Joseph Priestley

How to use this course

Joseph Priestley wrote more than 150 works, most of them on theology, education and politics. Science was, by his own account, a pastime. Yet within about ten years he identified more new gases than anyone before him, isolated oxygen, showed that green plants restore air spoiled by breathing and burning, invented carbonated water and argued from a simple observation that electrical force falls off as the inverse square of distance. He interpreted almost all of it through the phlogiston theory and defended that theory to the end of his life, after Antoine Lavoisier had used his discoveries to overthrow it. This course presents the experiments in modern terms and also takes his own reading of them seriously.

Each technical lesson opens with a paragraph for the general reader, then states the key results precisely, reconstructs Priestley’s experiments with modern chemistry and ends with exercises whose solutions are folded. Priestley called gases “airs”, and the course keeps his names alongside the modern ones: fixed air (CO\(_2\)), nitrous air (NO), marine acid air (HCl), alkaline air (NH\(_3\)), dephlogisticated air (O\(_2\)), phlogisticated air (mostly N\(_2\)), inflammable air (H\(_2\)).

Timeline

Yorkshire, 1733–1752

Joseph Priestley was born on 24 March 1733 at Birstall Fieldhead, a farming and cloth-making district a few miles south-west of Leeds. His father, Jonas Priestley, was a cloth dresser; his mother, Mary Swift, died in 1739. Within a few years the boy was sent to live with his father’s sister, Sarah Keighley, at the Old Hall in Heckmondwike; she was widowed in 1745 and raised him as her own.

The household was strictly Calvinist, and the Keighley home was a meeting place for Dissenting ministers, Protestants outside the Church of England. Dissenters could not graduate from Oxford or Cambridge or hold most public offices. Priestley was a precocious student of languages: at local schools and with ministers he learned Latin, Greek and Hebrew, and taught himself French, Italian and German, along with some Aramaic and Arabic. He also read logic, geometry and natural philosophy.

Illness interrupted his studies in his late teens, and for a time he was intended for trade. Theology won. By eighteen he had already moved away from his aunt’s Calvinism toward the more liberal Arminian view, which made it hard for him to be accepted into her congregation. Instead of an orthodox academy he went to one of the most open-minded in the country.

Daventry and two failed pulpits, 1752–1761

In 1752 Priestley entered the Daventry Academy, successor to Philip Doddridge’s academy at Northampton. Its tutors set the arguments for each side of a question before the students and let them decide, and Priestley usually found himself on the heterodox side. In these years he also absorbed David Hartley’s Observations on Man, whose theory of the association of ideas and philosophical necessity shaped his thought for the rest of his life.

In 1755 he became assistant minister to a small congregation at Needham Market in Suffolk. His views were too liberal for it, and his stammer made his preaching hard to follow. In 1758 he moved to Nantwich in Cheshire, where he was happier. He opened a school there and bought scientific instruments so that his pupils could see experiments for themselves. Writing for his pupils, he began his long career as an author of textbooks.

Warrington, Franklin and Leeds, 1761–1773

In 1761 Priestley became tutor in languages and belles-lettres at Warrington Academy in Lancashire, then the leading Dissenting academy. He published The Rudiments of English Grammar (1761), which freed the teaching of English grammar from Latin models. He also lectured on history and politics. In 1762 he married Mary Wilkinson, sister of the ironmaster John Wilkinson. His Chart of Biography (1765) and New Chart of History (1769) drew lives and events as lines on a time axis, one of the first modern timelines.

On visits to London from 1765 he met Benjamin Franklin, John Canton, William Watson and Richard Price, the leading electricians and natural philosophers of the city. With their encouragement and access to their papers, he wrote a full history of electrical science, repeating many experiments and adding his own. The History and Present State of Electricity appeared in 1767. He was elected a Fellow of the Royal Society in 1766, while it was in preparation.

In 1767 he became minister of Mill Hill Chapel in Leeds. His house stood next to a brewery, and he began experimenting with the gas that lay over the fermenting vats. That curiosity grew into the work on airs that occupies most of Part II. In 1772 the Royal Society heard his long paper “Observations on Different Kinds of Air”, and in 1773 it awarded him the Copley Medal. The same year the Earl of Shelburne, a leading Whig politician, offered him a post as librarian, tutor to his sons and general intellectual companion, with a house and a salary that left him time for research.

Electricity and the inverse-square law

For the general reader. Franklin noticed that a small cork ball hung inside a charged metal cup feels no electrical force. Priestley repeated the experiment and saw what it meant. Newton had proved that inside a hollow sphere there is no gravitational pull, precisely because gravity weakens with the square of distance. If electricity behaves the same way inside a hollow conductor, its force too should follow an inverse-square law. Charles-Augustin Coulomb measured the law directly eighteen years later.

Priestley’s argument

In December 1766 Priestley electrified a tin quart vessel standing on an insulating stand and found that cork balls suspended inside it were neither attracted nor charged, while outside they were strongly repelled. In the History he asked whether one might not infer that electrical attraction obeys the same law as gravitation, since a hollow sphere of matter exerts no force on a body inside it. He put the point as a question and did not pursue it, but it is the first statement of the inverse-square law for electricity grounded in an experiment.

Proof sketch. Through a point \(P\) inside the shell draw a thin double cone of solid angle \(d\Omega\). It cuts the shell in two patches at distances \(r_1\) and \(r_2\). Both patches meet the cone axis at the same angle to the shell’s surface (the chord makes equal angles with the sphere at its two ends), so their areas are \(r_1^2\,d\Omega/\cos\theta\) and \(r_2^2\,d\Omega/\cos\theta\). With \(F\propto r^{-n}\) the two opposite pulls are proportional to

equal for every cone exactly when \(n=2\). For \(n>2\) the nearer patch wins, for \(n<2\) the farther one. Summing over all cones gives the result. In modern terms, \(n=2\) is equivalent to Gauss’s law \(\nabla\cdot\mathbf E=\rho/\varepsilon_0\), and a charge-free cavity in a conductor at constant potential then has \(\mathbf E=0\).

Later tests

Henry Cavendish, around 1773, turned Priestley’s inference into a null experiment with two concentric spheres and found the exponent equal to 2 within about one part in 50. He did not publish, and James Clerk Maxwell repeated the experiment a century later with better precision. Coulomb’s torsion balance measurements of 1785 gave the law its name. Modern versions of the concentric-sphere test, writing \(F\propto r^{-(2+\delta)}\), bound \(|\delta|\) below about \(10^{-16}\).

Fixed air and soda water

For the general reader. Joseph Black had shown in the 1750s that chalk and magnesia contain a “fixed air” that can be driven off by heat or acid. At Leeds, Priestley found an endless supply of it over the fermenting vats of the brewery next door. He found that water absorbs it and acquires a pleasant sharp taste. His 1772 pamphlet on making such water started the soft-drinks industry.

Over the vats Priestley found that a candle went out, that smoke rolled along the surface of the invisible layer, and that water held in the layer for a while acquired the taste of Pyrmont water, a fashionable natural sparkling water. For a better method he made fixed air from chalk and oil of vitriol (sulphuric acid) and shook water with it, or let the gas bubble through water in a bladder-and-tube apparatus:

He hoped the water might help prevent scurvy on long voyages, which it does not. The pamphlet Directions for Impregnating Water with Fixed Air appeared in 1772. Johann Jacob Schweppe set up a commercial process in Geneva in the 1780s.

New airs over mercury

For the general reader. Before Priestley, only three distinct gases were known: common air, Black’s fixed air and Cavendish’s inflammable air. Priestley found at least eight more in a decade. One reason was technical. Chemists collected gases by bubbling them into a jar of water, which loses any gas that dissolves in water. Cavendish had used mercury for a few experiments; Priestley made collection over mercury routine.

Priestley developed the pneumatic trough introduced by Stephen Hales and refined by Cavendish, and his inventive, cheap apparatus (gun barrels, clay pipes, kitchen vessels) was widely copied. His “Observations on Different Kinds of Air” (1772) and the six volumes of Experiments and Observations on Different Kinds of Air and its sequel (1774–1786) describe the work.

Mixing alkaline air with marine acid air in a jar over mercury produced a white cloud that settled as a solid salt, sal ammoniac: \(\mathrm{NH_3+HCl\to NH_4Cl}\). Two invisible airs disappeared completely, a vivid lesson that gases are chemical substances that combine.

Dephlogisticated air

For the general reader. On 1 August 1774 Priestley focused sunlight with a large lens onto a red powder of mercury calx and collected the air it gave off. A candle in this air burned with an astonishing flame. Over the next months he found that a mouse lived longer in it than in ordinary air. He had isolated oxygen. He called it dephlogisticated air, and told Lavoisier about it over dinner in Paris that October.

The experiment

At Calne Priestley had a burning lens 12 inches in diameter with a focal distance of 20 inches. He heated mercurius calcinatus per se, the red calx made by heating mercury in air, in a vessel standing over mercury, and collected a large volume of air. In modern terms:

He obtained the same air from red precipitate and from red lead (minium). At first he took it for the dephlogisticated nitrous air (N\(_2\)O) he already knew, in which a candle also burns brightly, and he did not test it further until March 1775. Then, with the nitrous air test (Lesson 08), he found it to be several times “better” than common air, and on 8 March 1775 a mouse placed in two ounce-measures of it lived a full half-hour, where he expected a quarter of an hour in common air. He breathed it himself and reported that his lungs felt light and easy for some time afterward.

Carl Wilhelm Scheele in Uppsala had prepared the same gas, which he called “fire air”, probably by 1772, but his book appeared only in 1777. Lavoisier, after hearing Priestley in Paris, repeated and reinterpreted the experiments, and by 1777–79 had made the new air the basis of a chemistry without phlogiston (see the course on Lavoisier).

Measuring the goodness of air

For the general reader. How “good” is a sample of air for breathing? Priestley found a chemical test. Nitrous air combines with the respirable part of air to form a red vapour that dissolves in water, so the mixture shrinks. The more it shrinks, the better the air. The test became the basis of eudiometry, a fashion for measuring the healthiness of air in cities, hospitals and mountains.

Priestley reported that common air takes up about half its bulk of nitrous air before the mixture stops shrinking: \(2x\approx0.42\) for \(x=0.21\). His new air took more than four half-measures before it stopped shrinking. Pure oxygen should take \(2V\), four half-measures, so the ratio of the two is \(2/0.42\approx4.8\), matching his own estimate that the new air was between four and five times as good as common air. He later obtained samples he judged five or six times as good.

In practice NO\(_2\) reacts further with water and with more NO, in part by \(\mathrm{4\,NO+3\,O_2+2\,H_2O\to4\,HNO_3}\), which consumes NO and O\(_2\) in the ratio 4:3 instead of 2:1. The proportion of the two paths depends on how fast the gases are mixed and how long they stand over water. Different experimenters got different numbers for the same air, and many concluded wrongly that the air of cities and marshes differed greatly in composition. Cavendish, with careful standardized procedure, found in 1783 that the composition of air hardly varies.

Plants restore the air

For the general reader. Burning candles and breathing animals both spoil air. Why, then, is the atmosphere not spoiled after thousands of years? In 1771 Priestley put a sprig of mint into air in which a candle had burned out. Some days later a candle burned in it again. Plants, he concluded, repair the air. This was the first step toward understanding photosynthesis.

In 1771 Priestley found that a sprig of mint growing in a sealed jar of air in which a candle had gone out made the air fit to support a flame again within days. He found the same with other plants, and also that a mouse could live in air restored by plants. He presented the result in his 1772 paper, and when the Copley Medal was presented, John Pringle, president of the Royal Society, singled out the discovery that vegetation keeps the atmosphere wholesome. But Priestley’s results were inconsistent: sometimes plants made the air worse. He had not noticed the role of light.

Jan Ingenhousz, a Dutch physician at the Austrian court, showed in 1779 that only the green parts of plants restore the air, and only in sunlight; in the dark plants spoil air as animals do. Jean Senebier in Geneva showed in the 1780s that the restoration needs fixed air, and Nicolas-Théodore de Saussure in 1804 showed that the plant gains weight from carbon dioxide and water.

Phlogiston and the chemical revolution

For the general reader. Priestley discovered oxygen but never accepted Lavoisier’s explanation of it. He held that metals and combustibles contain phlogiston, the principle of fire, which they release when they burn. By the 1790s almost every chemist had changed sides, but Priestley published a defence of phlogiston as late as 1800. His objections were not foolish; some of them pointed to real gaps in the new chemistry.

Calx in inflammable air

In the early 1780s Priestley heated minium (red lead) with a burning lens in a jar of inflammable air over water. The calx turned to metallic lead and the inflammable air disappeared. To him this was direct proof that the calx absorbs phlogiston to become metal, with inflammable air as phlogiston. In modern terms the hydrogen removes oxygen from the oxide and forms water, which condensed unnoticed in the moist apparatus:

The composition of water

In the spring of 1781 Priestley and John Warltire fired mixtures of inflammable air and common air with an electric spark in a closed vessel. Warltire noticed moisture on the glass; Priestley thought it unimportant. Cavendish repeated the experiment carefully and showed that the two airs combine into their own weight of water, with about two volumes of inflammable air to one of dephlogisticated air. Lavoisier, in 1783–84, synthesized water and decomposed it over red-hot iron, and declared it a compound of hydrogen and oxygen. Once water was a compound, the phlogistic explanation of the minium experiment collapsed: the “absorbed” inflammable air had gone into the water. Priestley answered that water was present in the airs all along, and continued to argue the point in The Doctrine of Phlogiston Established (Northumberland, 1800).

Calne and Birmingham, 1773–1791

The years with Shelburne, from 1773 to 1780, were Priestley’s most productive in science. He lived at Calne in Wiltshire, near Shelburne’s house at Bowood, and spent the winters in London. In 1774 he travelled with Shelburne on the Continent and met the Paris chemists. He also wrote philosophy: an edition of Hartley (1775), Disquisitions Relating to Matter and Spirit (1777), and The Doctrine of Philosophical Necessity Illustrated (1777), in which he argued that the mind is a function of matter and that human actions are determined. The materialism alarmed his patron’s political friends, and in 1780 the two parted, Shelburne paying him an annuity.

Priestley moved to Birmingham and became minister of the New Meeting, one of the largest Dissenting congregations in England. He joined the Lunar Society, an informal club that met near the full moon and included Matthew Boulton, James Watt, Erasmus Darwin and Josiah Wedgwood. They supplied him with apparatus, materials and money for his experiments. He continued his work on airs and on the composition of water. He was elected a foreign associate of the Paris Académie des sciences in 1784.

His theology grew more radical. He became, with his friend Theophilus Lindsey, a founder of Unitarianism in England, denying the Trinity and the divinity of Christ. An History of the Corruptions of Christianity (1782) provoked fierce replies from Anglican writers. He campaigned for the repeal of the laws that excluded Dissenters from public life, and he welcomed the French Revolution. His earlier Essay on the First Principles of Government (1768) had argued that government should be judged by the happiness of the majority; Jeremy Bentham later said he had found in it the phrase “the greatest happiness of the greatest number”.

Riot, exile and legacy, 1791–1804

On 14 July 1791 Birmingham Dissenters held a dinner to celebrate the second anniversary of the fall of the Bastille. Priestley did not attend. A Church-and-King mob burned the New Meeting, then Priestley’s house at Fair Hill, Sparkbrook, with his library, manuscripts and laboratory. Over three days other Dissenters’ houses and chapels were destroyed. Priestley escaped and went to London, where he succeeded Richard Price as minister at the Gravel Pit Meeting in Hackney and lectured at New College, Hackney. In 1792 the French Legislative Assembly made him an honorary French citizen, which deepened his isolation in England as the two countries went to war.

In April 1794 he sailed for America with his wife, following his sons, and settled at Northumberland, Pennsylvania, on the Susquehanna. He declined an offer of the chair of chemistry at the University of Pennsylvania. He built a house with a laboratory and continued to experiment and write, including on carbon monoxide, which he made but did not recognize as distinct. His son Henry died in 1795 and his wife Mary in 1796. In 1798, as relations between the United States and France worsened, he was publicly accused of being a French agent. The election of Thomas Jefferson, his admirer and correspondent, in 1800 ended his political troubles. He died at Northumberland on 6 February 1804, aged 70.

Legacy

• Pneumatic chemistry: oxygen, nitric oxide, nitrous oxide, ammonia, hydrogen chloride, sulphur dioxide and other gases; collection over mercury; the experimental basis of Lavoisier’s revolution.
• Biology: the first evidence that plants and animals exchange gases in complementary ways, the root of the carbon cycle.
• Physics: the experimental inference of the inverse-square law of electric force, and the first comprehensive history of electricity.
• Education and thought: modern English grammar teaching, the historical timeline, utilitarian political theory, and English Unitarianism.
• Honours: the Copley Medal (1773); the American Chemical Society’s Priestley Medal, its highest award, since 1923; his Northumberland house is a museum.

Connections

People linked to Joseph Priestley across this collection. A name without a link has no page here yet. See Joseph Priestley in the network of all 337 lives →

Influenced by

• Joseph Black
• David Hartley
• Stephen Hales

Influenced

• Charles-Augustin de Coulomb
• Jan Ingenhousz

Colleagues and collaborators

• Alessandro Volta 1775
• Henry Cavendish
• James Watt
• Erasmus Darwin
• John Canton
• John Warltire 1781
• Josiah Wedgwood
• Matthew Boulton

Rivals

• Antoine Lavoisier 1774-1794
• Carl Wilhelm Scheele

Friends

• Benjamin Franklin 1765-1790
• James Watt
• Richard Price
• Theophilus Lindsey
• Thomas Jefferson

Also mentioned in this life

• Carl Friedrich Gauss
• Josiah Willard Gibbs
• James Clerk Maxwell
• Isaac Newton

Fields

• chemistry 21 lives
• electricity 5 lives
• natural philosophy 12 lives
• theology 8 lives

Sources and further reading

Biography

1. Joseph Priestley, Wikipedia.
2. R. E. Schofield, Priestley, Joseph, Complete Dictionary of Scientific Biography, via Encyclopedia.com.
3. Science History Institute, Joseph Priestley.
4. J. Priestley, Memoirs of Dr. Joseph Priestley, to the Year 1795, Written by Himself, with a continuation by his son, Northumberland and London, 1806.
5. R. E. Schofield, The Enlightenment of Joseph Priestley (1997) and The Enlightened Joseph Priestley (2004), Pennsylvania State University Press.
6. S. Johnson, The Invention of Air, Riverhead, 2008.

Science

1. J. Priestley, The History and Present State of Electricity, with Original Experiments, London, 1767.
2. J. Priestley, Directions for Impregnating Water with Fixed Air, London, 1772.
3. J. Priestley, Observations on different kinds of air, Philosophical Transactions 62 (1772) 147–264.
4. J. Priestley, Experiments and Observations on Different Kinds of Air, 3 vols., London, 1774–77; excerpt on the discovery of dephlogisticated air, Classic Chemistry, Le Moyne College.
5. J. Priestley, The Doctrine of Phlogiston Established, and that of the Composition of Water Refuted, Northumberland, 1800.
6. J. Ingenhousz, Experiments upon Vegetables, London, 1779.
7. J. G. McEvoy, The Chemical Revolution in Context, Cambridge University Press, 2010.
8. J. L. Heilbron, Electricity in the 17th and 18th Centuries, University of California Press, 1979.