Joseph Priestley

Priestley eventually decided to return to his theological studies and, in 1752, matriculated at Daventry, a Dissenting academy.[19] Because he had already read widely, Priestley was allowed to skip 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 the rational analysis of the natural world and the Bible.[20]

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".[21]

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".[33]

A redacted version of A New Chart of History (1765); Priestley believed this chart would "impress" upon students "a just image of the rise, progress, extent, duration, and contemporary state of all the considerable empires that have ever existed in the world"[34]

In his Essay on a Course of Liberal Education for Civil and Active Life (1765),[35] 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.[36] 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.[37] Lectures on History was well received and was employed by many educational institutions, such as New College at Hackney, Brown, Princeton, Yale, and Cambridge.[38] Priestley designed two Charts to serve as visual study aids for his Lectures.[39] These charts are in fact timelines; they have been described as the most influential timelines published in the 18th century.[40] 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.[41] 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.[42]

Priestley's "electrical machine for amateur experimentalists", illustrated in the first edition of his Familiar Introduction to the Study of Electricity (1768)

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.[43] 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. Priestly also consulted with Franklin during the latter's kite experiments.[44][45] In the process of replicating others' experiments, Priestley became intrigued by unanswered questions and was prompted to undertake experiments of his own design.[46] (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.)[47]

In 1767, the 700-page The History and Present State of Electricity was published to positive reviews.[48] 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.[49] 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.[50] 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.[51][52] He did not generalise or elaborate on this,[49] and the general law was enunciated by French physicist Charles-Augustin de Coulomb in the 1780s.[53]

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).[54] He marketed the book with his brother Timothy, but unsuccessfully.[55]

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.[60] Priestley believed that by educating the young, he could strengthen the bonds of the congregation.[61]

In his three-volume Institutes of Natural and Revealed Religion (1772–74),[62] 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 belief that a divine being acts in accordance with necessary metaphysical laws).[63]

Priestley had been working on Institutes of Natural and Revealed Religion since his Daventry days.

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. Because his views of religion were deeply tied to his understanding of nature, the text's theism rested on the argument from design.[64] 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."[65] 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.[66]

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",[67] but as Schofield points out Priestley rarely altered his opinion as a result of these debates.[67] 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.[68]

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 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.[69] He revived it in 1784 with similar results.[70]

His Essay on the First Principles of Government (1768) influenced early 19th-century political philosophers, including Jeremy Bentham.[71]

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.[72]

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).[73] 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.[74]

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.[75] 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.[76]

Optics: The History and Present State of Vision, Light, and Colours, published in 1772, London

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".[77] Priestley's science was eminently practical and he rarely concerned himself with theoretical questions; his model was his clse friend, Benjamin Franklin.[78] 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.[79]

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.[80] 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.[81] 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.[82]

External audio
“Fizzy Water”, Distillations Podcast Episode 217, Science History Institute

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).[83] Priestley did not exploit the commercial potential of carbonated water, but others such as J. J. Schweppe made fortunes from it.[84] For his discovery of carbonated water Priestley has been labelled "the father of the soft drink",[85] with the beverage company Schweppes regarding him as “the father of our industry”.[86] In 1773, the Royal Society recognised Priestley's achievements in natural philosophy by awarding him the Copley Medal.[3][87]

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.[88]

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.[92]

By 1782, at least a dozen hostile refutations were published to Disquisitions relating to Matter and Spirit, and Priestley was branded an atheist.[93]

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.[94]

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.[95]

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.[96]

Although Priestley's philosophical work has been characterised as "audacious and original",[12][97] it partakes of older philosophical traditions on the problems of free will, determinism, and materialism.[98] For example, the 17th-century philosopher Baruch Spinoza argued for absolute determinism and absolute materialism.[99] Like Spinoza[100] and Priestley,[101] Leibniz argued that human will was completely determined by natural laws;[102] 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.[103] Leibniz[104] and Priestley[105] 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,[12] whereas for Leibniz the entire chain of events was optimal in and of itself, as compared with other conceivable chains of events.[106]

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),[107] 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.[108] 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.[109]

Equipment used by Priestley in his experiments on gases, 1775

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).[110][111] 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.[112]

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".[113] 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".[114]

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. 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.[115] 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."[116] Priestley also described his cheap and easy-to-assemble experimental apparatus; his colleagues therefore believed that they could easily reproduce his experiments.[117] 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".[118]

The laboratory at Lord Shelburne's estate, Bowood House in Wiltshire, in which Priestley discovered 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.[119] 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".[120] He had discovered oxygen gas (O₂).

William Petty-Fitzmaurice, 1st Marquess of Landsdowne – a fellow Unitarian – built a laboratory for the famous dissenter at Bowood House.

Reproduction of Joseph Priestley's oxygen apparatus

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.[121] 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).[122]

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.[123]

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.[124]

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.[125]

One of a set of Lunar Society Moonstones commemorating Priestley at Great Barr, Birmingham

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.[128] 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".[129] 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.[130]

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;[131] 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, color, 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."[132]

Antoine Lavoisier and his wife, Marie-Anne Pierrette Paulze, by Jacques-Louis David, 1788

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.[133]

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.[134] 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."[135] 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.[136] 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."[137] Priestley himself claimed in the last volume of Experiments and Observations that his most valuable works were his theological ones because they were "superior [in] dignity and importance".[138]

DOCTOR PHLOGISTON, The PRIESTLEY politician or the Political Priest: An anti-Priestley cartoon shows him trampling on the Bible and burning documents representing English freedom. "Essays on Matter and Spirit", "Gunpowder", and "Revolution Toasts" bulge from his pockets.

Although Priestley was busy defending phlogiston theory from the "new chemists", most of what he published in Birmingham was theological. 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.[139] Schofield describes the work as "derivative, disorganized, wordy, and repetitive, detailed, exhaustive, and devastatingly argued".[140] The text addresses issues ranging from the divinity of Christ to the proper form for the Lord's Supper. Priestley followed up in 1786 with the provocatively titled book, An History of Early Opinions concerning Jesus Christ, compiled from Original Writers, proving that the Christian Church was at first Unitarian. Thomas Jefferson would later write 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."[141] Although a few readers such as Jefferson and other Rational Dissenters approved of the work, it was harshly reviewed because of its extreme theological positions, particularly its rejection of the Trinity.[142]

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.[143] 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 ....[144]

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.[145]

"A Word of Comfort" by William Dent (dated 22 March 1790). Priestley is preaching in front of Charles James Fox who asks "Pray, Doctor, is there such a thing as a Devil?", to which Priestley responds "No" while the devil prepares to attack Priestley from behind.

In 1787, 1789, and 1790, Dissenters again tried to repeal the Test and Corporation Acts. Although initially it looked as if they might succeed, 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.[146] 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[147] and Letters to Burke[148] in an attempt to persuade them otherwise, but these publications only further inflamed the populace against him.

Dissenters such as Priestley who supported the French Revolution came under increasing suspicion as scepticism regarding the revolution grew.[149] 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".[150] 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.[151] 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.[152]

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".[153]

The attack on Priestley's home, Fairhill, Sparkbrook, Birmingham

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 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.[154] 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."[155]

• "Joseph Priestley: Discoverer of Oxygen" at the American Chemical Society
• Joseph Priestley at the Woodrow Wilson National Fellowship Foundation
• Joseph Priestley from the Encyclopædia Britannica
• Chisholm, Hugh, ed. (1911). "Priestley, Joseph" . Encyclopædia Britannica (11th ed.). Cambridge University Press.
• "Priestley, Joseph" . Dictionary of National Biography. London: Smith, Elder & Co. 1885–1900.
• ExplorePAHistory.com
• Poliakoff, Martyn. "Joseph Priestley". The Periodic Table of Videos. University of Nottingham.