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Priestley, Joseph


(b. Birstal Fieldhead, Yorkshire, England, 13 March 1733; d. Northumberland, Pennsylvania, 6 February 1804)

chemistry, electricity, natural philosophy, theology.

Educated for the dissenting ministry and employed most of his adult life as a teacher or preacher, Priestley wrote books, pamphlets, and articles on theology, history, education, metaphysics, language, aesthetics, and politics, as well as on scientific subjects. In his own day he was as well known for his religious and political views as for his science, although he is chiefly remembered as the discoverer of oxygen.

Priestley was the eldest son of a Yorkshire cloth dresser, Jonas Priestley, and his first wife, Mary Swift. He spent much of his infancy with his maternal grandparents. Shortly after the death of his mother in 1739, he was sent to live with an aunt, Sarah Priestley Keighley, who was left a propertied, childless widow in 1745. Priestley lived with his aunt until he was nineteen. Perhaps some of his lifelong independence from authority can be credited to this continual separation from his immediate family. Certainly, by the age of eighteen he had rejected the sterner Calvinism of his family’s Independency for the Arminianism of eighteenth-century English Presbyterians, whose clergy were sometimes entertained in his aunt’s house.

Having early demonstrated an affinity for learning, Priestley was encouraged to study for the ministry. He attended local parish schools and supplemented his formal training by private lessons and self-directed studies in ancient languages, mathematics, and introductory natural philosophy, reading on his own ’s Gravesande’s elementary text, Mathematical Elements of Natural Philosophy Confirmed by Experiments. When ill health prompted a change in plans, Priestley learned modern languages in preparation for employment as a clerk, but his health returned and at the age of nineteen he entered the dissenting academy at Daventry, the successor to the famous Northampton Academy of Philip Doddridge, which was continued under two of Doddridge’s former students, Caleb Ashworth and Samuel Clark. Because of his assiduous self-education, he was excused the first year of the course of studies and part of the second, but subsequent years were to disclose some weaknesses in his preparation. Although he claimed to have read Latin, Greek, Hebrew, some Syriac and Arabic, French, Italian, and High Dutch (German) and to have learned various branches of theoretical and practical mathematics (all prior to his admission to Daventry), he later admitted having to relearn German in order to read scientific works in that language, and his mathematics was always weak.

At Daventry, Priestley was sufficiently grounded in Latin and Greek to hold his own in subsequent disputes with university-trained scholars. He was more generally introduced to a range of subjects in natural philosophy, but more significantly, he was there formally instructed in logic and metaphysics. Doddridge had favored an educational method of free inquiry on controversial subjects, based on a study and comparison of conflicting texts. This method was continued at Daventry through the use of Doddridge’s lectures, which were later published as A Course of Lectures on the Principal Subjects in Pneumatology, Ethics, and Divinity (1763). From these lectures Priestley acquired a knowledge of the empirical philosophy of Locke, the theology of Richard Baxter, the speculations of the Cambridge Neoplatonists, and the Newtonian natural philosophy and theology of the early Boyle lecturers. At Daventry he also read Rowning’s Compendious System of Natural Philosophy, Hartley’s Observations on Man, His Frame, His Duty, and His Expectations, and one of the English translations of Boerhaave’s Elementa chemiae. By the time he left Daventry in 1755, Priestley was tending toward an Arian position in theology and had begun compounding a metaphysics of Locke, Newtonian corpuscular philosophy, and the determinism and associationist psychology of Hartley. In all of his subsequent work (theological, educational, political, and scientific) Priestley illustrated the utilitarjan, reductionist approach based on this system.

Completing his studies, Priestley went to preach, first at Needham Market, Suffolk, and then at Nantwich, Cheshire. He was successful in neither of these posts. Difficulties arose in the former because of his heterodox position on the Trinity, and in both pulpits he was handicapped by an inherited speech impediment. At Nantwich he opened a school that proved so successful that he was invited to become tutor of languages and belles lettres at the recently founded dissenting academy at Warrington, to which he moved in 1761. There he seems to have taught languages, modern history, law, oratory and criticism, and even anatomy—nearly everything, that is, but theology and experimental sciences, which, he said, he should most like to have taught. While at Warrington he published The Rudiments of English Grammar (1761) and A Course of Lectures on the Theory of Language (1762), both cited by modern scholars for their early recognition of recently rediscovered linguistic principles and for their singular utilitarian insistence on usage as the only proper guide to correct English. He also prepared A Course of Lectures on Oratory and criticism, published several years later (1777). It emphasizes, on Hartleian grounds, practical rules of oratory and an associationist aesthetic criticism, which—as further developed by Archibald Alison— was greatly to influence the poetry o Coleridge and Wordsworth.

Priestley’s general philosophy of education was described in an Essay on a course of Liberal Education for civil and Active Life (1765) and Miscellaneous Observations Relating to Education (1778), and his teaching in history and law was outlined in Lectures on History and General Policy, prepared at Warrington but not published until 1788. The Essay on the First Principles of Government (1768), from which Jeremy Bentham declared that he had derived his Utilitarian formula: “The greatest happiness of the greatest number,” was, therefore, but one of many protoutilitarian works written by Priestley.

While at Warrington, Priestley was ordained and obtained an LL.D. from the University of Edinburgh (1764) in recognition of his work in education. There he also began his scientific career, with the writing of his History of Electricity for which he enlisted the support of Benjamin Franklin, John Canton, Richard Price, and William Watson, whom he met in London late in 1765. At their suggestion—before the History was published but after some of his experiments were known privately to his sponsors—he was nominated and elected F.R.S. in 1766.

In 1762 Priestley married Mary Wilkinson, the daughter of Isaac and the sister of John and William Wilkinson, three of the great ironmasters of eighteenth-century England. William had been a pupil of Priestley at Nantwich and Warrington. Early in 1767, because of growing family responsibilities and the perennial financial and sectarian problems of Warrington, Priestley resigned his teaching position to become minister of Mill-Hill Chapel, a major Presbyterian congregation in Leeds. The History of Electricity (1767) and the History of Optics (1772) were published while he was at Leeds, and he there began his most famous scientific researches, those into the nature and properties of gases. He also completed his itinerary from orthodoxy by adopting Unitarian (then called Socinian) principles and commenced his writing of controversial theological works.

In 1773 William Petty, Earl of Shelburne and later Marquis of Lansdowne, prevailed on Priestley to enter his service. Priestley became Shelburne’s resident intellectual, although officially he served as librarian and adviser to the household tutor. He probably also was useful as unofficial liaison between Shelburne and the politically active dissenting interest. Although Priestley regularly disclaimed any particular concern with politics, his disclaimer must be viewed against the Test and Corporation Acts, which virtually forced all eighteenth-century nonconformists to become involved in politics. Priestley had already written some politico-religious pamphlets (for example, Remarks on… Dr. Blackstone’s Commentaries on the Laws of England [1769], regarding the legality of dissent); he was to continue with others, particularly at the suggestion of his friend Franklin, in support of the American colonies.

Shelburne, a disciple of Lord Chatham and sometime associate of the Rockingham Whigs, excited the animosity and distrust of most professional politicians; but toward Priestley his actions were always honorable and friendly. Together they toured the continent in the summer and fall of 1774 —Priestley’s only visit to Europe, during which he met many scientists in Paris. Priestley and his family had a house in Calne, near Bowood, the Shelburne estate in Wiltshire, and during the season lived at Shelburne House in London. During this period Priestley did most of his scientific work, preparing five of the six major volumes of experiments on gases. He also wrote his major philosophical works, including the only books that come close to relating explicitly his theological and scientific philosophies, the Disquisitions Relating to Matter and Spirit (1777) and the Doctrine of Philosophical Necessity, Illustrated (1777). During 1779 the relationship between Priestley and Shelburne cooled (possibly as a result of Shelburne’s second marriage in that year) and Priestley chose to leave Shelburne’s service in 1780, retaining till his death the annuity promised him should such a separation occur.

At the suggestion of his brother-in-law, John Wilkinson, Priestley settled with his family in Birmingham for what were to be the happiest years of his life. He became preacher at New Meeting House, one of the most liberal congregations in England, and was soon associated with the Lunar Society, an informal collection of provincial intellectuals, scientists, and industrialists. The Lunar Society—comprised during Priestley’s years of Matthew Boulton, Erasmus Darwin, Richard Lovell Edgeworth, Samuel Galton, Jr., Robert Augustus Johnson, James Keir, Jonathan Stokes, James Watt, Josiah Wedgwood, and William Withering—combined widely ranging curiosity about nature with pragmatic concerns that would most appeal to Priestley. The members supported his researches intellectually and financially. He began there his opposition to the new chemical system of Lavoisier and devoted much of his experimentation to the application of scientific phenomena to practical Pursuits.

Priestley’s major preoccupations, however, were increasingly theological. He became the chief propagandist and protagonist for Unitarian beliefs in England, writing annual defenses against attack, and developing in various historical and polemical works (for example, An History of the Corruptions of christianity [1782] and An History of Early Opinions concerning Jesus Christ [1786]) a rationalist theology that suggests, in some measure, the ideas of textual and “higher criticism” of the New Testament. In the eyes of the church establishment, he came to represent the intolerable encroachments of dissent, and on him was focused their theological and political animus.

The opportunity to silence Priestley came in 1791, as a result of his support of early phases of the French Revolution and his related criticism of continued political discrimination against theological dissent in England. A “Church-and-King” mob in Birmingham destroyed the New Meeting House and Priestley’s house and laboratory and threatened his personal safety. He removed to London and was briefly associated with the dissenting academy at Hackney, where he taught natural philosophy and preached; but increasing signs of political persecution, including economic sanctions against his sons, prompted him to emigrate to the United States in 1794.

Although he was warmly received on his arrival and offered a position as professor of chemistry at the University of Pennsylvania, he chose to settle in Northumberland, Pennsylvania, near what was intended to become a settlement for British emigres fleeing political repression. The settlement failed to develop, but Priestley and his family remained. Increasingly remote from current developments, he continued his theological writings and his scientific work. Taking the side of Jeffersonian opposition to the administration of John Adams, Priestley was vilifed in the Federalist press; but the election of Jefferson to the presidency in 1800 changed his situation. Priestley declared that for the first time in his life he was living in a country in which the political authorities were friendly to him. Jefferson personally befriended the aging Priestley, who was ill and lonely; his youngest and favorite son had died in 1795 and his wife in 1796. Priestley died in 1804, at the age of seventy-one, leaving two sons in the United States and a married daughter in England. He is buried at Northumberland, where his house has been turned into a museum.

Scientific Work . Priestley’s scientific work was begun as a logical extension of his interests in education. The history and Present Stare of Electricity, With Original Experiments was conceived as a methodized account of previous discoveries and an assessment of contemporary electrical studies, to encourage further work on the subject. That is, the work was to be a “history” in the Baconian sense; and as a chronicle of near- contemporary and contemporary electrical researches, lucidly and simply described, it was very successful. During Priestley’s lifetime the work went through five English editions and was translated into Dutch, French, and German.

The first edition was marred by Priestley’s slight access to the work of German and Scandinavian electricians (a deficiency corrected in later editions through reference to the historical accounts by Daniel Gralath in the Versuche und Abhandlungen der Naturforschenden Gesellschaft in Danzig), but he made a serious attempt to consult primary sources and his account is fairly impartial, although it understandably favors the currently popular one-fluid theory of electricity as developed and demonstrated by his friend Benjamin Franklin and the Franklinian school.

His advisers, with whom he regularly exchanged correspondence and to whom he sent sections of the History for criticism, supplied Priestley with books and references and gave him hints for experiments, which he initially performed to verify or elucidate the work of others. From these experiments he was soon drawn into original investigations, which he described in the Philosophical Transactions of the Royal Society and included in successive editions of the History. His experiments relate primarily to conductivities of different substances, although he also examined other modes of the motion of the electrical fluid. He discovered the conductivities of charcoal and of metallic salts, ranged the metals in a table of comparative conductivities, first noted the distinctive marks left by spark discharges on metallic surfaces—now known as “Priestley’s rings”—and examined the phenomena of “electric wind” and sideflash. His most remarkable electrical discovery came as an interpretation of an experiment by Franklin. From the observation that pith balls lowered within an electrified metallic cup were not influenced by electricity, Priestley deduced, on Newtonian grounds, the inverse-square form of the force law between electrical charges. The publication of this deduction in the History passed nearly unnoticed (as had that of Daniel Bernoulli in 1760), but it probably inspired Cavendish’s subsequent experimental determination of the force law.

The success of the History of Electricity involved Priestley in a scientific career in which his original experiments had, initially, little place. Almost at once he commenced an international correspondence with scientists such as Bergman and Volta, who were anxious either to correct misunderstandings in the History or to win a place in a continuation of that work beyond 1766, which Priestley had promised. This continuation never appeared; in subsequent revised editions Priestley merely corrected and improved accounts contained in the first edition and added additional experiments of his own.

The educational function of the History was extended by A Familiar Introduction to the Study of Electricity (1768), which was intended for beginners; and this was followed by A Familiar Introduction to the Theory and Practise of Perspective (1770), written from Priestley’s own experience in drawing illustrations of apparatus for the History. The reception given the History encouraged Priestley to undertake a multivolumed history of all the experimental sciences. After some hesitation he determined that the next volume should be on optics and in 1772 published his History and Present State of Discoveries Relating to Vision, Light, and Colours (usually referred to as the History of Optics).

Although the History of Optics contains much useful information, it was considerably less successful than the History of Electricity. Until recently optics had not attracted substantial historical interest, and although Priestley’s History of Optics had but one English edition and a translation into German, it remained the only English work on the subject for a hundred and fifty years and the only one in any language for over fifty. Eighteenth-century optical concerns were primarily ontological and mathematical; even Priestley’s own experimental speculations on the variation of optical parameters with electrification and on the indices of refraction of different gases, relate to his electrical or pneumatic studies and do not appear in the History of Optics. Yet Priestley was not mathematically minded and avowed his intention of presenting the material so as to make it perfectly accessible to readers with little or no knowledge of mathematics. His Baconian design of an exhaustive chronicle was precluded by the masses of material requiring condensation, and he lacked the discrimination to select wisely a ruling principle around which the work might be organized.

Priestley permitted himself one theoretical judgment in expressing strong reservations on the reality of Newton’s optical ether, but the most interesting and original sections of the History of Optics were derived from the work and suggestions of John Michell, rector of the neighboring parish of Thornhill. Michell, a geologist, astronomer, and student of magnetism, assisted Priestly in the preparation of the History of Optics, advised him in the interpretation of optical experiments and phenomena, and provided him with an account of an experiment to measure the momentum of light, which appeared to confirm its particulate nature. Apparently, it was also Michell who called Priestley’s attention to Bos˘ković’s theory of matter, as contained in the Philosophiae natural is theoria (1763 edition); however, this was not Priestley’s first introduction to a theory involving alternating, concentric-spheres of repulsion and attraction surrounding fundamental particles of matter. Rowning’s Compendious System of Natural Philosophy (1735-1743), which he had read at Daventry and from which he borrowed plates to illustrate the History of Optics, contains a discussion of just such a theory. Bo˘ković described the details and implications of the theory of matter with far more completeness than anyone had earlier done and he extended the theory to the dissolution of any material substratum for the particles, contracting them into geometrical points.

Here was the ultimate in reduction of phenomena to simple elements; the prospect fascinated Priestley. He described Bo˘ković’s theory in detail in the History of Optics and used a variation of the theory—conceived by Michell—in explanation of the phenomenon of Newton’s rings, for which Newton had introduced the reciprocal interaction of vibrations in an ether and in refracting bodies. Priestley also used the theory in his metaphysics and theology.

From the publication of the History of Optics, Priestley continued into his major metaphysical writings. His Examination of Dr. Reid’s Inquiry Into the Human Mind.. (1774) criticized the Scottish philosophy of common sense for its multiplication of entities (in this case independent instincts or affections of the mind) in contradiction to Newton’s “Rules of Reasoning” and to the reductionist principles of Hartley, whose work, minus the mechanistic physiology, Priestley edited as Hartley’s Theory of the Human Mind… in 1775. His edition of Hartley emphasized the deterministic and associationist aspects of Hartley’s psychology and led him to the Disquisitions Relating to Matter and Spirit (1777).

The Disquisitions is primarily a theological work, but in the first part Priestley borrowed from Bos˘ković and also from Hartley in order to construct a theory of matter and of mind and soul as material substance. As the “material” is that of Bo˘ković’s geometrical points surrounded by alternating concentric spheres of attracting and repelling force, it would appear that Priestley had really dissolved all matter in a matrix of forces. These forces were, at least to the Cambridge Neoplatonists and early Newtonian natural philosophers, identifiable with the omnipresent will of God. As Priestley further insisted (on the grounds of Revelation) that body and soul were to be reconstituted on a resurrection of man at the Second Coming of Christ, the storm of criticism that stigmatized Priestley as an atheistic materialist is not easy to understand. Although he defended himself vigorously—for example, in a friendly controversy with Richard Price (A Free Discussion of the Doctrines of Materialism and Philosophical Necessity [1778]) in which he declared that matter is not self-existent hut has meaning only in terms of its properties—Priestley seldom again ventured into an extended discussion of his theory of matter.

This was particularly unfortunate, for an understanding of his theoretical position might help to clarify what appear to be anomalies in Priestley’s work on gases (“different kinds of air”), which he had begun concurrently with continued researches on electricity and the writing of the History of Optics. The failure of that History to pay the costs of the books that were collected to write it, and the evident problems involved in condensing the profusion of materials for use in any other subject, prompted Priestley to drop the idea of a history of all the experimental sciences. Besides, he had already embarked on those researches on gases, which were more than sufficient to occupy all the time he had to spare for his scientific work.

Priestley’s concentration on pneumatic studies began comparatively late in his career (at the age of thirty-seven). This interest lasted for the remainder of his life; and during those thirty-odd years, and chiefly in the first ten of them, he was to establish himself as one of the world’s foremost pneumatic chemists. His discoveries of new gases and new processes were to make the chemistry of his day seem untenable; but Priestley never developed a new system to encompass his discoveries, and he refused to adopt the system developed by Lavoisier, which did so.

Attempts to explain this apparent failure of creative synthesis have generated a legend for which Priestley is nearly as responsible as his critics and biographers. Accounts of his scientific work (left by Priestley in his Memoirs), supported by memorable quotations out of the context of his books and papers, have made it appear that he worked in ignorance of the chemical writings of others, that he used the crudest of apparatus, and that the discoveries of this most prolific of eighteenth-century British chemists were made haphazardly and accidentally. Detailed reading of all of Priestley’s scientific writings and of his correspondence suggests that the germs of truth in this legend were exaggerated by Priestley in order to emphasize the contrast between his career and that of his great rival, Lavoisier. Subsequent writers enhanced the exaggeration in order to explain anomalies in his work as a chemist, but Priestley as a scientist is not be understood so easily.

As early as 1755 Priestley had read Boerhaave’s textbook on chemistry. In 1762 he participated in planning, and then assisted in, a course of chemical lectures given at Warrington Academy by Matthew Turner, a physician-chemist of Manchester. It appears that chemical apparatus was acquired by the Academy and that the course of lectures was repeated in subsequent years. In 1766, in his first letter on electricity to Canton, Priestley refers to his reading of William Lewis’ translation of Caspar Neumann’s lectures, while further studies on electrical conductivity of gases and carbon in 1770 cite the opinion of “Macquer and other chemists. “ Finally, the “Catalogue of Books of Which Dr. Priestley Is Already Possessed or to Which He Has Access, for Compiling the History of Experimental Philosophy,” appended to the History of Optics and published the year of his first paper on gases, included over fifty titles (exclusive of papers in the listed scientific journals), which relate primarily to chemistry. Clearly Priestley was aware, at least, of contemporary chemical literature as he commenced his own studies.

Yet in a sense most of this literature was irrelevant to Priestley’s enterprise, for it relates primarily to that activity of separating substances into their constituents and recombining them, which defined the nature of eighteenth-century chemistry. Priestley showed so little interest for this activity that he repeatedly denied any particular knowledge of or concern with chemistry. Priestley was interested in the nature of gases; this was true as early as 1766 when he began his study of common, mephitic, and inflammable airs, and the relationships of the airs to one another, During 1769 a fourth edition of Hales’s Vegetable Staticks was published, and early in 1770 Priestley wrote his friend Theophilus Lindsey, “I am now taking up some of Dr. Hales’ inquiries concerning air.” From this time Priestley’s experimental interests were almost exclusively related to the study of “different kinds of air.”

“Dr. Hales’ inquiries concerning air” were those described in a long chapter, “A Specimen of an Attempt to Analyse the Air,” in Vegetable Staticks, which had already been the inspiration of British pneumatic chemists before Priestley: Brownrigg, Black, Cavendish, and Macbride. Of Brownrigg’s work, Priestley can have known very little prior to his own first paper on “airs,” as the most significant parts were unpublished before 1774. Black’s work was available in the Essays and Observations, Physical and Literary. Read Before a Society in Edinburgh, which Priestley cited in the appendix of the History of Optics. Although he knew of Black’s work, Priestley seems never seriously to have studied it. Cavendish is cited in Priestley’s correspondence by 1771, while Priestley knew of Macbride at least as early as 1767, when a laudatory review of the History of Electricity referred him to Macbride’s studies on fixed air as an antiscorbutic. This, and the accident of Priestley’s first settling in Leeds next to a brewery, with its ample supply of carbon dioxide, may explain why his first “chemical” publication was a pamphlet: Directions for Impregnating Water With Fixed Air, in Order to Communicate to it the Peculiar Spirit and Virtue of Pyrmont Water, … (1772).

The pamphlet was widely and favorably noticed; within the year it was translated into French, and it was an important factor in the awarding to Priestley of the Copley Medal of the Royal Society for 1773. The incongruity of a science honor awarded in part for the discovery of artificially carbonated water is more than balanced by the appearance of Priestley’s magisterial “Observations on Different Kinds of Air,” read to the Society in March 1772, with a supplement in November, and printed in the Philosophical Transactions of the Royal Society for that year. This paper reports Priestley’s pneumatic researches since 1770, including the isolation and identification of nitric oxide and anhydrous hydrochloric acid gases; the beginnings of the discovery of photosynthesis; and a scarcely noted reference to an “air extracted from nitre,” which appeared extraordinary and important to him, but on which he was not to experiment further for several years. Both the range and quality of the research described in this paper illustrate the major influence on Priestley of Hales’s work.

Without in any way detracting from the personal characteristics of Priestley’s work as a scientist—the ingenuity with which he diversified his experiments, the increasingly skilled manipulation of simple apparatus, and the tenacity with which he followed minor variations in results—it is necessary to emphasize the great difference in his mode of working before and after his reading of Hales. In the paper of 1772 and thereafter, the experiments performed, the instruments used, and the way of using them—but particularly the thinking that informed the experiments and guided their interpretation—are all developed from the chapter on airs of the Vegetable Staticks.

Hales had interpreted gases in terms of a Newtonian mechanical model in which a single elastic substance, air, could be fixed and made inelastic in other substances, and then could be released and made elastic again by processes of distillation (heating) or fermentation (mixing with acids, alkalies, or other fluids). He had thus released a variety of gases, but in spite of the title of his chapter, he had not chemically analyzed any of them; for he regarded them all as a single air made various by differing amounts of differing impurities. Black and Cavendish had each identified a particular species of air, but no one had examined the natures of all of the airs that might be released from substances.

This examination was the task Priestley set for himself. First he examined the airs as generated by Hales; and then, as his confidence grew, he set about generating new varieties—by heating substances, mixing them, or taking the residues from containers in which such processes as calcination, vegetation, or electrical discharge had taken place. He adapted the pneumatic trough with inverted receiver, pedestal apparatus, and supporting rack that had been used and improved by Hales, Brownrigg, and Cavendish. The occasional advantage of substituting mercury for water in the pneumatic trough was, for example, first noted by Cavendish. For his early experiments Priestley transformed household utensils (a laundry tub, beer and wine glasses, clay tobacco pipes) into chemical apparatus; but soon he was designing equipment to meet his particular requirements. Josiah Wedgwood freely supplied him with ceramic tubes, dishes, crucibles, and mortars; and the London firm of William Parker and Sons was his supplier of glassware, including the burning lenses that he frequently used to heat substances within the receivers of the pneumatic trough. An inventory of his apparatus destroyed in the Birmingham riots of 1791 (published as an appendix to H. C. Bolton’s edition of Priestley’s correspondence) reveals a well-stocked laboratory of sophisticated apparatus and a variety of reagents.

The tests Priestley initially used in distinguishing the airs that he produced were quite simple: Did they turn lime water turbid? Would they burn or support combustion? What was their appearance and taste? How long would a mouse live in a container filled with one of them? As he gained in knowledge his tests became more comprehensive. He developed techniques of eudiometry using nitric oxide, noted flame size and color in gases that burned or supported combustion, and even recorded the different colors of electric sparks through different gases. Most of his experiments were qualitative; when he did quantitative work it was generally volumetric and not gravimetric. For however skilled in experimental manipulation Priestley became, he never lost the conviction that the important pneumatic parameters were physical and mechanical rather than substantive and chemical.

Priestley’s emphasis on mechanical considerations provided the rationale for an experimental program, which, from a chemist’s viewpoint, appears chaotic. Of course, any successful set of experiments generates a kind of momentum in which one operation suggests another; this can frequently be seen in Priestley’s career, as when the surprising reactivity of marine acid air (anhydrous hydrochloric acid) led to attempts to produce other highly active anhydrous acids and alkalies, and thus to the discovery of sulfur dioxide and ammonia. Moreover, any systematic investigation of the differences between airs must almost of necessity lead to the discovery of new airs. Priestley’s well-reported “accidental” discovery of oxygen in 1774- 1775 was not an accident of producing an air. Priestley had expected the discovery, having deliberately created the conditions for it when he placed a piece of mercuric oxide within a receiver inverted in a pneumatic trough and heated it with his newly acquired burning lens. The surprise was in the unexpected nature of the gas released, which he had expected to be the same as the carbon dioxide he had found in heating impure red lead. Routine examination of substances could produce new airs but did not permit predictions as to their natures.

Internal momentum of experimentation and routine investigation of substances aside, it was the mechanistic mode of interpretation that provided continuity for Priestley’s pneumatic investigations. This mode of investigation could be, and ultimately was, a weakness. All of his life Priestley persisted in believing that physical operations on gases (compression or rare-faction, agitation in water, electrification) would somehow transform one kind of air into another kind of air;and his search for mechanical-force explanations in a kind of premature physical chemistry blinded him to the prior necessities of classifying elements and compounds. Yet the same ideas were also involved in leading him in 1766 from a generalized concern to know the nature of the changes by which combustion or respiration made common air mephitic, to the development of eudiometry, the differentiation of oxygen from nitrous oxide, the work on photosynthesis, and his experiments that led Cavendish and Watt to discover the compound nature of water.

Priestley’s experiments were carried on at such a prolific rate, that following the paper of 1772, it was decided that he should publish his accounts of them in book form. The first volume of Experiments and Observations on Different Kinds of Air appeared in 1774, the second in 1775, and the third in 1777. In 1779 Priestley began a new series, Experiments and Observations Relating to Various Branches of Natural Philosophy, continued with a second volume in 1781 and a third in 1786. (These six volumes are generally cited as forming a single series; in 1790 they were combined and edited in three volumes as Experiments and Observations on Different Kinds of Air, and Other Branches of Natural Philosophy.) These works were supplemented by an occasional paper in the Philosophical Transactions (including the “Account of Further Observations on Air” [1775], in which he announced his discovery of “dephlogisticated air,” later to be defined as oxygen), and an extensive correspondence with other scientists in Britain and on the Continent.

During this period—in addition to his discovery of oxygen—Priestley described the isolation and identification of ammonia, sulfur dioxide, nitrous oxide and nitrogen dioxide, and silicon tetrafluoride. He discussed the properties of mineral acids; further extended the knowledge of photosynthesis; defined the role of the blood in respiration: and noted, unknowingly, the differential diffusion of gases through porous containers. More than any other person, he established the experimental techniques of pneumatic chemistry. For over a decade Priestley dominated the scientific scene in Britain and attracted the attention of scientists throughout Europe. In 1784 he was elected one of the eight foreign associates of the Royal Academy of Sciences in Paris, and he was similarly honored by nearly a score of memberships in other scientific societies from Boston and Philadelphia to Stockholm and St. Petersburgh. His reign came to an end with the development of Lavoisier’s new chemistry.

Priestley had met Lavoisier during his visit to Paris in 1774, when he exhibited the gas (as yet unidentified) that he extracted from mercuric oxide. Their first clash occurred the following year when Priestley asserted his claim to the discovery of a new gas and set Lavoisier right as to its essential properties. Subsequently, there were minor disagreements, but the confrontation did not become a major one until after the discovery in 1783, by Cavendish and Watt, of the compound nature of water. It was these two discoveries, of oxygen and the composition of water, that formed the experimental basis of Lavoisier’s new, oxidation, chemistry; yet Priestley refused to accept Lavoisier’s interpretation of either of them. First in papers in the Philosophical Transactions then in privately printed pamphlets, and. finally, from the United States in papers in the Transactions of the American Philosophical Society and the New York Medical Repository (papers frequently republished in Nicholson’s Journal of Natural Philosophy and the Monthly Magazine and British Register), Priestley presented experimental arguments to counter those of Lavoisier and his growing school of disciples. Many of these papers continue and repeat confusions that had dogged Priestley from his earliest experiments on gases: impurities in his reagents, for example, or difficulties produced by gaseous diffusions. Some of the objections that he raised did have merit. Not all acids do contain oxygen, as he demonstrated in the case of hydrochloric acid. Some of Priestley’s experiments required the definition of yet another new gas, carbon monoxide, to which he has some claim of discovery.

Priestley’s opposition to Lavoisier is frequently described as the conservatism of an old man unable to give up the doctrine of phlogiston, the principle of combustion whose use he had learned early in his career. There is no doubt but that Priestley employed both the language and the concepts of phlogiston chemistry in his arguments; but he regularly insisted that he was ready to abandon phlogiston should the advantage of doing so be demonstrated to him, and his entire career is that of a man not bound to tradition or convention.

Yet to penetrate behind the obvious to a deeper understanding of the differences between Priestley and Lavoisier remains a conjectural operation. Priestley always concealed the theoretical considerations that prompted his experimental investigations in a “Baconian” conviction that only “facts” were important, and he therefore adopted a mode of argument in which Lavoisier’s experiments were countered by his own. But experiments do not stand independent of interpretation, and in the implications of their variant interpretation it is possible to see a fundamental ontological difference between Priestley and Lavoisier.

In his Heads of Lectures on a Course of Experimental Philosophy, Particularly Including Chemistry (1794), Priestley declared that changes in the properties of bodies may result from the addition of substances, from a change in the texture of the substance itself, or from the addition of something not a substance. It was the first of these methods of interpretation that had introduced the imponderable fluids of electricity, heat, and phlogiston and it was in this mode of explanation that Lavoisier’s chemistry achieved its revolution, through its emphasis on mass as a parameter and on gravimetrics as a technique for defining the elements that entered into chemical composition. Priestley’s training and instincts led him to prefer the second method of explanation as he twice declared—in the Experiments on the Generation of Air From Water (1793) and again in “Miscellaneous Observations Relating to the Doctrine of Air,” in New York Medical Repository (1802), when he emphasized that the principle and mode of arrangement of elements in substances was the object of his investigations.

From the beginning of his scientific labors, Priestley was concerned to elucidate the dynamic, corpuscular view of matter outlined by Newton in the queries to the first edition and to the Latin edition of the Opticks. Although he adopted the fluid theory of electricity as the one that was most successful currently, his own experiments led him (at least in correspondence) to doubt the existence of an electric fluid, sui generis. In the same manner he was ambivalent about heat as a fluid, admitting the concept into his Heads of Lectures but earlier suggesting that heat was the vibratory motion of the parts of bodies. In the preface and in a concluding section to the History of Electricity he explicitly noted that electricity, optics, and chemistry combine to give information on the internal structure of bodies, on which their sensible properties depend.

In 1777 Priestley described his reason for experimenting with gases as the exhibiting of substances in the form of air, thus advancing nearer to their primitive elements. These are the elements, which, in the Heads of Lectures, he was to define as combinations of shared properties of extension and powers of attraction and repulsion. Except in the History of Optics and the Disquisitions Relating to Matter and Spirit, there is no indication that Priestley had adopted the theory of matter outlined by Bo˘ković, but there are many suggestions that he had retained the idea—learned at school and reinforced by much of his reading—that a final scientific explanation required the reduction of phenomena into terms of the sizes, shapes, and motions of the fundamental particles of matter and the forces of attraction and repulsion between them. He could accept phlogiston, as he had tentatively accepted the fluids of electricity and heat, prior to the ultimate achievement of true, mechanical explanations. But he could not have accepted Lavoisier’s antiphlogistic chemistry, for this was based upon a ratification of a multiplicity of substances. Priestley’s work in science is thus consistent with than in his other endeavors, where his publications on language, aesthetics, psychology, politics, and particularly on theology, all reveal an attempt to reduce these subjects to a few basic elements interacting according to determinant laws.


I. Original Works. The major source for Priestley’s personal life is the Memoirs of Dr. Joseph Priestley, to the Year 1795, Written by Himself, With a Continuation to the Time of His Decease, by His Son, Joseph Priestley: and Observations on His Writings by Thomas Cooper (Northumberland, Pa., 1805; London, 1806), which has been reprinted many times. No collected ed. of all of Priestley’s writings exists. His Theological and Miscellaneous Works, John Towill Rutt, ed., 25 vols. (London 1817–1831), includes prefaces to the scientific works; Rutt annotated the various works included in his ed. but used no consistent policy for the eds. included and sometimes modified their form. A reissue of Rutt’s ed. has been published.

Single-volume collections of extracts of Priestley’s writings have been edited by Ira V. Brown, Joseph Priestley, Selections From His Writings (University Park, Penn., 1962); John A.Passmore, Priestley’s Writings on Philosophy, Science and Politics (New York, 1965); and P. Kovaly, Joseph Priestley Vybrané Splsy (Prague, 1960), a Czech trans, of selections. A complete listing of all of Priestley’s publications, even in 1st eds. alone, would require more space than can reasonably be devoted to it. An essentially complete bibliography of titles can be found in Ronald E. Crook, A Bibliography of Joseph Priestley, 1733–1804 (London, 1966). Recent eds. of the works cited in the text above are Lectures on Oratory and Criticism (Carbondale, III., 1965); Directions for Impregnating Water With Fixed Air (Washington, D.C., 1945); and the History and Present State of Electricity, 3rd. ed. (London, 1775), with electrical papers from the Philosophical Transactions of the Royal Society (New York, 1966). Extracts from Experiments and Observations on Different Kinds of Air, II (1775), describing the discovery of oxygen, reprinted in Alembic Club Reprints, no. 7 (1901). One of Priestley’s pamphlets. Considerations on the Doctrine of Phlogiston and the Decomposition of Water (1796), with the contemporary response of John Maclean, was reprinted (Princeton, 1929). The published articles are listed by Crook, cited above, and in an appendix to R. E. Schofield, ed., Scientific Autobiography, cited below.

The major collection of scientific MSS (all published) is that of the Royal Society, the archives of which also contain a few MS letters, nearly all published in Bolton or Schofield (cited below). Manchester College, Oxford, possesses some MS sermons, notes in the shorthand of Peter Annet, and a few letters. The Pennsylvania State University Library holds 2 copies of the Memoirs, in the hand of an amanuensis, as are many other “Priestley” MSS. Priestley letters are in various public and private collections around the world. The primary collection of nonscientific correspondence is in the Dr. Williams Library, London, from which J. T. Rutt took the extracts included in his ed. of Priestley’s Memoirs, vol. I, pts. 1, 2, of the Works. Rutt’s editing of the letters is even worse than that of the printed materials, and his versions cannot be depended upon. A substantially complete ed. of Priestley’s scientific correspondence—drawn from major collections of the Royal Society, the American Philosophical Society, the Bodleian Library, and other archives listed in an appendix—is R. E. Schofield, ed., A Scientific Autobiography of Joseph Priestley, 1733–1804 (Cambridge, Mass., 1963). An earlier, smaller ed. of letters, including some not in the Scientific Autobiography; was edited by H. C. Bolton and privately printed as Scientific Correspondence of Joseph Priestley (New York, 1892).

II. Secondary Literature. Books and articles about Priestley are almost as profuse as those by Priestley. Probably the best full-length biography is F. W. Gibbs, Joseph Priestley: Adventurer in Science and Champion of Truth, in the British Men of Science Series (London, 1965). Anne D, Holt, A Life of Joseph Priestley (London, 1931), is perhaps the most knowledgeable treatment of Priestley’s theology. The most detailed examination of his scientific philosophy is in the commentary in Robert E. Schofield, ed., A Scientific Autobiography of Joseph Priestley, 1733-1804 (Cambridge, Mass., 1963).

Robert E. Schofield

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Priestley, Joseph

Priestley, Joseph


Joseph Priestley was a dissenting Unitarian minister in England at a time when adherence to the established Church of England was of great importance. Preaching was a difficult career for Priestleybecause his Unitarian views were unpopular and because he spoke with a stammer. Priestley published widely in a variety of subjects, including theology, education, history, politics, and science. Most often, Priestley is remembered as one of the discoverers of oxygen, but his impact on other lives went much further than this.

His first major science publication was The History and Present State of Electricity (1767), which gained him admission to the Royal Society ; it was followed by The History and Present State of Discoveries Relating to Vision, Light and Colours (1772). Both light and electricity were regarded as aspects of the Newtonian "imponderable fluid" or "ether." They were called "imponderable fluids" (or subtle fluids) because they had no detectable weight (no "poundage"), but still had some fluidlike characteristics. Another form of this "ether" was "phlogiston." Phlogiston was the postulated substance of fire, the active principle of acids, and the driving force behind chemical reactions.

As Priestley expanded his studies in chemistry he became active in the field of pneumatic chemistry, the study of air and gases. Priestley was the first to isolate and characterize a number of gases, including oxygen, nitrogen, hydrogen chloride, ammonia, sulfur dioxide, carbon monoxide, nitric oxide , and nitrous oxide. Priestley's names for these compounds were different from the modern names, in part because he never adopted the oxygen theory of chemistry. The names he used were in terms of the older "phlogiston theory." Priestley did this work using very simple apparatuses, such as saucers, glasses, tubes, cylinders, and tubs of water or mercury.

Among the chemical phenomena he investigated was the behavior of a gas or other substance in contact with fire. If fire was the visible escape of phlogiston from a burning substance, then some gases had a greater affinity for phlogiston than ordinary air and encouraged the flame. Other gases had a lesser affinity for phlogiston than ordinary air (or no affinity at all) and would extinguish the flame.

One gas was found to be especially able to support a flame. Priestley called this gas "eminently respirable air." He later called this same substance "dephlogisticated air," reasoning that because it had a large affinity for phlogiston, it must be particularly devoid of it, or dephlogisticated. Priestley found that the heating of a sample of "red precipitate" (a calx of mercury) to produce pure mercury generated very pure dephlogisticated air. Priestley's discovery of the large amount of "air" generated during the heating of red precipitate was similar to Joseph Black's discovery of "fixed air." The production of dephlogisticated air also fit Priestley's belief that a metal is phlogiston compounded with a calc. The dephlogisticated air liberated from red precipitate also fit well with the observation that when a candle burned out in a closed vessel, the volume of the air was diminished. It was thought that the presence of phlogiston decreased the "springiness" of air. Thus, adding phlogiston to air would cause it to contract, and removing phlogiston from air would cause it to expand. Priestley also found that air saturated with phlogiston could be "revivified" (or dephlogisticated) by green plants in the presence of sunlight. Dephlogisticated air would be renamed "oxygen" by Antoine Lavoisier, who made it the cornerstone of his theory of chemistry.

Priestley resisted the oxygen theory of chemistry to the end of his life. For Priestley, phlogiston was more than just the active principle of fireit was the active principle of life. Here Priestley's scientific theory merged with some of his religious beliefs. If phlogiston were the active principle of fire, heat, light, electricity, acids, chemical reactivity, and life, then it might also be the active principle of spirit. This accorded well with his Unitarian belief in one omnipresent active principle in the universe. In his book Disquisition on Matter and Spirit (1777), he asserted that there was only matter and void in the universethere were no immaterial spiritual influences. Thus, the material existence of phlogiston corresponded well with his religious beliefs.

Priestley also had strong convictions in favor of broad-based democratic reforms and freedom of thought. He advocated wider religious toleration in England. He supported the American colonists in their revolution against the British Crown and supported the French Revolution, even in the face of atrocities such as the Reign of Terror. Priestley made enemies as a result of his political beliefs, and in 1791 his house and laboratory in Birmingham were attacked and burned by a mob. Priestley fled to London and was able to emigrate from there to the United States in 1794. In the United States he was a renowned international figure. When he landed in New York, both the mayor and the governor greeted him. When he arrived in Philadelphia, he was received by President Washington.

In 1794 Priestley declined an offer to be a professor of chemistry at the University of Pennsylvania. He retired from public life in Northumberland, Pennsylvania, and died there in 1804. His home in Northumberland is now preserved as a historical landmark. According to Peter Miller (1993), "A work entitled 'Joseph Priestley in Context' would . . . far surpass the competence of any single chronicler."


There never was a widespread coherent theory of phlogiston. German chemist Johann J. Becher (16351682) brought the term "phlogiston" into use among European chemists in the middle 1600s. The word is based upon a Greek word used by Aristotle in his writings on matter. German chemist Georg Stahl (16601734) further articulated the phlogiston theory in the early 1700s.

According to the phlogiston theory, a flame was thought to be the visible escape of matter called phlogiston from a burning substance. Another key feature of the theory was that a metal was thought to be composed of phlogiston and earth. Luster, high heat conductivity, malleability, and ductility are all unusual characteristics for metals, but according to the theory, metals share these features because of their postulated phlogiston content. If the phlogiston was removed from a metal, the result was an earth called calc (plural, calx), often the metal's naturally occurring ore. Under certain conditions, phlogiston might even exhibit a negative weight! This anomaly became problematic after Sir Isaac Newton's 1687 Law of Universal Gravitation.

David A. Bassett

see also Gases; Lavoisier, Antoine; Nitrogen; Oxygen.

David A. Bassett


Lindsay, Jack, ed. (1970). Autobiography of Joseph Priestley. Cranbury, NJ: Associated University Presses.

Miller, Peter, ed. (1993). Joseph Priestley: Political Writings. New York: Cambridge University Press.

Partington, J. R. (1962; reprint 1996). A History of Chemistry, Vol. 3. New York: Martino Publishing.

Partington, J. R., and McKie, Douglas (1981). Historical Studies on the Phlogiston Theory. New York: Arno Press.

Priestley, Joseph (1767). The History and Present State of Electricity. London.

Priestley, Joseph (1772). The History and Present State of Discoveries Relating to Vision, Light and Colours. London.

Priestley, Joseph (17741777). Experiments and Observations on Different Kinds of Air, Vols. 13. London.

Priestley, Joseph (1777). Disquisition on Matter and Spirit. London.

Priestley, Joseph (17791786). Experiments and Observations Relating to Various Branches of Natural Philosophy, Vols. 13. London.

Priestley, Joseph (1796). Considerations on the Doctrine of Phlogiston and the Composition of Water. Philadelphia.

Priestley, Joseph (1800). The Doctrine of Phlogiston Established and that of the Composition of Water Refuted. Northumberland, PA.

White, J. H. (1932). The History of the Phlogiston Theory. London: Edward Arnold.

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Priestley, Joseph (1733–1804)


PRIESTLEY, JOSEPH (17331804), English cleric, chemist, historian, theologian, philosopher, and social and political critic. Joseph Priestley, the eldest son of a maker and dresser of woolen cloth, was born in Fieldhead near Leeds, Yorkshire. As a boy, Joseph was exposed to strict Calvinism and tutored by local clergymen. Because his religious Nonconformity barred him from Oxford and Cambridge, his formal education was completed at the dissenting academy at Daventry. However, it was largely through his own efforts that Priestley learned Latin, Greek, French, Italian, German, Hebrew, Chaldean, Syriac, and Arabic.

Over the course of his life, Priestley's religious beliefs evolved from Calvinism to Socinianism (Unitarianism), but religion always remained of pivotal importance. His chief formal occupation was as a minister, and he served liberal congregations in various parts of England. In addition, he taught for six years at the dissenting academy in Warrington, and he tutored private students. During all this time, his prolific pen seldom stopped moving. His collected works fill over twenty-five volumes and include such titles as A Chart of Bibliography, Rudiments of English Grammar, A Course of Lectures on Oratory and Criticism, An Essay on the First Principles of Government, History of the Corruptions of Christianity, Disquisitions Relating to Matter and Spirit, Institutes of Natural and Revealed Religion, and Experiments on Air.

Although today Priestley is best known for his contributions to chemistry, he was only an amateur scientist. His first scientific publication, The History and Present State of Electricity (1767), was stimulated and encouraged by his friend Benjamin Franklin. Priestley reported in his posthumously published memoir that his interest in chemistry was a consequence of living adjacent to a brewery during his ministry at Leeds (17671773). His first publication on pneumatic chemistry (1772) provided directions for impregnating water with the "fixed air" generated by fermenting beer. In modern terms, Priestley described the carbonation of water. In addition, he isolated and identified ten gases, most of them previously unknown, and he discovered photosynthesis independently of Jan Ingenhousz.

Joseph Priestley's most famous discovery occurred on 1 August 1774, while he was serving as the "literary companion" of William Petty, the second Earl of Shelburne. On that date, Priestley used a burning glass to focus the rays of the sun on a sample of the red calx of mercury, which evolved a colorless, odorless, and tasteless gas. He ultimately found that this new gas was "between five and six times as good as the best common air" in supporting combustion. The name he chose, "dephlogisticated air," reflects the Phlogiston Theory, an explanation of combustion widely held in the eighteenth century. According to this theory, flammable substances contained phlogiston, the principle of combustibility, which escaped during burning. Air was necessary as a reservoir to absorb the escaping phlogiston, and when the air became saturated with it, burning ceased. Because the newly isolated gas had an enhanced capacity for supporting combustion, Priestley concluded that its phlogiston content must be lower than that of air.

Unbeknown to Priestley, Karl Wilhelm Scheele (17421786), a Swedish apothecary, had prepared the same gas in 1771. But the correct interpretation of the essential role of this gas in combustion and in chemistry was one of the major contributions of the French chemist, Antoine Laurent Lavoisier (17431794). Lavoisier gave the name "oxygen" to Priestley's dephlogisticated air and included it among the thirty-three simple substances listed in his Elements of Chemistry (Traitéélémentaire de chimie, 1789). Oxygen was literally a key element in the revolution that transformed chemistry and established the modern science, but Priestley never accepted the new "French chemistry."

Priestley's chemical conservatism seems to stand in stark contrast to his religious, political, and social radicalism. He was a severe critic of traditional Trinitarian Christianity, an outspoken advocate of freedom of religion and speech, and an ardent supporter of the American and French Revolutions. It was especially his espousal of the latter cause that led to criticism and caricature in the popular press and to the sacking of his Birmingham home in 1791. Continuing opposition in England contributed to Priestley's decision to move to Pennsylvania in 1794. He and his family settled in the village of Northumberland, where he lived quietly until his death in 1804.

Most modern scholars have found considerable consistency in the great diversity of Priestley's work. The unifying themes are his materialistic world view, his acceptance of a benign form of determinism known as philosophical necessity, his commitment to the power of reason, and his Unitarian beliefs. From this foundation Priestley inferred (in his own words) that "a wise Providence [disposes] everything for the best"; "the human species itself is capable of . . . unbounded improvement"; "the great instrument in the hand of divine providence of this progress of the species towards perfection, is society and consequently government"; and, "the good and happiness of the . . . majority of the members of any state is the great standard by which everything relating to that state must finally be determined." Ultimately, even Priestley's refusal to accept the chemical revolution that he helped start is consistent with his status as an "honest heretic."

See also Chemistry ; Lavoisier, Antoine ; Petty, William .


Primary Sources

Priestley, Joseph. Autobiography of Joseph Priestley. Introduction by Jack Lindsay. Bath, U.K., 1970.

. Experiments and Observations on Different Kinds of Air. 2nd ed. London, 1775.

. The Theological and Miscellaneous Works of Joseph Priestley, L.L.D., F.R.S., etc. 25 vols. Edited by J. T. Rutt. London, 18171835.

Secondary Sources

Schofield, Robert E. The Enlightenment of Joseph Priestley: A Study of His Life and Work from 1733 to 1773. University Park, Pa., 1997.

Schwartz, A. Truman, and John G. McEvoy, eds. Motion Toward Perfection: The Achievement of Joseph Priestley. Boston, 1990.

A. Truman Schwartz

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Joseph Priestley

Joseph Priestley

The English clergyman and chemist Joseph Priestley (1733-1804) contributed to the foundation of the chemistry of gases and discovered the role of oxygen in the animal-plant metabolic system.

Joseph Priestley was born on March 13, 1733, at Fieldhead. His mother died when he was 6, and he was reared by an aunt. Because of ill health he was unable to go to school and was educated partly by a Nonconformist minister and partly by private study. He had a gift for languages and learned about 10. He became a minister when he was 22.

Priestley moved about the country a great deal, preaching and teaching. About 1758 he began to add experiments in "natural philosophy" to his students' activities. In 1761 he moved to Warrington to teach languages in an academy established by Dissenters. There he began to take even more interest in science in general and had an opportunity to attend a few lectures in elementary chemistry.

On a trip to London in 1766 Priestley met Benjamin Franklin, who interested him in electricity. This led to fruitful experimentation—Priestley discovered the conductivity of carbon in 1766, found that an electrical charge stays on the surface of a conductor, and studied the conduction of electricity by flames—and his History and Present State of Electricity (1767), which at that time was definitive.

In 1767 Priestley moved to Leeds, where he lived next to a brewery. He became interested in the gases evolved during fermentation and soon discovered that carbon dioxide was being formed. He began preparing this gas at home for study and found that it could be absorbed by water. This discovery of "soda water" brought him much attention and the Royal Society's Copley Medal.

Thus stimulated, Priestley turned his attention to the preparation and study of other gases. He decided to collect them over mercury rather than water and was therefore able to prepare for the first time a variety of gases at random. His greatest discovery came in 1774, when he prepared oxygen by using a burning glass and solar heat to heat red oxide of mercury in a vacuum and collected the evolved gas over mercury. In accordance with the phlogiston doctrine, to which he remained loyal to his death, he called the new gas "dephlogisticated air, " for he found that it greatly improved combustion. He realized that this gas must be the active component in the atmosphere and that the concept of air being a single substance was incorrect. Three years earlier he had discovered that plants had the capacity to restore to air the ability to support combustion after a candle had been burned in it. He could now identify oxygen as the agent involved in the animal-plant metabolic cycle.

Between 1772 and 1780 Priestley held the not very demanding post of librarian and companion to Lord Shelburne, and much of his best work was done through this patronage. Priestley then settled in Birmingham, where he became a member of the Lunar Club.

Priestley hated all oppression, openly supported the American and French revolutions, and denounced the slave trade and religious bigotry. As a result of his continued attacks on the government, public resentment rose against Priestley and in 1791 a mob sacked and burnt his house and laboratory. He and his family escaped to London, where he encountered harassment and snubs, and in 1794 he emigrated to the United States. He was offered various positions, including that of the presidency of the University of Pennsylvania, all of which he declined, but he did pass on much of his experimental techniques to American chemists and preached from time to time. President John Adams was among those who attended his sermons, and George Washington made him a welcome visitor to his home. Priestley died at his home in Northumberland, Pa., on Feb. 6, 1804.

Further Reading

Among the biographies of Priestley are Anne Holt, A Life of Joseph Priestley (1931); John G. Gillam, The Crucible: The Story of Joseph Priestley (1954); and Frederick W. Gibbs, Joseph Priestley: Revolutions of the Eighteenth Century (1967). Bernard Jaffe's treatment of Priestley in his Crucibles: The Lives and Achievement of the Great Chemists (1930) is readable and interesting. There is also a study of Priestley in James G. Crowther, Scientists of the Industrial Revolution (1963).

Additional Sources

Clark, John Ruskin, Joseph Priestley, a comet in the system: biography, San Diego, Calif.: Torch Publications, 1990.

McLachlan, John, Joseph Priestley, man of science, 1733-1804: an iconography of a great Yorkshireman, Braunton, Devon: Merlin Books, 1983.

Priestley, Joseph, Memoirs of Dr. Joseph Priestley to the year 1795, written by himself; with a continuation to the time of his decease by his son, Joseph Priestley, and observations on his writings by Thomas Cooper and William Christi, Millwood, N.Y.: Kraus Reprint Co., 1978.

Smith, Edgar Fahs, Priestley in America, 1794-1804, New York: Arno Press, 1980.

Thorpe, Thomas Edward, Sir, Joseph Priestley, New York: AMS Press, 1976. □

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Priestley, Joseph

Joseph Priestley, 1733–1804, English theologian and scientist. He prepared for the Presbyterian ministry and served several churches in England as pastor but gradually rejected orthodox Calvinism and adopted Unitarian views. He settled in London in 1765 and moved to Birmingham some years later. In both cities he became associated with some of the outstanding figures of his day, such as Benjamin Franklin, James Boswell, James Watt, Josiah Wedgwood, and Erasmus Darwin. Priestley's Essay on Government (1768) suggested the idea of "the greatest happiness of the greatest number" to Jeremy Bentham. In 1769 he founded the Theological Repository for critical discussion. In his History of Electricity (1767), he explained the rings (known as Priestley's rings) formed by a discharge upon a metallic surface. His improvements in the manipulation of gases enabled him to investigate the properties of gases and to discover new ones, including sulfur dioxide, ammonia, and what Priestley called "dephlogisticated air," the gas that Lavoisier named oxygen and made the basis of experiments that were the foundation of modern chemistry. Priestley himself, however, failed to realize the importance of his discovery of oxygen.

Priestley's Examination of Scottish Philosophy appeared in 1774; his History of the Corruptions of Christianity, published in 1782, was officially burned in 1785; and his History of Early Opinions concerning Jesus Christ appeared in 1786. In 1790 he wrote two volumes of a General History of the Christian Church to the Fall of the Western Empire, and four volumes of the later history of the church appeared between 1802 and 1803. In the meantime he pursued his scientific and philosophical studies; opposed orthodox doctrines, the government's colonial policy, and slave trade; advocated the repeal of the Test Act and Corporation Act; and carried on a seven-year controversy (1783–90) with the Rev. Samuel Horsley.

Priestley's sympathy with the aims of the French Revolution aroused popular prejudice against him, which led in 1791 to the wrecking of his house and the destruction of his library and scientific apparatus. Priestley emigrated to the United States in 1794 and lived at Northumberland, Pa., for the remainder of his life. He became friendly with many of the nation's founders, forming a particularly close tie with Thomas Jefferson, and continued his chemical experimentation, engaging in a controversy on the phlogiston theory with leading American chemists. Priestley, who was fluent in seven languages, wrote a total of nearly 500 books and pamphlets.

See his letters, ed. by R. E. Schofield (1966); his memoirs (2 vol., 1806, repr. 1970); L. Kieft and B. R. Willeford, Jr., Joseph Priestley: Scientist, Theologian, and Metaphysician (1979); J. J. Huecher, Joseph Priestley and the Idea of Progress (1987); S. Johnson, The Invention of Air (2008); bibliography by R. E. Crook (1966).

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Priestley, Joseph

Priestley, Joseph (1733–1804). Chemist, clergyman, and political theorist. Priestley was born in Yorkshire and educated at Batley Grammar School and at Daventry dissenting academy. An amateur scientist of great renown, his discovery of ‘dephlogisticated air’, later named oxygen by Lavoisier, transformed the study of chemistry. As a theologian, Priestley moved from presbyterianism via arianism to a unitarian position. He argued against the atonement; questioned traditional views of the person of Christ; and rejected the doctrine of Christ's perfection. He was a lively polemicist. His Essay on the Principles of Government (1768) was a strong plea for liberty, and he campaigned for the repeal of the Test and Corporation Acts and for the abolition of the slave trade. An incautious phrase in his Letter to Edward Burn (1790) caused him to be satirized as ‘Gunpowder Priestley’, plotting to blow up the British constitution, and brought down upon him the wrath of the Birmingham ‘church and king’ mob. In July 1791 it burned his house, wrecked his laboratory, and destroyed most of his papers. Priestley left England and spent the rest of his life in Pennsylvania.

Tim S. Gray

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Priestley, Joseph

Priestley, Joseph (1733–1804) English chemist and clergyman who discovered oxygen in 1774. He also discovered a number of other gases, including ammonia and oxides of nitrogen. He studied the properties of carbon dioxide. Priestley advocated the later discredited phlogiston theory.

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