any thoughts of entering the ministry at that time. In preparation for joining a relative in trade in Lisbon, he studied French, Italian, and German in addition to Aramaic, and Arabic. He was tutored by the Reverend George Haggerstone, who first introduced him to higher mathematics, natural philosophy, logic, and metaphysics through the works of Isaac Watts, Willem 's Gravesande, and John Locke.

Daventry Academy

Priestley eventually decided to return to his theological studies and, in 1752, matriculated at Daventry, a Dissenting academy. Because he was already widely read, Priestley was allowed to omit the first two years of coursework. He continued his intense study; this, together with the liberal atmosphere of the school, shifted his theology further leftward and he became a Rational Dissenter. Abhorring dogma and religious mysticism, Rational Dissenters emphasised rational analysis of the natural world and the Bible.

Priestley later wrote that the book that influenced him the most, save the Bible, was David Hartley's '' Observations on Man'' (1749). Hartley's psychological, philosophical, and theological treatise postulated a material theory of mind. Hartley aimed to construct a Christian philosophy in which both religious and moral "facts" could be scientifically proven, a goal that would occupy Priestley for his entire life. In his third year at Daventry, Priestley committed himself to the ministry, which he described as "the noblest of all professions".

Needham Market and Nantwich (1755–1761)

Robert Schofield, Priestley's major modern biographer, describes his first "call" in 1755 to the Dissenting parish in Needham Market, Suffolk, as a "mistake" for both Priestley and the congregation. Priestley yearned for urban life and theological debate, whereas Needham Market was a small, rural town with a congregation wedded to tradition. Attendance and donations dropped sharply when they discovered the extent of his heterodoxy. Although Priestley's aunt had promised her support if he became a minister, she refused any further assistance when she realised he was no longer a Calvinist. To earn extra money, Priestley proposed opening a school, but local families informed him that they would refuse to send their children. He also presented a series of scientific lectures titled "Use of the Globes" that was more successful.

Priestley's Daventry friends helped him obtain another position and in 1758 he moved to Nantwich, Cheshire, living at Sweetbriar Hall in the town's Hospital Street; his time there was happier. The congregation cared less about Priestley's heterodoxy and he successfully established a school. Unlike many schoolmasters of the time, Priestley taught his students natural philosophy and even bought scientific instruments for them. Appalled at the quality of the available English grammar books, Priestley wrote his own: '' The Rudiments of English Grammar'' (1761). His innovations in the description of English grammar, particularly his efforts to dissociate it from Latin grammar, led 20th-century scholars to describe him as "one of the great grammarians of his time". After the publication of ''Rudiments'' and the success of Priestley's school, Warrington Academy offered him a teaching position in 1761.

Warrington Academy (1761–1767)

In 1761, Priestley moved to Warrington in Cheshire and assumed the post of tutor of modern languages and rhetoric at the town's Dissenting academy, although he would have preferred to teach mathematics and natural philosophy. He fitted in well at Warrington, and made friends quickly. These included the doctor and writer John Aikin, his sister the children's author Anna Laetitia Aikin, and the potter and businessman Josiah Wedgwood. Wedgwood met Priestley in 1762, after a fall from his horse. Wedgwood and Priestley met rarely, but exchanged letters, advice on chemistry, and laboratory equipment. Wedgwood eventually created a medallion of Priestley in cream-on-blue jasperware.

On 23 June 1762, Priestley married Mary Wilkinson of Wrexham. Of his marriage, Priestley wrote:

This proved a very suitable and happy connexion, my wife being a woman of an excellent understanding, much improved by reading, of great fortitude and strength of mind, and of a temper in the highest degree affectionate and generous; feeling strongly for others, and little for herself. Also, greatly excelling in every thing relating to household affairs, she entirely relieved me of all concern of that kind, which allowed me to give all my time to the prosecution of my studies, and the other duties of my station.

On 17 April 1763, they had a daughter, whom they named Sarah after Priestley's aunt.

Educator and historian

All of the books Priestley published while at Warrington emphasised the study of history; Priestley considered it essential for worldly success as well as religious growth. He wrote histories of science and Christianity in an effort to reveal the progress of humanity and, paradoxically, the loss of a pure, "primitive Christianity".

In his '' Essay on a Course of Liberal Education for Civil and Active Life'' (1765), '' Lectures on History and General Policy'' (1788), and other works, Priestley argued that the education of the young should anticipate their future practical needs. This principle of utility guided his unconventional curricular choices for Warrington's aspiring middle-class students. He recommended modern languages instead of classical languages and modern rather than ancient history. Priestley's lectures on history were particularly revolutionary; he narrated a providentialist and naturalist account of history, arguing that the study of history furthered the comprehension of God's natural laws. Furthermore, his millennial perspective was closely tied to his optimism regarding scientific progress and the improvement of humanity. He believed that each age would improve upon the previous and that the study of history allowed people to perceive and to advance this progress. Since the study of history was a moral imperative for Priestley, he also promoted the education of middle-class women, which was unusual at the time. Some scholars of education have described Priestley as the most important English writer on education between the 17th-century John Locke and the 19th-century Herbert Spencer. ''Lectures on History'' was well received and was employed by many educational institutions, such as New College at Hackney, Brown, Princeton, Yale, and Cambridge. Priestley designed two ''Charts'' to serve as visual study aids for his ''Lectures''. These charts are in fact timelines; they have been described as the most influential timelines published in the 18th century. Both were popular for decades, and the trustees of Warrington were so impressed with Priestley's lectures and charts that they arranged for the University of Edinburgh to grant him a Doctor of Law degree in 1764. During this period Priestley also regularly delivered lectures on rhetoric that were later published in 1777 as ''A Course of Lectures on Oratory and Criticism''.

History of electricity

The intellectually stimulating atmosphere of Warrington, often called the "Athens of the North" (of England) during the 18th century, encouraged Priestley's growing interest in natural philosophy. He gave lectures on anatomy and performed experiments regarding temperature with another tutor at Warrington, his friend John Seddon. Despite Priestley's busy teaching schedule, he decided to write a history of electricity. Friends introduced him to the major experimenters in the field in Britain— John Canton, William Watson, Timothy Lane, and the visiting Benjamin Franklin who encouraged Priestley to perform the experiments he wanted to include in his history. Priestley also consulted with Franklin during the latter's kite experiments. In the process of replicating others' experiments, Priestley became intrigued by unanswered questions and was prompted to undertake experiments of his own design. (Impressed with his ''Charts'' and the manuscript of his history of electricity, Canton, Franklin, Watson, and Richard Price nominated Priestley for a fellowship in the Royal Society; he was accepted in 1766.)

In 1767, the 700-page '' The History and Present State of Electricity'' was published to positive reviews. The first half of the text is a history of the study of electricity to 1766; the second and more influential half is a description of contemporary theories about electricity and suggestions for future research. The volume also contains extensive comments on Priestley's views that scientific inquiries be presented with all reasoning in one's discovery path, including false leads and mistakes. He contrasted his narrative approach with Newton's analytical proof-like approach which did not facilitate future researchers to continue the inquiry. Priestley reported some of his own discoveries in the second section, such as the conductivity of charcoal and other substances and the continuum between conductors and non-conductors.Schofield (1997), 144–56. This discovery overturned what he described as "one of the earliest and universally received maxims of electricity", that only water and metals could conduct electricity. This and other experiments on the electrical properties of materials and on the electrical effects of chemical transformations demonstrated Priestley's early and ongoing interest in the relationship between chemical substances and electricity. Based on experiments with charged spheres, Priestley was among the first to propose that electrical force followed an inverse-square law, similar to Newton's law of universal gravitation. He did not generalise or elaborate on this, and the general law was enunciated by French physicist Charles-Augustin de Coulomb in the 1780s.

Priestley's strength as a natural philosopher was qualitative rather than quantitative and his observation of "a current of real air" between two electrified points would later interest Michael Faraday and James Clerk Maxwell as they investigated electromagnetism. Priestley's text became the standard history of electricity for over a century; Alessandro Volta (who later invented the battery), William Herschel (who discovered infrared radiation), and Henry Cavendish (who discovered hydrogen) all relied upon it. Priestley wrote a popular version of the ''History of Electricity'' for the general public titled ''A Familiar Introduction to the Study of Electricity'' (1768). He marketed the book with his brother Timothy, but unsuccessfully.

Leeds (1767–1773)

Perhaps prompted by Mary Priestley's ill health, or financial problems, or a desire to prove himself to the community that had rejected him in his childhood, Priestley moved with his family from Warrington to Leeds in 1767, and he became Mill Hill Chapel's minister. Two sons were born to the Priestleys in Leeds: Joseph, Junior, on 24 July 1768 and William three years later. Theophilus Lindsey, a rector at Catterick, Yorkshire, became one of Priestley's few friends in Leeds, of whom he wrote: "I never chose to publish any thing of moment relating to theology, without consulting him." Although Priestley had extended family living around Leeds, they do not appear to have communicated. Schofield conjectures that they considered him a heretic. Each year, Priestley travelled to London to consult with his close friend and publisher, Joseph Johnson, and to attend meetings of the Royal Society.

Minister of Mill Hill Chapel

When Priestley became its minister, Mill Hill Chapel was one of the oldest and most respected Dissenting congregations in England; however, during the early 18th century the congregation had fractured along doctrinal lines and was losing members to the charismatic Methodist movement. Priestley believed that he could strengthen the bonds of the congregation by educating the young people there.

In his three-volume '' Institutes of Natural and Revealed Religion'' (1772–74), Priestley outlined his theories of religious instruction. More importantly, he laid out his belief in Socinianism. The doctrines he explicated would become the standards for Unitarians in Britain. This work marked a change in Priestley's theological thinking that is critical to understanding his later writings—it paved the way for his materialism and necessitarianism (the latter being the belief that a divine being acts in accordance with necessary metaphysical laws).

Priestley's major argument in the ''Institutes'' was that the only revealed religious truths that could be accepted were those that matched one's experience of the natural world. Since his views of religion were tied deeply to his understanding of nature, the text's theism rested on the argument from design. The ''Institutes'' shocked and appalled many readers, primarily because it challenged basic Christian orthodoxies, such as the divinity of Christ and the miracle of the Virgin Birth. Methodists in Leeds penned a hymn asking God to "the Unitarian fiend expel / And chase his doctrine back to Hell." Priestley wanted to return Christianity to its "primitive" or "pure" form by eliminating the "corruptions" which had accumulated over the centuries. The fourth part of the ''Institutes'', '' An History of the Corruptions of Christianity'', became so long that he was forced to issue it separately in 1782. Priestley believed that the ''Corruptions'' was "the most valuable" work he ever published. In demanding that his readers apply the logic of the emerging sciences and comparative history to the Bible and Christianity, he alienated religious and scientific readers alike—scientific readers did not appreciate seeing science used in the defence of religion and religious readers dismissed the application of science to religion.

Religious controversialist

Priestley engaged in numerous political and religious pamphlet wars. According to Schofield, "he entered each controversy with a cheerful conviction that he was right, while most of his opponents were convinced, from the outset, that he was willfully and maliciously wrong. He was able, then, to contrast his sweet reasonableness to their personal rancor",Schofield (1997), 181. but as Schofield points out Priestley rarely altered his opinion as a result of these debates. While at Leeds he wrote controversial pamphlets on the Lord's Supper and on Calvinist doctrine; thousands of copies were published, making them some of Priestley's most widely read works.

Priestley founded the '' Theological Repository'' in 1768, a journal committed to the open and rational inquiry of theological questions. Although he promised to print any contribution, only like-minded authors submitted articles. He was, therefore, obliged to provide much of the journal's content himself. This material also became the basis for many of his later theological and metaphysical works. After only a few years, due to a lack of funds, he was forced to cease publishing the journal. However, he did revive it briefly in 1784 with similar results.

Defender of Dissenters and political philosopher

Many of Priestley's political writings supported the repeal of the Test and Corporation Acts, which restricted the rights of Dissenters. They could not hold political office, serve in the armed forces, or attend Oxford and Cambridge unless they subscribed to the Thirty-nine Articles of the Church of England. Dissenters repeatedly petitioned Parliament to repeal the Acts, arguing that they were being treated as second-class citizens.

Priestley's friends, particularly other Rational Dissenters, urged him to publish a work on the injustices experienced by Dissenters; the result was his '' Essay on the First Principles of Government'' (1768). An early work of modern liberal political theory and Priestley's most thorough treatment of the subject, it—unusually for the time—distinguished political rights from civil rights with precision and argued for expansive civil rights. Priestley identified separate private and public spheres, contending that the government should have control only over the public sphere. Education and religion, in particular, he maintained, were matters of private conscience and should not be administered by the state. Priestley's later radicalism emerged from his belief that the British government was infringing upon these individual freedoms.

Priestley also defended the rights of Dissenters against the attacks of William Blackstone, an eminent legal theorist, whose '' Commentaries on the Laws of England'' (1765–69) had become the standard legal guide. Blackstone's book stated that dissent from the Church of England was a crime and that Dissenters could not be loyal subjects. Furious, Priestley lashed out with his ''Remarks on Dr. Blackstone's Commentaries'' (1769), correcting Blackstone's interpretation of the law, his grammar (a highly politicised subject at the time), and history. Blackstone, chastened, altered subsequent editions of his ''Commentaries'': he rephrased the offending passages and removed the sections claiming that Dissenters could not be loyal subjects, but he retained his description of Dissent as a crime.

Natural philosopher: electricity, ''Optics'', and carbonated water

Although Priestley claimed that natural philosophy was only a hobby, he took it seriously. In his ''History of Electricity'', he described the scientist as promoting the "security and happiness of mankind". Priestley's science was eminently practical and he rarely concerned himself with theoretical questions; his model was his close friend, Benjamin Franklin. When he moved to Leeds, Priestley continued his electrical and chemical experiments (the latter aided by a steady supply of carbon dioxide from a neighbouring brewery). Between 1767 and 1770, he presented five papers to the Royal Society from these initial experiments; the first four papers explored coronal discharges and other phenomena related to electrical discharge, while the fifth reported on the conductivity of charcoals from different sources. His subsequent experimental work focused on chemistry and pneumatics.

Priestley published the first volume of his projected history of experimental philosophy, ''The History and Present State of Discoveries Relating to Vision, Light and Colours'' (referred to as his ''Optics''), in 1772. He paid careful attention to the history of optics and presented excellent explanations of early optics experiments, but his mathematical deficiencies caused him to dismiss several important contemporary theories. He followed the (corpuscular) particle theory of light, influenced by the works of Reverend John Rowning and others. Furthermore, he did not include any of the practical sections that had made his ''History of Electricity'' so useful to practising natural philosophers. Unlike his ''History of Electricity'', it was not popular and had only one edition, although it was the only English book on the topic for 150 years. The hastily written text sold poorly; the cost of researching, writing, and publishing the ''Optics'' convinced Priestley to abandon his history of experimental philosophy.

Priestley was considered for the position of astronomer on James Cook's second voyage to the South Seas, but was not chosen. Still, he contributed in a small way to the voyage: he provided the crew with a method for making carbonated water, which he erroneously speculated might be a cure for scurvy. He then published a pamphlet with ''Directions for Impregnating Water with Fixed Air'' (1772). Priestley did not exploit the commercial potential of carbonated water, but others such as made fortunes from it. For his discovery of carbonated water Priestley has been labelled "the father of the soft drink", with the beverage company Schweppes regarding him as "the father of our industry". In 1773, the Royal Society recognised Priestley's achievements in natural philosophy by awarding him the Copley Medal.

Priestley's friends wanted to find him a more financially secure position. In 1772, prompted by Richard Price and Benjamin Franklin, Lord Shelburne wrote to Priestley asking him to direct the education of his children and to act as his general assistant. Although Priestley was reluctant to sacrifice his ministry, he accepted the position, resigning from Mill Hill Chapel on 20 December 1772, and preaching his last sermon on 16 May 1773.

Calne (1773–1780)

In 1773, the Priestleys moved to Calne in Wiltshire, and a year later Lord Shelburne and Priestley took a tour of Europe. According to Priestley's close friend Theophilus Lindsey, Priestley was "much improved by this view of mankind at large". Upon their return, Priestley easily fulfilled his duties as librarian and tutor. The workload was intentionally light, allowing him time to pursue his scientific investigations and theological interests. Priestley also became a political adviser to Shelburne, gathering information on parliamentary issues and serving as a liaison between Shelburne and the Dissenting and American interests. When the Priestleys' third son was born on 24 May 1777, they named him Henry at the lord's request.

Materialist philosopher

Priestley wrote his most important philosophical works during his years with Lord Shelburne. In a series of major metaphysical texts published between 1774 and 1780—''An Examination of Dr. Reid's Inquiry into the Human Mind'' (1774), ''Hartley's Theory of the Human Mind on the Principle of the Association of Ideas'' (1775), '' Disquisitions relating to Matter and Spirit'' (1777), '' The Doctrine of Philosophical Necessity Illustrated'' (1777), and '' Letters to a Philosophical Unbeliever'' (1780)—he argues for a philosophy that incorporates four concepts: determinism, materialism, causation, and necessitarianism. By studying the natural world, he argued, people would learn how to become more compassionate, happy, and prosperous.

Priestley strongly suggested that there is no mind-body duality, and put forth a materialist philosophy in these works; that is, one founded on the principle that everything in the universe is made of matter that we can perceive. He also contended that discussing the soul is impossible because it is made of a divine substance, and humanity cannot perceive the divine. Despite his separation of the divine from the mortal, this position shocked and angered many of his readers, who believed that such a duality was necessary for the soul to exist.

Responding to Baron d'Holbach's '' Système de la Nature'' (1770) and David Hume's '' Dialogues Concerning Natural Religion'' (1779) as well as the works of the French ''philosophers'', Priestley maintained that materialism and determinism could be reconciled with a belief in God. He criticised those whose faith was shaped by books and fashion, drawing an analogy between the scepticism of educated men and the credulity of the masses.

Maintaining that humans had no free will, Priestley argued that what he called "philosophical necessity" (akin to absolute determinism) is consonant with Christianity, a position based on his understanding of the natural world. Like the rest of nature, man's mind is subject to the laws of causation, Priestley contended, but because a benevolent God created these laws, the world and the people in it will eventually be perfected. Evil is therefore only an imperfect understanding of the world.

Although Priestley's philosophical work has been characterised as "audacious and original", it partakes of older philosophical traditions on the problems of free will, determinism, and materialism.McEvoy and McGuire, 341–45. For example, the 17th-century philosopher Baruch Spinoza argued for absolute determinism and absolute materialism. Like Spinoza and Priestley,
Leibniz argued that human will was completely determined by natural laws;
Adams, Robert Merrihew. ''Leibniz: Determinist, Theist, Idealist''. New York: Oxford University Press (1998), 10–13, 1–20, 41–44. .
unlike them, Leibniz argued for a "parallel universe" of immaterial objects (such as human souls) so arranged by God that its outcomes agree exactly with those of the material universe.
Leibniz
and Priestley
share an optimism that God has chosen the chain of events benevolently; however, Priestley believed that the events were leading to a glorious millennial conclusion, whereas for Leibniz the entire chain of events was optimal in and of itself, as compared with other conceivable chains of events.

Founder of British Unitarianism

When Priestley's friend Theophilus Lindsey decided to found a new Christian denomination that would not restrict its members' beliefs, Priestley and others hurried to his aid. On 17 April 1774, Lindsey held the first Unitarian service in Britain, at the newly formed Essex Street Chapel in London; he had even designed his own liturgy, of which many were critical. Priestley defended his friend in the pamphlet ''Letter to a Layman, on the Subject of the Rev. Mr. Lindsey's Proposal for a Reformed English Church'' (1774), claiming that only the form of worship had been altered, not its substance, and attacking those who followed religion as a fashion. Priestley attended Lindsey's church regularly in the 1770s and occasionally preached there. He continued to support institutionalised Unitarianism for the rest of his life, writing several ''Defenses'' of Unitarianism and encouraging the foundation of new Unitarian chapels throughout Britain and the United States.

Experiments and Observations on Different Kinds of Air

Priestley's years in Calne were the only ones in his life dominated by scientific investigations; they were also the most scientifically fruitful. His experiments were almost entirely confined to "airs", and out of this work emerged his most important scientific texts: the six volumes of '' Experiments and Observations on Different Kinds of Air'' (1774–86). These experiments helped repudiate the last vestiges of the theory of four elements, which Priestley attempted to replace with his own variation of phlogiston theory. According to that 18th-century theory, the combustion or oxidation of a substance corresponded to the release of a material substance, ''phlogiston''.

Priestley's work on "airs" is not easily classified. As historian of science Simon Schaffer writes, it "has been seen as a branch of physics, or chemistry, or natural philosophy, or some highly idiosyncratic version of Priestley's own invention". Furthermore, the volumes were both a scientific and a political enterprise for Priestley, in which he argues that science could destroy "undue and usurped authority" and that government has "reason to tremble even at an air pump or an electrical machine".

Volume I of ''Experiments and Observations on Different Kinds of Air'' outlined several discoveries: "nitrous air" (nitric oxide, NO); "vapor of spirit of salt", later called "acid air" or "marine acid air" ( anhydrous hydrochloric acid, HCl); "alkaline air" (ammonia, NH₃); "diminished" or "dephlogisticated nitrous air" (nitrous oxide, N₂O); and, most famously, "dephlogisticated air" (oxygen, O₂) as well as experimental findings that showed plants revitalised enclosed volumes of air, a discovery that would eventually lead to the discovery of photosynthesis by Jan Ingenhousz. Priestley also developed a "nitrous air test" to determine the "goodness of air". Using a pneumatic trough, he would mix nitrous air with a test sample, over water or mercury, and measure the decrease in volume—the principle of eudiometry.Fruton, 20, 29 After a small history of the study of airs, he explained his own experiments in an open and sincere style. As an early biographer writes, "whatever he knows or thinks he tells: doubts, perplexities, blunders are set down with the most refreshing candour." Priestley also described his cheap and easy-to-assemble experimental apparatus; his colleagues therefore believed that they could easily reproduce his experiments. Faced with inconsistent experimental results, Priestley employed phlogiston theory. This led him to conclude that there were only three types of "air": "fixed", "alkaline", and "acid". Priestley dismissed the burgeoning chemistry of his day. Instead, he focused on gases and "changes in their sensible properties", as had natural philosophers before him. He isolated carbon monoxide (CO), but apparently did not realise that it was a separate "air".

Discovery of oxygen

In August 1774 he isolated an "air" that appeared to be completely new, but he did not have an opportunity to pursue the matter because he was about to tour Europe with Shelburne. While in Paris, Priestley replicated the experiment for others, including French chemist Antoine Lavoisier. After returning to Britain in January 1775, he continued his experiments and discovered "vitriolic acid air" ( sulphur dioxide, SO₂).

In March he wrote to several people regarding the new "air" that he had discovered in August. One of these letters was read aloud to the Royal Society, and a paper outlining the discovery, titled "An Account of further Discoveries in Air", was published in the Society's journal ''Philosophical Transactions''. Priestley called the new substance "dephlogisticated air", which he made in the famous experiment by focusing the sun's rays on a sample of mercuric oxide. He first tested it on mice, who surprised him by surviving quite a while entrapped with the air, and then on himself, writing that it was "five or six times better than common air for the purpose of respiration, inflammation, and, I believe, every other use of common atmospherical air". He had discovered oxygen gas (O₂).

Priestley assembled his oxygen paper and several others into a second volume of ''Experiments and Observations on Air'', published in 1776. He did not emphasise his discovery of "dephlogisticated air" (leaving it to Part III of the volume) but instead argued in the preface how important such discoveries were to rational religion. His paper narrated the discovery chronologically, relating the long delays between experiments and his initial puzzlements; thus, it is difficult to determine when exactly Priestley "discovered" oxygen. Such dating is significant as both Lavoisier and Swedish pharmacist Carl Wilhelm Scheele have strong claims to the discovery of oxygen as well, Scheele having been the first to isolate the gas (although he published after Priestley) and Lavoisier having been the first to describe it as purified "air itself entire without alteration" (that is, the first to explain oxygen without phlogiston theory).

In his paper "Observations on Respiration and the Use of the Blood", Priestley was the first to suggest a connection between blood and air, although he did so using phlogiston theory. In typical Priestley fashion, he prefaced the paper with a history of the study of respiration. A year later, clearly influenced by Priestley, Lavoisier was also discussing respiration at the Académie des sciences. Lavoisier's work began the long train of discovery that produced papers on oxygen respiration and culminated in the overthrow of phlogiston theory and the establishment of modern chemistry.

Around 1779 Priestley and Shelburne – soon to be the 1st Marquess of Landsdowne – had a rupture, the precise reasons for which remain unclear. Shelburne blamed Priestley's health, while Priestley claimed Shelburne had no further use for him. Some contemporaries speculated that Priestley's outspokenness had hurt Shelburne's political career. Schofield argues that the most likely reason was Shelburne's recent marriage to Louisa Fitzpatrick—apparently, she did not like the Priestleys. Although Priestley considered moving to America, he eventually accepted Birmingham New Meeting's offer to be their minister.

Both Priestley and Shelburne's families upheld their Unitarian faith for generations. In December 2013, it was reported that Sir Christopher Bullock—a direct descendant of Shelburne's brother, Thomas Fitzmaurice (MP)—had married his wife, Lady Bullock, née Barbara May Lupton, at London's Unitarian Essex Church in 1917. Barbara Lupton was the second cousin of Olive Middleton, née Lupton, the great-grandmother of Catherine, Duchess of Cambridge. In 1914, Olive and Noel Middleton had married at Leeds' Mill Hill Chapel, which Priestley, as its minister, had once guided towards Unitarianism.

Birmingham (1780–1791)

In 1780 the Priestleys moved to Birmingham and spent a happy decade surrounded by old friends, until they were forced to flee in 1791 by religiously motivated mob violence in what became known as the Priestley Riots. Priestley accepted the ministerial position at New Meeting on the condition that he be required to preach and teach only on Sundays, so that he would have time for his writing and scientific experiments. As in Leeds, Priestley established classes for the youth of his parish and by 1781, he was teaching 150 students. Because Priestley's New Meeting salary was only 100 guineas, friends and patrons donated money and goods to help continue his investigations. He was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1782.

Chemical Revolution

Many of the friends that Priestley made in Birmingham were members of the Lunar Society, a group of manufacturers, inventors, and natural philosophers who assembled monthly to discuss their work. The core of the group included men such as the manufacturer Matthew Boulton, the chemist and geologist James Keir, the inventor and engineer James Watt, and the botanist, chemist, and geologist William Withering. Priestley was asked to join this unique society and contributed much to the work of its members. As a result of this stimulating intellectual environment, he published several important scientific papers, including "Experiments relating to Phlogiston, and the seeming Conversion of Water into Air" (1783). The first part attempts to refute Lavoisier's challenges to his work on oxygen; the second part describes how steam is "converted" into air. After several variations of the experiment, with different substances as fuel and several different collecting apparatuses (which produced different results), he concluded that air could travel through more substances than previously surmised, a conclusion "contrary to all the known principles of hydrostatics". This discovery, along with his earlier work on what would later be recognised as gaseous diffusion, would eventually lead John Dalton and Thomas Graham to formulate the kinetic theory of gases.

In 1777, Antoine Lavoisier had written ''Mémoire sur la combustion en général'', the first of what proved to be a series of attacks on phlogiston theory; it was against these attacks that Priestley responded in 1783. While Priestley accepted parts of Lavoisier's theory, he was unprepared to assent to the major revolutions Lavoisier proposed: the overthrow of phlogiston, a chemistry based conceptually on elements and compounds, and a new chemical nomenclature. Priestley's original experiments on "dephlogisticated air" (oxygen), combustion, and water provided Lavoisier with the data he needed to construct much of his system; yet Priestley never accepted Lavoisier's new theories and continued to defend phlogiston theory for the rest of his life. Lavoisier's system was based largely on the ''quantitative'' concept that mass is neither created nor destroyed in chemical reactions (i.e., the conservation of mass). By contrast, Priestley preferred to observe ''qualitative'' changes in heat, colour, and particularly volume. His experiments tested "airs" for "their solubility in water, their power of supporting or extinguishing flame, whether they were respirable, how they behaved with acid and alkaline air, and with nitric oxide and inflammable air, and lastly how they were affected by the electric spark."

By 1789, when Lavoisier published his '' Traité Élémentaire de Chimie'' and founded the '' Annales de Chimie'', the new chemistry had come into its own. Priestley published several more scientific papers in Birmingham, the majority attempting to refute Lavoisier. Priestley and other Lunar Society members argued that the new French system was too expensive, too difficult to test, and unnecessarily complex. Priestley in particular rejected its "establishment" aura. In the end, Lavoisier's view prevailed: his new chemistry introduced many of the principles on which modern chemistry is founded.

Priestley's refusal to accept Lavoisier's "new chemistry"—such as the conservation of mass—and his determination to adhere to a less satisfactory theory has perplexed many scholars. Schofield explains it thus: "Priestley was never a chemist; in a modern, and even a Lavoisierian, sense, he was never a scientist. He was a natural philosopher, concerned with the economy of nature and obsessed with an idea of unity, in theology and in nature." Historian of science John McEvoy largely agrees, writing that Priestley's view of nature as coextensive with God and thus infinite, which encouraged him to focus on facts over hypotheses and theories, prompted him to reject Lavoisier's system. McEvoy argues that "Priestley's isolated and lonely opposition to the oxygen theory was a measure of his passionate concern for the principles of intellectual freedom, epistemic equality and critical inquiry." Priestley himself claimed in the last volume of ''Experiments and Observations'' that his most valuable works were his theological ones because they were "superior ndignity and importance".

Defender of English Dissenters and French revolutionaries

Although Priestley spent much of this time defending phlogiston theory from the "new chemists", most of what he published in Birmingham was theological. For example, in 1782, he published the fourth volume of his ''Institutes'', '' An History of the Corruptions of Christianity'', describing how he thought the teachings of the early Christian church had been "corrupted" or distorted. Schofield describes the work as "derivative, disorganized, wordy, and repetitive, detailed, exhaustive, and devastatingly argued". The text addresses issues ranging from the divinity of Christ to the proper form for the Lord's Supper. In 1786, Priestley published its provocatively titled sequel, ''An History of Early Opinions concerning Jesus Christ, compiled from Original Writers, proving that the Christian Church was at first Unitarian''. Thomas Jefferson later wrote of the profound effect that these two books had on him: "I have read his Corruptions of Christianity, and Early Opinions of Jesus, over and over again; and I rest on them ... as the basis of my own faith. These writings have never been answered." Although a few readers such as Jefferson and other Rational Dissenters approved of the work, many others reviewed it harshly because of its extreme theological positions, particularly its rejection of the Trinity.

In 1785, while Priestley was engaged in a pamphlet war over ''Corruptions'', he also published ''The Importance and Extent of Free Enquiry'', claiming that the Reformation had not really reformed the church. In words that would boil over into a national debate, he challenged his readers to enact change:

Let us not, therefore, be discouraged, though, for the present, we should see no great number of churches professedly unitarian .... We are, as it were, laying gunpowder, grain by grain, under the old building of error and superstition, which a single spark may hereafter inflame, so as to produce an instantaneous explosion; in consequence of which that edifice, the erection of which has been the work of ages, may be overturned in a moment, and so effectually as that the same foundation can never be built upon again ....

Although discouraged by friends from using such inflammatory language, Priestley refused to back down from his opinions in print and he included it, forever branding himself as "Gunpowder Joe". After the publication of this seeming call for revolution in the midst of the French Revolution, pamphleteers stepped up their attacks on Priestley and he and his church were even threatened with legal action.

In 1787, 1789, and 1790, Dissenters again tried to repeal the Test and Corporation Acts. Although they might have succeeded initially, by 1790, with the fears of revolution looming in Parliament, few were swayed by appeals to equal rights. Political cartoons, one of the most effective and popular media of the time, skewered the Dissenters and Priestley. In Parliament, William Pitt and Edmund Burke argued against the repeal, a betrayal that angered Priestley and his friends, who had expected the two men's support. Priestley wrote a series of ''Letters to William Pitt'' and ''Letters to Burke''.

Dissenters such as Priestley who supported the French Revolution came under increasing suspicion as scepticism regarding the revolution grew. In its propaganda against " radicals", Pitt's administration used the "gunpowder" statement to argue that Priestley and other Dissenters wanted to overthrow the government. Burke, in his famous '' Reflections on the Revolution in France'' (1790), tied natural philosophers, and specifically Priestley, to the French Revolution, writing that radicals who supported science in Britain "considered man in their experiments no more than they do mice in an air pump". Burke also associated republican principles with alchemy and insubstantial air, mocking the scientific work done by both Priestley and French chemists. He made much in his later writings of the connections between "Gunpowder Joe", science, and Lavoisier—who was improving gunpowder for the French in their war against Britain. Paradoxically, a secular statesman, Burke, argued against science and maintained that religion should be the basis of civil society, whereas a Dissenting minister, Priestley, argued that religion could not provide the basis for civil society and should be restricted to one's private life.

Priestley also supported the campaign to abolish the British slave trade and published a sermon in 1788 in which he declared that nobody treated enslaved people "with so much cruelty as the English".

Birmingham riots of 1791

The animus that had been building against Dissenters and supporters of the American and French Revolutions exploded in July 1791. Priestley and several other Dissenters had arranged to have a celebratory dinner on Bastille Day, the anniversary of the storming of the Bastille, a provocative action in a country where many disapproved of the French Revolution and feared that it might spread to Britain. Amid fears of violence, Priestley was convinced by his friends not to attend. Rioters gathered outside the hotel during the banquet and attacked the attendees as they left. The rioters moved on to the New Meeting and Old Meeting churches, and burned both to the ground. Priestley and his wife fled from their home; although their son William and others stayed behind to protect their property, the mob overcame them and torched Priestley's house "Fairhill" at Sparkbrook, destroying his valuable laboratory and all of the family's belongings. Twenty-six other Dissenters' homes and three more churches were burned in the three-day riot. Priestley spent several days hiding with friends until he was able to travel safely to London. The carefully executed attacks of the "mob" and the farcical trials of only a handful of the "leaders" convinced many at the time—and modern historians later—that the attacks were planned and condoned by local Birmingham magistrates. When George III was eventually forced to send troops to the area, he said: "I cannot but feel better pleased that Priestley is the sufferer for the doctrines he and his party have instilled, and that the people see them in their true light."

Hackney (1791–1794)

Unable to return to Birmingham, the Priestleys eventually settled in Lower Clapton, a district in Hackney, Middlesex where he gave a series of lectures on history and natural philosophy at the Dissenting academy, the New College at Hackney. Friends helped the couple rebuild their lives, contributing money, books, and laboratory equipment. Priestley tried to obtain restitution from the government for the destruction of his Birmingham property, but he was never fully reimbursed. He also published ''An Appeal to the Public on the Subject of the Riots in Birmingham'' (1791), which indicted the people of Birmingham for allowing the riots to occur and for "violating the principles of English government".

The couple's friends urged them to leave Britain and emigrate to either France or the new United States, even though Priestley had received an appointment to preach for the Gravel Pit Meeting congregation. Priestley was minister between 1793 and 1794 and the sermons he preached there, particularly the two Fast Sermons, reflect his growing millenarianism, his belief that the end of the world was fast approaching. After comparing Biblical prophecies to recent history, Priestley concluded that the French Revolution was a harbinger of the Second Coming of Christ. Priestley's works had always had a millennial cast, but after the beginning of the French Revolution, this strain increased. He wrote to a younger friend that while he himself would not see the Second Coming, his friend "may probably live to see it ... It cannot, I think be more than twenty years way"

Daily life became more difficult for the family: Priestley was burned in effigy along with Thomas Paine; vicious political cartoons continued to be published about him; letters were sent to him from across the country, comparing him to the devil and Guy Fawkes; tradespeople feared the family's business; and Priestley's Royal Academy friends distanced themselves. As the penalties became harsher for those who spoke out against the government, Priestley examined options for removing himself and his family from England.

Joseph Priestley's son William was presented to the French Assembly and granted letters of naturalisation on 8 June 1792. Priestley learned about it from the ''Morning Chronicle''. A decree of 26 August 1792 by the French National Assembly conferred French citizenship on Joseph Priestley and others who had "served the cause of liberty" by their writings. Priestley accepted French citizenship, considering it "the greatest of honours". In the French National Convention election on 5 September 1792, Joseph Priestley was elected to the French National Convention by at least two departments, (Orne

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