Joseph Priestley FRS (pronounced /ˈpriːstli/; born March 24, 1733; died February 6, 1804) was an English scientist, religious leader, teacher, and thinker who studied many subjects. He wrote over 150 books and conducted experiments in different areas of science.

Priestley discovered oxygen independently by heating mercury oxide in 1774. During his lifetime, he was well-known for creating carbonated water, writing about electricity, and identifying several gases, including "dephlogisticated air," which is now known as oxygen. His support for an old scientific idea called the phlogiston theory and his rejection of new scientific ideas caused him to lose favor with many scientists.

Priestley believed that understanding the natural world could help people improve and eventually lead to a better future. He tried to combine scientific thinking with religious beliefs, especially by connecting Enlightenment ideas with Christian teachings. He wrote books that explored how God, the physical world, and the idea that events happen for a reason could work together. Priestley also supported religious freedom and equal rights for people who did not follow the main church in England, which helped shape the Unitarian movement. His writings on politics, religion, and science, along with his support for the American and French Revolutions, caused some people to dislike him. In 1791, after a mob destroyed his home and church in Birmingham, he moved to London and later to the United States.

Priestley was a lifelong teacher and wrote important books on grammar, history, and timelines. His works on education were widely read. His writings on philosophy, especially his book Essay on the First Principles of Government (1768), are still studied today as early examples of modern liberal political ideas.

Early life and education (1733–1755)

Joseph Priestley was born in Birstall, which is near Batley in the West Riding of Yorkshire, to a family that did not follow the Church of England. He was the oldest of six children born to Mary Swift and Jonas Priestley, who worked with cloth. At about one year old, Priestley lived with his grandfather. He returned home five years later after his mother passed away. In 1741, when his father remarried, Priestley moved to live with his aunt and uncle, Sarah and John Keighley, who were wealthy and childless. Their home, the Old Hall in Heckmondwike, was three miles from Fieldhead.

Priestley was a very bright child. At age four, he could perfectly recite all 107 questions and answers from the Westminster Shorter Catechism. His aunt wanted him to receive the best education possible, hoping he would become a minister. During his youth, he attended local schools where he studied Greek, Latin, and Hebrew.

Around 1749, Priestley became seriously ill and believed he was dying. Raised as a devout Calvinist, he thought a spiritual experience was needed for salvation but doubted he had one. This caused him emotional pain and led him to question his religious beliefs. He began to reject the idea of divine election and accepted the belief in universal salvation. Because of this, the leaders of his church, the Independent Upper Chapel of Heckmondwike near Leeds, refused to let him become a full member.

Priestley’s illness left him with a permanent stutter, and he gave up the idea of becoming a minister at that time. To prepare for joining a relative in trade in Lisbon, he studied French, Italian, and German, as well as Aramaic and Arabic. He was taught by Reverend George Haggerstone, who introduced him to advanced mathematics, natural philosophy, logic, and metaphysics through the works of Isaac Watts, Willem 's Gravesande, and John Locke.

Later, Priestley returned to theological studies and, in 1752, enrolled at Daventry, a Dissenting academy in Northamptonshire, England. Because he had already read widely, he was allowed to skip the first two years of coursework. His intense studies, combined with the school’s open-minded environment, led him to adopt more progressive religious views. He became a Rational Dissenter, a group that rejected religious dogma and mysticism, instead focusing on using reason and science to understand the Bible and the natural world.

Priestley later said the book that most influenced him, aside from the Bible, was David Hartley’s Observations on Man (1749). Hartley’s work explored the mind as a physical process and aimed to create a scientific Christian philosophy. Priestley dedicated his life to this goal. In his third year at Daventry, he committed himself to the ministry, calling it “the noblest of all professions.”

Needham Market and Nantwich (1755–1761)

Robert Schofield, a major modern biographer of Priestley, called Priestley’s first "call" in 1755 to the Dissenting parish in Needham Market, Suffolk, a problem for both Priestley and the congregation. Priestley wanted to live in a city and take part in theological debates, but Needham Market was a small, rural town where the congregation valued tradition. Attendance and donations decreased quickly when the congregation learned about Priestley’s different beliefs. His aunt had promised to support him if he became a minister, but she stopped helping when she realized he no longer followed Calvinist beliefs. To earn more money, Priestley suggested opening a school, but local families told him they would not send their children. He also gave a series of scientific lectures called "Use of the Globes," which were more successful.

Priestley’s friends in Daventry helped him find another job, 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 in Nantwich cared less about Priestley’s different beliefs, and he successfully started a school. Unlike many schoolmasters at the time, Priestley taught his students about natural philosophy and even bought scientific instruments for them. Unhappy with the quality of English grammar books, Priestley wrote his own: The Rudiments of English Grammar (1761). His new ways of describing English grammar, especially his effort to separate it from Latin grammar, led 20th-century scholars to call him "one of the great grammarians of his time." After publishing Rudiments and the success of his school, Warrington Academy offered him a teaching position in 1761.

Warrington Academy (1761–1767)

In 1761, Priestley moved to Warrington in Cheshire and became a teacher of modern languages and rhetoric at the town’s Dissenting academy, though he preferred to teach mathematics and natural philosophy. He adjusted well to life in Warrington and made friends quickly. These included the doctor and writer John Aikin, his sister Anna Laetitia Aikin, a children’s author, and Josiah Wedgwood, a potter and businessman. Wedgwood met Priestley in 1762 after falling from his horse. Though they met rarely, they exchanged letters and shared advice on chemistry and laboratory equipment. Wedgwood later created a medallion of Priestley in cream-on-blue jasperware.

On June 23, 1762, Priestley married Mary Wilkinson of Wrexham. Priestley wrote about his marriage:
On April 17, 1763, the couple had a daughter named Sarah, after Priestley’s aunt.

Beyond his work on "airs," Priestley is credited with inventing carbonated water (soda water) in 1767 after developing a method to mix water with carbon dioxide gas.

All books Priestley published while in Warrington emphasized the study of history. He believed history was important for success in life and for religious growth. He wrote histories of science and Christianity to show how humanity had progressed and, paradoxically, how "primitive Christianity" had been lost.

In works like Essay on a Course of Liberal Education for Civil and Active Life (1765) and Lectures on History and General Policy (1788), Priestley argued that education should prepare young people for practical needs. He believed modern languages and history were more useful than classical languages or ancient history. His lectures on history were revolutionary, as he described history as showing both God’s natural laws and human progress. He also believed studying history was a moral duty and supported the education of middle-class women, which was unusual at the time. Scholars have called Priestley the most important English writer on education between the 17th-century John Locke and the 19th-century Herbert Spencer. His Lectures on History was widely used by schools, including New College at Hackney, Brown, Princeton, Yale, and Cambridge. Priestley created two visual charts to help students understand history. These charts, which are timelines, were described as the most influential timelines of the 18th century. Impressed by his work, the University of Edinburgh granted Priestley a Doctor of Law degree in 1764. During this time, Priestley also gave lectures on rhetoric, which were published in 1777 as A Course of Lectures on Oratory and Criticism.

The intellectual environment in Warrington, often called the "Athens of the North" (of England), encouraged Priestley’s interest in natural philosophy. He taught anatomy and performed temperature experiments with a friend, John Seddon. Despite his busy schedule, Priestley decided to write a history of electricity. Friends introduced him to leading scientists in Britain, including John Canton, William Watson, Timothy Lane, and Benjamin Franklin, who encouraged Priestley to conduct experiments. Priestley also observed Franklin’s kite experiments. While repeating others’ work, Priestley noticed unanswered questions and designed his own experiments. His charts and manuscript on electricity impressed scientists, who nominated him for a Royal Society fellowship, which he received in 1766.

In 1767, Priestley published The History and Present State of Electricity, a 700-page book that received praise. The first half described the study of electricity up to 1766, while the second half explained modern theories and suggested future research. Priestley emphasized that scientific discoveries should include all steps, even mistakes, unlike Newton’s approach, which focused only on final results. He reported his own findings, such as the conductivity of charcoal and the connection between conductors and non-conductors, which challenged the belief that only water and metals could conduct electricity. His experiments on materials and chemical changes showed his early interest in the relationship between electricity and chemistry. Based on experiments with charged spheres, Priestley proposed that electrical force followed an inverse-square law, similar to Newton’s law of gravity. Though he did not expand on this idea, French physicist Charles-Augustin de Coulomb later formalized the law in the 1780s.

Priestley’s strength as a natural philosopher was in observation rather than numbers. His discovery of "a current of real air" between electrified points later interested scientists like Michael Faraday and James Clerk Maxwell as they studied electromagnetism. His book became the standard history of electricity for over a century, used by scientists such as Alessandro Volta (who invented the battery), William Herschel (who discovered infrared radiation), and Henry Cavendish (who discovered hydrogen). Priestley also wrote a simpler version of his electricity history for the public, titled A Familiar Introduction to the Study of Electricity (1768). He promoted the book with his brother Timothy, but it was not successful.

Leeds (1767–1773)

Joseph Priestley moved with his family from Warrington to Leeds in 1767. The reasons for this move are not certain, but they may have included his wife Mary’s poor health, financial difficulties, or a desire to prove himself to the community that had rejected him in his youth. In Leeds, two sons were born to the Priestleys: Joseph, Junior, on July 24, 1768, and William three years later. Theophilus Lindsey, a clergyman in Yorkshire, became one of Priestley’s few friends in Leeds. Priestley wrote that he always sought Lindsey’s advice before publishing important theological works. Although Priestley had relatives living near Leeds, they did not seem to communicate with him. Schofield suggested that they may have avoided him because of his religious beliefs, which some considered heretical. Each year, Priestley traveled to London to meet his friend and publisher, Joseph Johnson, and to attend meetings of the Royal Society.

When Priestley became the minister of Mill Hill Chapel in Leeds, the chapel was one of the oldest and most respected Dissenting congregations in England. However, during the early 18th century, the congregation had split into groups with different beliefs and was losing members to the Methodist movement. Priestley believed that teaching young people in the congregation could help strengthen its unity.

In his three-volume work, Institutes of Natural and Revealed Religion (1772–74), Priestley explained his ideas about religious education. He also described his belief in Socinianism, a religious philosophy that would later influence Unitarians in Britain. This work marked a shift in Priestley’s religious thinking, as it laid the foundation for his later ideas about materialism and necessitarianism (the belief that a divine being acts according to fixed laws). Priestley argued that only religious truths that matched natural experiences could be accepted. He based his belief in God on the idea that the natural world shows evidence of design. His work upset many readers because it challenged traditional Christian beliefs, such as the divinity of Christ and the Virgin Birth. Methodists in Leeds wrote a hymn calling for God to remove Priestley’s ideas from the world. Priestley believed that Christianity should return to its original, uncorrupted form. The fourth part of his Institutes, An History of the Corruptions of Christianity, was so long that it was published separately in 1782. Priestley considered this work his most valuable. By urging readers to use science and history to examine the Bible and Christianity, he angered both religious and scientific audiences.

Priestley wrote many political and religious pamphlets. Schofield noted that Priestley believed he was always correct in debates, while his opponents thought he was deliberately wrong. Priestley often contrasted his calm reasoning with his opponents’ anger, but he rarely changed his views. In Leeds, he wrote pamphlets on the Lord’s Supper and Calvinist doctrine. These works were widely read.

In 1768, Priestley founded the Theological Repository, a journal that encouraged open and rational discussion of religious questions. Although he promised to publish any submission, only people who shared his views sent articles. Priestley therefore wrote much of the journal himself. This material later became the basis for his theological and philosophical works. Due to lack of money, he stopped publishing the journal after a few years. He briefly revived it in 1784, but it failed again.

Many of Priestley’s writings supported the repeal of the Test and Corporation Acts, which limited the rights of Dissenters. These laws prevented Dissenters from holding political office, serving in the military, or attending Oxford and Cambridge unless they agreed with the Church of England’s Thirty-nine Articles. Dissenters repeatedly asked Parliament to remove these laws, arguing that they were being treated unfairly.

Priestley’s friends, who were also Rational Dissenters, encouraged him to write about the injustices faced by Dissenters. This led to his Essay on the First Principles of Government (1768), an early work of modern liberal political theory. In it, Priestley clearly separated political rights from civil rights and argued for strong civil rights. He believed that the government should only control public matters, while education and religion should be private decisions. His later political views grew more radical because he believed the British government was violating individual freedoms.

Priestley defended Dissenters against William Blackstone, a famous legal theorist. Blackstone’s Commentaries on the Laws of England (1765–69) claimed that dissent from the Church of England was a crime and that Dissenters could not be loyal citizens. Priestley responded with Remarks on Dr. Blackstone’s Commentaries (1769), correcting Blackstone’s legal interpretations, grammar, and historical claims. Blackstone revised later editions of his book, removing statements that labeled Dissenters as disloyal but keeping his description of Dissent as a crime.

Although Priestley said natural philosophy was just a hobby, he studied it seriously. In his History of Electricity, he described scientists as working to improve human life. His scientific work was practical, and he focused on experiments rather than theory, following the example of his friend Benjamin Franklin. In Leeds, Priestley continued his experiments in electricity and chemistry, aided by carbon dioxide from a nearby brewery. Between 1767 and 1770, he presented five papers to the Royal Society, exploring electrical discharges and the conductivity of different types of charcoal. His later experiments focused on chemistry and pneumatics.

In 1772, Priestley published the first volume of his planned history of experimental philosophy, The History and Present State of Discoveries Relating to Vision, Light and Colours (known as his Optics). He carefully explained early experiments in optics but avoided complex mathematics. He followed the particle theory of light, influenced by Reverend John Rowning and others. Unlike his History of Electricity, his Optics did not include practical sections that had made the earlier work useful to scientists.

Calne (1773–1780)

In 1773, the Priestleys moved to Calne in Wiltshire, southwestern England, and a year later, Lord Shelburne and Priestley traveled across Europe. According to Priestley's close friend Theophilus Lindsey, Priestley was "much improved by this view of mankind at large." After returning, Priestley easily completed his duties as librarian and tutor. His workload was intentionally light, giving him time to study science and explore religious ideas. Priestley also became a political adviser to Shelburne, collecting information about parliamentary matters and acting as a connection between Shelburne and groups like the Dissenting and American communities. When their third son was born on May 24, 1777, they named him Henry at the lord's request.

During his time with Lord Shelburne, Priestley wrote some of his most important philosophical works. Between 1774 and 1780, he published several major texts, including 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). In these works, he argued for a philosophy based on four ideas: determinism, materialism, causation, and necessitarianism. He believed that by studying the natural world, people could learn to be more compassionate, happy, and prosperous.

Priestley claimed there is no mind-body duality, meaning he believed the mind and body are not separate. He supported a materialist philosophy, which holds that everything in the universe is made of matter we can perceive. He also argued that discussing the soul is impossible because it is made of a divine substance, which humans cannot see. This belief surprised and upset many readers, who thought a mind-body duality was necessary for the soul to exist.

Priestley responded to works like Baron d'Holbach's Système de la Nature (1770) and David Hume's Dialogues Concerning Natural Religion (1779), as well as writings by French philosophers. He believed materialism and determinism could coexist with a belief in God. He criticized people whose faith was shaped by books and fashion, comparing the skepticism of educated men to the blind belief of the masses.

Priestley argued that humans have no free will, claiming that "philosophical necessity" (similar to absolute determinism) aligns with Christianity. He believed the mind, like the rest of nature, follows the laws of causation. However, since a kind God created these laws, the world and its people will eventually be perfected. Evil, he said, is simply a lack of understanding of the world.

Although Priestley's philosophy was seen as "audacious and original," it built on older ideas about free will, determinism, and materialism. For example, the 17th-century philosopher Baruch Spinoza also argued for absolute determinism and materialism. Like Spinoza, Leibniz believed human will was determined by natural laws, but he also proposed a "parallel universe" of immaterial objects, such as human souls, arranged by God. Both Leibniz and Priestley believed God chose events benevolently, but Priestley saw these events leading to a glorious future, while Leibniz thought the chain of events was already perfect.

Priestley rejected René Descartes's idea that animals are like machines without feelings or consciousness. Using David Hartley's associationist psychology, he argued that mental functions come from physical processes and that human and animal minds are similar. He believed animals have the same abilities as humans "in kind, though not in degree," with differences in behavior due to brain structure, not the absence of soul or reason.

In Disquisitions Relating to Matter and Spirit (1777), Priestley criticized Descartes and Ralph Cudworth for underestimating animal intelligence. He rejected Cudworth's idea of an immaterial "plastic nature" guiding animal behavior, insisting that both human and animal minds operate through physical laws. Priestley also suggested animals might share in a future life, especially because they can suffer, and that divine justice must apply to all creatures capable of suffering.

When Priestley's friend Theophilus Lindsey started a new Christian group that allowed members to believe freely, Priestley supported him. On April 17, 1774, Lindsey held the first Unitarian service in Britain at the Essex Street Chapel in London. Priestley defended Lindsey in a pamphlet, arguing that only worship practices changed, not the core beliefs. He regularly attended Lindsey's church in the 1770s and preached there sometimes. Priestley supported Unitarianism for the rest of his life, writing defenses of the movement and encouraging new Unitarian chapels in Britain and the United States.

Priestley's time in Calne was the most scientifically productive of his life. His work focused on "airs," leading to his most important scientific texts: six volumes of Experiments and Observations on Different Kinds of Air (1774–86). These experiments helped reject the old theory of four elements and replaced it with a version of the phlogiston theory. According to that 18th-century theory, burning a substance released a material called phlogiston.

Priestley's work on "airs" is hard to classify. Historian Simon Schaffer noted it "has been seen as a branch of physics, chemistry, natural philosophy, or something unique to Priestley." The volumes were also a political effort, arguing that science could challenge unfair authority and that governments should fear scientific tools like air pumps or electrical machines.

Volume I of Experiments and Observations on Different Kinds of Air described discoveries such as "nitrous air" (nitric oxide, NO), "vapor of spirit of salt" (anhydrous hydrochloric acid, HCl), "alkaline air" (ammonia, NH₃), "diminished" or "dephlogisticated nitrous air" (nitrous oxide, N₂O), and "dephlogisticated air" (oxygen, O₂). Priestley also showed that plants can improve enclosed air, a finding that later helped discover photosynthesis by Jan Ingenhousz. He developed a "nitrous air test" to measure air quality using a pneumatic trough, mixing nitrous air with samples over water or mercury and measuring volume changes—the principle of eudiometry.

Birmingham (1780–1791)

In 1780, the Priestleys moved to Birmingham and lived there happily for ten years with friends until they had to leave in 1791 because of violent attacks by a mob motivated by religious differences, known as the Priestley Riots. Priestley accepted a job as a minister at New Meeting with the condition that he only preach and teach on Sundays, so he could focus on his writing and scientific experiments. Like in Leeds, he started classes for young people in his community, teaching 150 students by 1781. His salary from New Meeting was only 100 guineas, so friends and supporters gave him money and supplies to continue his research. In 1782, he was chosen as a Foreign Honorary Member of the American Academy of Arts and Sciences.

Many of Priestley’s friends in Birmingham were part of the Lunar Society, a group of inventors, scientists, and businesspeople who met monthly to share ideas. Key members included Matthew Boulton, a manufacturer; James Keir, a chemist and geologist; James Watt, an inventor and engineer; and William Withering, a botanist, chemist, and geologist. Priestley joined the society and helped its members with their work. Because of the lively discussions, he wrote several important scientific papers, including "Experiments relating to Phlogiston, and the seeming Conversion of Water into Air" (1783). This paper had two parts: the first tried to explain why his work on oxygen was correct despite challenges from Antoine Lavoisier; the second described how steam could be turned into air. After testing different fuels and tools, Priestley concluded that air could pass through more materials than previously thought, a finding that went against known scientific ideas. This discovery, along with his earlier work on gas movement, later helped scientists like John Dalton and Thomas Graham develop the kinetic theory of gases.

In 1777, Antoine Lavoisier published "Mémoire sur la combustion en général," the first of many writings that criticized the phlogiston theory. Priestley responded to these challenges in 1783. While Priestley agreed with some parts of Lavoisier’s ideas, he refused to accept Lavoisier’s major changes, such as rejecting the phlogiston theory, using a new system of chemistry based on elements and compounds, and introducing a new way to name chemicals. Priestley’s experiments on "dephlogisticated air" (oxygen), combustion, and water gave Lavoisier the data needed to build his system. However, Priestley never accepted Lavoisier’s new theories and continued to support the phlogiston theory for the rest of his life. Lavoisier’s system was based on the idea that mass is not created or destroyed in chemical reactions, known as the conservation of mass. Priestley, however, focused more on observing changes in heat, color, and volume during experiments. His tests looked at how different "airs" dissolved in water, supported or stopped fire, could be breathed, and reacted with acids, bases, nitric oxide, and flammable air, as well as how they responded to electric sparks.

By 1789, when Lavoisier published his "Traité Élémentaire de Chimie" and started the "Annales de Chimie," his new chemistry became widely accepted. Priestley continued writing scientific papers in Birmingham, most of which tried to argue against Lavoisier’s ideas. Priestley and other members of the Lunar Society believed the French system was too costly, hard to test, and overly complicated. Priestley especially disliked how it seemed too formal. In the end, Lavoisier’s ideas won out, forming the basis of modern chemistry.

Priestley’s refusal to accept Lavoisier’s new chemistry, such as the conservation of mass, and his continued support for the less accurate phlogiston theory have puzzled many scholars. Historian Schofield explained that Priestley was not a chemist in the modern sense but a natural philosopher who focused on nature’s balance and believed in unity in both religion and science. Historian John McEvoy added that Priestley saw nature as infinite and connected to God, which made him value facts over theories. He also argued that Priestley’s opposition to Lavoisier’s system showed his strong belief in intellectual freedom and equal rights to question ideas. Priestley himself said in the last volume of his "Experiments and Observations" that his most important works were his religious writings because they were "superior in dignity and importance."

Although Priestley spent much time defending the phlogiston theory against new chemists, most of his published works in Birmingham were religious. For example, in 1782, he released the fourth volume of his "Institutes," titled "An History of the Corruptions of Christianity," where he argued that early Christian teachings had been changed or distorted. Schofield described the book as "derivative, disorganized, wordy, and repetitive, detailed, exhaustive, and devastatingly argued." It covered topics like the divinity of Christ and the proper way to celebrate the Lord’s Supper. In 1786, Priestley published a follow-up titled "An History of Early Opinions concerning Jesus Christ," which claimed the early Christian Church was Unitarian. Thomas Jefferson later said these books deeply influenced him, writing that he relied on them as the foundation of his own beliefs.

While some readers, like Jefferson and other Rational Dissenters, supported Priestley’s work, others criticized it for its extreme religious views, especially its rejection of the Trinity. In 1785, while Priestley was involved in a debate over his book "Corruptions," he also published "The Importance and Extent of Free Enquiry," arguing that the Reformation had not truly changed the church. He challenged readers to push for change, even though friends warned him against using strong language. This led to him being nicknamed "Gunpowder Joe." After publishing this call for change during the French Revolution, critics attacked Priestley and his church, even threatening legal action.

In 1787, 1789, and 1790, religious groups called Dissenters tried again to repeal the Test and Corporation Acts, which limited their rights. Though they might have succeeded at first, by 1790, fear of revolution in Parliament made most people oppose the changes. Political cartoons, a popular form of media at the time, mocked Priestley and the Dissenters. In Parliament, William Pitt and Edmund Burke argued against repealing the acts, disappointing Priestley and his friends, who had expected their support. Priestley wrote letters to both Pitt and Burke to express his views.

Dissenters like Priestley, who supported the French Revolution, faced growing suspicion as people became more skeptical of the revolution. The government used Priestley’s "gunpowder" statement to claim he and others were radicals, further fueling tensions.

Hackney (1791–1794)

Joseph Priestley, a respected man known for his patriotism, wisdom, and scientific work, was driven from his home in Birmingham by violent riots. The riots were fueled by lies that spread among the people, leading to attacks on Priestley and others who supported ideas of freedom and reform. Unable to return to Birmingham, Priestley and his wife settled in Lower Clapton, a neighborhood in Hackney, Middlesex. There, Priestley taught history and natural philosophy at the Dissenting academy, also known as the New College at Hackney. Friends supported the Priestleys by giving them money, books, and laboratory tools to help rebuild their lives. Priestley tried to get the British government to compensate him for the damage to his property in Birmingham, but he was never fully paid. He also wrote a book titled An Appeal to the Public on the Subject of the Riots in Birmingham (1791), in which he criticized the people of Birmingham for allowing the riots to happen and for breaking the principles of English government.

Priestley’s friends encouraged him and his wife to leave Britain and move to France or the United States, even though he had been offered a position as a preacher for the Gravel Pit Meeting congregation. From 1793 to 1794, Priestley served as a minister there, and his sermons, especially the two Fast Sermons, showed his belief that the end of the world was near. He connected events in history to Biblical prophecies and believed the French Revolution was a sign of the Second Coming of Christ. Priestley had always written about the idea of a coming age of peace and justice, but this belief grew stronger after the French Revolution began. He once wrote to a younger friend that while he would not live to see the Second Coming, his friend might, and that it could happen within twenty years.

Life became harder for Priestley and his family. He was mocked in public, with people burning effigies of him and Thomas Paine. Political cartoons showed him in a negative light, and letters from strangers compared him to the devil and Guy Fawkes, a historical figure known for treason. Some businesses avoided dealing with the Priestley family, and Priestley’s friends from the Royal Academy stopped speaking to him. As the government punished people who criticized it, Priestley considered leaving England to protect his family.

In 1792, Priestley’s son William was welcomed by the French government and given French citizenship. Priestley learned about this from a newspaper called the Morning Chronicle. A French law in August 1792 granted citizenship to people like Priestley who had supported freedom through their writing. Priestley accepted this honor, calling it the greatest recognition he had ever received. In September 1792, Priestley was elected to the French National Convention by two regions, Orne and Rhône-et-Loire, but he refused the position because he did not speak French fluently.

As tensions between England and France grew, moving to France became impossible. After England declared war on France in February 1793 and passed a law in March 1793 that banned communication between the two countries, Priestley’s son William left France and moved to America. In August 1793, Priestley’s sons Harry and Joseph also left England for America. Finally, in April 1794, Priestley and his wife boarded a ship called the Sansom in Gravesend, England, and sailed to America. Five weeks after Priestley left, the government of William Pitt began arresting people accused of spreading seditious ideas, leading to the famous 1794 Treason Trials.

Pennsylvania (1794–1804)

Joseph Priestley arrived in New York City on June 4, 1794. He was celebrated by different political groups who wanted his support, but he refused their requests to avoid causing disagreements in his new country. Before moving to a new home in the remote areas of Northumberland County, Pennsylvania (now the Borough of Northumberland), Priestley and his wife stayed in Philadelphia. There, Priestley gave a series of sermons that led to the founding of the First Unitarian Church of Philadelphia. He also turned down an offer to teach chemistry at the University of Pennsylvania.

Priestley’s eldest son, Joseph Jr., was part of a group that bought 300,000 acres of untouched forest near the forks of Loyalsock Creek. They planned to sell or lease the land in 400-acre plots, with payments spread over seven years and interest. His brothers, William and Henry, purchased a 284-acre plot of woodland and tried to turn it into a farm called "Fairhill." They cut down trees and built lime kilns to improve the soil. Henry died on December 11, 1795, possibly from malaria he may have caught after arriving in New York. Mary Priestley, who was already in poor health, worsened further. Her daughter-in-law, Margaret Foulke-Priestley, cared for her around the clock, but Mary died on September 17, 1796.

Since arriving in America, Priestley continued to support his Christian Unitarian beliefs. As he became more influenced by Thomas Cooper and his son Joseph Jr.’s wife, Elizabeth Ryland-Priestley, he became involved in political debates. In 1798, after President Adams expanded the navy and military in response to the Pinckney affair, Priestley wrote an anonymous article titled Maxims of political arithmetic, criticizing Adams, supporting free trade, and promoting a form of Jeffersonian isolationism. That same year, a package addressed to "Dr. Priestley in America" was seized by the Royal Navy on a Danish ship. It contained three letters, one signed by the printer John Hurford Stone. One letter was addressed to "MBP," with a note asking the recipient to seal and send it. This mystery made the letters seem suspicious. Fearing he might be accused of being a spy for France, Priestley sent an awkward letter to newspaper editors, revealing that "MBP" was Mr. Benjamin Vaughan, who had also left England. William Cobbett, in his Porcupine’s Gazette, claimed Priestley and Vaughan were spies working for France.

Joseph Priestley Jr. traveled to England in December 1798 and returned in August 1800. During his absence, his wife, Elizabeth Ryland-Priestley, and Thomas Cooper grew closer, writing political essays together. Priestley continued to be influenced by them, even helping distribute a controversial pamphlet Cooper printed in Point township and Sunbury. In September 1799, William Cobbett published excerpts from the pamphlet, accusing Priestley of promoting it and challenging him to prove he was not a spy. Priestley defended himself in Letters to the inhabitants of Northumberland in November and December 1799.

Priestley’s son, William, who lived in Philadelphia, became increasingly upset by his father’s actions. He confronted Priestley, expressing concerns about his father’s relationship with Elizabeth Ryland-Priestley and Thomas Cooper. This led to more tension between William and his sister-in-law. Later, when Priestley’s household suffered food poisoning, possibly from milk sickness or a bacterial infection, Elizabeth falsely accused William of poisoning the family’s flour. Some modern historians have studied this claim, but it is believed to be untrue.

Priestley continued his educational work, helping establish the "Northumberland Academy" and donating his library to the school. He exchanged letters with Thomas Jefferson about university organization, advice Jefferson used when founding the University of Virginia. Jefferson and Priestley became close friends. After Priestley completed his General History of the Christian Church, he dedicated it to Jefferson, writing that "it is now only that I can say I see nothing to fear from the hand of power, the government under which I live being for the first time truly favourable to me."

Priestley tried to continue his scientific research in America with the help of the American Philosophical Society, to which he had been elected a member in 1785. However, he was unaware of the latest European scientific developments, so he was no longer at the forefront of discovery. Most of his writings defended the phlogiston theory, but he also did original work on spontaneous generation and dreams. Though his scientific output decreased, his presence encouraged American interest in chemistry.

By 1801, Priestley was too ill to write or conduct experiments. He died on February 6, 1804, at the age of 70, and was buried at Riverview Cemetery in Northumberland, Pennsylvania.

Priestley’s epitaph reads:

Degrees

JOSEPH PRIESTLEY, LL.D., University of Edinburgh. Preacher and librarian for the Earl of Shelburne from 1773 to 1780; pastor of a Unitarian congregation in Birmingham from 1780 to 1791 at the Gravel Pit meeting house; pastor in Hackney, London, from 1791 to 1794; resident of Northumberland, Penn, from 1794 to 1804. Discovered nitric oxide in 1772; oxygen, hydrochloric acid, and ammonia in 1774; sulfur dioxide and silicon tetrafluoride in 1775; and nitrous oxide. Member of the Royal Society in 1766 and the American Philosophical Society. Born in Fieldhead, near Leeds, Yorkshire, England, on March 24, 1733; died in Northumberland, Penn, on February 6, 1804.

Legacy

Joseph Priestley died in 1804. By that time, he had been accepted into every major scientific group in the Western world and had discovered many new substances. In a speech honoring Priestley, the 19th-century French naturalist George Cuvier praised his scientific achievements but also noted that Priestley never stopped believing in the outdated phlogiston theory, calling him "the father of modern chemistry [who] never acknowledged his daughter." Priestley wrote more than 150 works on subjects such as politics, education, religion, and science. He influenced British reformers in the 1790s, helped develop the idea of utilitarianism, and contributed to the founding of Unitarianism. Many philosophers, scientists, and poets became associationists because of his work on David Hartley’s Observations on Man, including Erasmus Darwin, Coleridge, William Wordsworth, John Stuart Mill, Alexander Bain, and Herbert Spencer. Immanuel Kant, in his Critique of Pure Reason (1781), wrote that Priestley "knew how to combine his paradoxical teaching with the interests of religion." Priestley aimed to use Enlightenment ideas to support a version of Christianity that was different from traditional beliefs but based on reason and scientific methods.

Although Priestley had a wide influence, few scholars have studied his life and work in detail. In the early 20th century, he was often described as a conservative scientist who also worked for political and religious change. Historian Simon Schaffer later described two main views of Priestley: one portrayed him as a curious and lucky scientist who made discoveries by accident, and the other saw him as naive for not understanding the bigger implications of his work. Scholars have found it hard to study Priestley’s life and writings as a whole because his interests were so varied. His scientific work was often separated from his religious and philosophical writings to make analysis easier, but some recent scholars, like John McEvoy and Robert Schofield, argue that this approach misses the connections between his ideas. Earlier scholars claimed his religious writings were distractions, but later research from the 1960s to the 1980s showed that his scientific and religious works formed a unified theory. However, as Schaffer explains, no clear summary of Priestley’s ideas has been created yet. In 2001, historian Dan Eshet argued that attempts to create an overview of Priestley’s work have only made his contradictions clearer, as these summaries focus on philosophical ideas and separate scientific thinkers from the social issues of their time.

Priestley is remembered in the towns where he worked as an educator and minister, as well as in the scientific groups he influenced. Two schools have been named after him: Priestley College in Warrington and Joseph Priestley College in Leeds (now part of Leeds City College). An asteroid, 5577 Priestley, discovered in 1986 by Duncan Waldron, is also named in his honor. Statues and plaques honoring him are located in Birstall, Leeds City Square, and Birmingham. In Birmingham, Calne, and Warrington, plaques commemorate his contributions. At the University of Leeds, the main undergraduate chemistry laboratories were renamed Priestley Laboratories in 2006 to honor his role as a prominent chemist. In 2016, the University of Huddersfield renamed the building housing its Applied Sciences department as the Joseph Priestley Building as part of an effort to honor local figures.

Since 1952, Dickinson College in Pennsylvania has given the Priestley Award to a scientist who has made important contributions to human well-being. The American Chemical Society honored Priestley’s discovery of oxygen with a National Historic Chemical Landmark designation at the Priestley House in Northumberland, Pennsylvania, on August 1, 1994. A similar honor was given at Bowood House in Wiltshire, England, on August 7, 2000. The society also awards its highest honor, the Priestley Medal, in his name.

Some of Priestley’s descendants became doctors, including James Taggart Priestley II, a noted American surgeon at the Mayo Clinic.

Selected works

• The Rudiments of English Grammar (1761)
• A Chart of Biography (1765)
• An Essay About Liberal Education for Active and Civil Life (1765)
• The History and Current State of Electricity (1767)
• An Essay About the Basic Ideas of Government (1768)
• A New Chart of History (1769)
• Institutes of Natural and Revealed Religion (1772–74)
• Experiments and Observations on Different Kinds of Air (1774–77)
• Disquisitions Relating to Matter and Spirit (1777)
• The Doctrine of Philosophical Necessity Illustrated (1777)
• Letters to a Philosophical Unbeliever (1780)
• An History of the Corruptions of Christianity (1782)
• Lectures on History and General Policy (1788)
• Theological Repository (from 1770 to 1773, from 1784 to 1788)