In transcribing the following passages from the The Dictionary of National Biography I have relied on the Hathi Trust’s online version and its invaluable OCR text, corrected scanning errors, and and added links to the material on this site. The DNB section on his early life, education, and religion appears in a separate section on this site. Please notify the webmaster if you encounter typographical errors. — George P. Landow

Joseph Priestley by Ellen Sharples. 1797. Courtesy Courtesy of the National Portrait Gallery, London. Click on image to enlarge it.

It is as a man of science, and chiefly as a chemist, the 'discoverer' of oxygen, that Priestley is most generally remembered; and except for certain references to religion in the prefaces to his 'Experiments . . . on . . . Air,' his scientific work has little connection with his other occupations. His fuller interest in science dates from 1758, when he bought a few scientific books, a small air-pump, an electric machine, and other instruments, with the help of which he made experiments for his pupils at Nantwich, as well as for his own amusement and that of his friends (Phil. Trans. 1770, p. 192). The delight in pretty experiments finds constant expression throughout his work. Although his preference for science over literature appears, in 1761, in his 'English Grammar' (p. 62), and in the introduction to the 'Chart, on Biography,' Priestley seems to have been long prevented by an unusual diffidence from attacking the subject on his own account. This diffidence was removed during his visit to London in January 1766, when he met Richard Price (1723-1791) [a. v.], Sir William Watson, M.D. [q.v.], John Canton [q.v.l and Benjamin Franklin (1706-1790). Franklin encouraged him to undertake the 'History of Electricity,' which Priestley intended as part of a general history of experimental philosophy. The book drew him 'into a large field of original experiments,' and on the strength of these he was elected F[ellow of the]. R[oyal]. S[ociety]. on 12 June 1766, on the proposition of Watson, Franklin, Canton, and Price. With the last three men he maintained a scientific correspondence till death. Franklin and Canton corrected the proofs of the 'History,' which was printed in 1767, within twelve months of its inception. Priestley's electrical work is mostly sound, and much of it brilliant; it shows him at his best, although the discoveries contained therein are of less importance in the history of science than his later discoveries in chemistry. The 'History of Electricity' supplies an excellent account of previous work both treated historically and summarised systematically, and his own reflections and experiments described in a 'simple, exact, and artless style' borrowed, as he admits, from Stephen Gray [q.v.]; the style contrasts with the excessive fluency of much of his purely literary work. In the second part Priestley enounces his views on scientific method (Hist. of Electricity, 3rd edit. ii. preface), which he derived from Locke and possibly in part from Condillac. The object of science is 'to comprehend things clearly, and to comprise as much knowledge as possible in the smallest compass;' hypotheses are useful only in order to ascertain facts, and must not be valued for their own sake. At this time Priestley, adhering to his principles, and showing a critical power that was not equally conspicuous in his later work, declined to adopt either of the two contending fluid theories, and suggested to Canton on 12 Nov. 1767 (quoted in Chemical News, 14 May 1869) that electrification may be only a modification of the body electrified; but he afterwards identified 'the electric matter' with phlogiston (Experiments . . . on . . . Air, i. 186). In his 'History' he anticipated Henry Cavendish [q. v.] and Charles Augustin de Coulomb in the important suggestion that the law of electric attraction is that of the inverse square, deducing this from an experiment suggested by Franklin. He found that an electrified body is discharged by the proximity of flame, that charcoal, blacklead, and red-hot glass are conductors : and satisfactorily explained the formation of rings (since known as Priestley's rings) when a discharge takes place on a metallic surface. He showed great insight by pointing out the need for the measure of electric resistance, and proposed a method for measuring what is now called 'impedance,' which at the time was not distinguished from resistance (Phil. Trans. 1769, 63). In February 1770 (ib. 1770, p. 192) he investigated the 'lateral explosion' produced in the discharge of a Leyden jar, and showed that it is of an oscillatory nature, thus anticipating in part recent discoveries on this subject, especially those of Dr. Oliver Lodge (The Electrician, 1888, vol. xxi. pp. 234 276, 302). In 1772 he corresponded with Volta at Como; and received a commission from Leopold, grand duke of Tuscany (afterwards the Emperor Leopold II), for an electrical machine, which was made under his direction by Edward Nairne [q.v.]

But after 1770 Priestley practically abandoned the study of electricity for that of chemistry, to which he had been led incidentally. He had attended a course of chemical lectures given in Warrington Academy by Dr. Turner of Liverpool. But he admitted that he 'knew very little of chemistry at this time,' and even attributed his success to the ignorance which forced him to devise apparatus and processes of his own (Memoirs, i. 61). Much later he declared himself 'no professed chemist.' It was precisely to this ignorance of chemical history and practice that was due his lasting incapacity to analyse experiments thoroughly, and to push them to their logical conclusion. He began his chemical work by attacking the problem of combustion, the solution of which created the science of modern chemistry (Phil. Trans. 1770, p. 211). He was led to study gases by watching the process of fermentation in a brewery next to his house ; and in March 1772 he read his first paper, 'On different Kinds of Air.' It was inspired by the work of Stephen Hales [q.v.], of Joseph Black [q. v.], and of Cavendish.

Despite its many wrong conclusions, and its records of unsatisfactory experiments, this essay marked an epoch in the history of the science. In the first place, Priestley set forth improvements in the methods of collecting gases, and especially the use of mercury in the pneumatic trough, which enabled him to deal for the first time with gases soluble in water. He announced the discovery of marine acid air (hydrochloric acid) and nitrous air (nitric oxide), and showed the feasibility of substituting the latter for living mice as a means of measuring the goodness of air, a suggestion which led, in the hands of Fontana, Landriani, Cavendish, and others, to exact eudiometry. He showed that in air exposed over water, one-fifth disappears in processes of combustion, respiration, and putrefaction, and that plants restore air vitiated by these processes; and that no known as conducted electricity. The paper also contained a proposal to saturate water with carbonic acid under either atmospheric or increased pressure, which has led to the creation of the mineral-water industry. Of this means of making 'Pyrmont water' (which he described in a pamphlet in June 1777), ho wrote: 'I can make better than you import, and what cost you five shillings will not cost me a penny. I might have turned quack' (Memoirs, i. 177). Certain experiments on this part of his work were made for Priestley by William Hey [q.v.]

Priestley likewise described the preparation of pure nitrogen, a gas to which he gave the vague name of 'phlogisticated air,' only recognising it later as a distinct species. Daniel Rutherford [q. v.] simultaneously and independently obtained a like result, which he first described in 'De Aere fixo' (p. 16), dated 12 Sept. 1772. In the same dissertation Priestley noted, without comment, that he had produced two other gases, which were subsequently recognised as new, and were designated respectively carbonic oxide and nitrous oxide, and that he had disengaged from nitre a gas which further examination would have proved to be identical with the as yet undiscovered oxygen. The paper was awarded the Copley medal of the Royal Society (30 Nov. 1773), and was at once abstracted at length by Lavoisier (CEuvres, L 512, 621) and criticised by him. Hence forward Lavoisier acted as a sieve to separate the inaccurate work and conclusions of Priestley from the accurate.

There followed in 1772 Priestley's 'History of . . . Light.' His knowledge of mathematics was insufficient to enable him to produce anything more than a clear but unoriginal narrative, and with its publication he abandoned his scheme of writing a general scientific history, owing to the financial failure of the work. He wrote to Canton (18 Nov. 1771), ' If I do work for nothing, it shall be on theological subjects.' In the History of Light' (pp. 390 sq.) he announced his adherence to Boscowich's theory of points of force (see supra). After 1772 Priestley decided,with the approbation of the president, Sir John Pringle, not to present his papers to the Royal Society, but to publish them separately, and from 1774 to 1786 he published six successive volumes of researches on air and kindred subjects (condensed into three volumes in 1790), occasionally contributing shorter accounts of his work to the 'Philosophical Transactions.' The first volume records the discoveries of alkaline air (ammonia gas) and dephlogisticated nitrous air (nitrous oxide), and the synthesis of sal-ammoniac, as well as (p. 258) his first general view of the then current hypothesis of Becher and Stahl — that fire is a decomposition, in which phlogiston is separated from all burning bodies. Priestley adopted modifications of detail in this view under the compulsion of facts and the in fluence of Richard Kirwan [q. v.] and Cavendish. At various periods he identified phlogiston with electricity and with hydrogen (Phil. Tran. 1785, p. 280). But his whole scientific energies from this time forward were devoted to the upholding of the phlogiatic theory, which his own experiments (and their completion by Cavendish) by a strange fate were destined, in the hands of Lavoisier, completely to overturn.

On 1 Aug. 1774, at Lansdowne House, Priestley obtained what was to him a new gas from mercurius calcinatus per se, in which a candle burnt vigorously, but he remained 'in ignorance of the real nature of this kind of air ... to 1 March following.' He then found it to be 'purer' than ordinary air, i.e. to support respiration, as well as combustion, better, and called it 'dephlogisticated air.' From its property of yielding acid compounds this gas was named oxygen by Lavoisier at a later date. As it both came from the atmosphere and could also be produced by heating certain metallic nitrates, Priestley concluded that the air is not an element, but 'consists of the nitrous [nitric] acid and earth, with so much phlogiston as is necessary to its elasticity '(Experiments ...on... Air, ii. 55), a mistaken opinion which he modified, but did not improve, in 1779 (Experiments and Observations on Natural Philosophy, i. 192).

Priestley's great discovery of oxygen contained the germ of the modern science of chemistry, but, owing to his blind faith in the phlogistic theory, the significance of the discovery was lost upon him. Priestley made the first public announcement of his discovery of oxygen in a letter to Sir John Pringle,dated 15 March 1775, which was read to the Royal Society on 25 May. But while in Paris, in October 1774, Priestley, according to his own account, spoke of the experiments he had already performed, and of those he meant to perform, in relation to the new gas (Experiments . ..on . . . Air, Nov. 1775, ii. 320). Fifteen years later — in the 1790 edition of 'Experiments on Air' (vol. ii. 108) — Priestley declared specifically that he told Lavoisier of his experiments during this visit to Paris. There doubt that immediately after that date Lavoisier made oxygen for himself, and in the May follow ing published the first of a long series of memoirs, in which he used his experments to explain the constitution of the air, com bustion and respiration, and to give an ex perimental interpretation of theQreek idea of the conservation of matter, thus founding chemistry on a new basis. Priestley refused to accept Lavoisier's sagacious views. The centenary of Priestley's discovery of oxygen was celebrated in Birmingham and in North umberland, Pennsylvania, on 1 Aug. 1874, but there is some divergence of opinion as to who is entitled to the full credit of the original discovery. Although Priestley was ' in pos session of the gas 'before November 1771' (Experiments on Natural Philosophy, i. 194), it is admitted that Karl Wilhelm Scheele, the great Swedish chemist, working quite independently, first recognised it as a distinct species 'before 1778' ( Nordenskjold and Thorpe), but Scheele did not publish his researches until after Priestley. Lavoisier's claim to subsequent but independent discovery, for which his own statement is the only evidence, offers greater difficulty. Lavoisier was possibly among the first chemists to whom Priestley's discovery was communicated before its public announcement. Priestley made no definite charge of plagiarism when Lavoisier published his memoir in May 1775. When, in 1790, Priestley first asserted that he had himself told Lavoisier of his discovery in October 1774, Lavoisier made no reply. Lavoisier died in 1794, and it was not until 1800, after twenty-five years had elapsed since the discovery, and memory was failing him, that Priestley made Lavoisier's pretensions a matter of complaint (Doctrine of Phlogiston established, 1800, p. 88).

In November 1774 Priestley discovered vitriolic acid air (sulphur dioxide), and before November 1775, continuing an investigation by Scheele (Kopp), fluor acid air (silicon tetra-fluoride). This completes the list of Priestley's great discoveries of gases (nine in all), of which only three species had been recognised before he began his researches.

Priestley's memoir on respiration, read in January 1776 (Phil. Trans, p. 226), in which he regards respiration as 'a true phlogistic process,' was not original in idea, but was acknowledged by Lavoisier as the starting-point of his own work on the subject (OEuvres, ii. 174), published in the next year. In the spring of 1778 Priestley returned to the important researches on vegetable physiology of 1772, and discovered oxygen in the bladders of seaweed. In June and the following months he found that this gas is given off in the light from the green confera in water but was doubtful as to the nature of the conferva until the following winter, when, with the help of William Bewley [q. v.] and others, he found it to be vegetable, and then extended his researches to other plants, but did not publish them till 1781. Meanwhile John Ingenhousz [q. v.] had published the main facts in 1779. Priestley accused him of plagiarism in 1800, after exonerating him from all suspicion in 1787 (Doctrine of Phlogiston established, pp. 80 sq). Priestley showed that the oxygen given off is due to the presence of gas in the water, and, also with the help of Bewley (Experiments on Natural Philosophy, i. 385 sq.), and in opposition to Ingenhousz, that the ' seeds ' (spores) of the conferva come from the air, or pre-exist in the water (ib. u. 17, 33), and are not spontaneously generated. He made numerous minor experiments of varying value on the effect of gases on plants.

In 1781 he decomposed ammonia by means of the electric spark; the experiments were interpreted later by Berthollet. In the same year Priestley, continuing with John Warltire of Birmingham certain observations of the latter on the burning of hydrogen in 1777, made experiments which led to the synthesis of nitric acid and water by Cavendish, and the interpretation of Cavendish's experiments by Lavoisier. Priestley and Warltire noticed that when hydrogen and air or oxygen are exploded, by means of an electric spark, a dew is formed; and Priestley had previously shown that when a spark is passed in air an acid is formed (Experiments . . .on. . . Air, i. 183 sq.) Cavendish repeated the experiments quantitatively in the summer of 1781, and told Priestley verbally of the formation of water without loss of weight when hydrogen and oxygen are exploded. Priestley in 1783, before Cavendish's paper was published, repeated the information to James Watt, who suggested to him that water was not an element, but a compound of dephlogisticated air and phlogiston. Hence arose a controversy on the relative claims of Watt and Cavendish with regard to priority, which Priestley might have settled, but did not. The repetition of Cavendish's experiments on a large scale in France, and Lavoisier's experiments on the action of steam on iron, made him waver for a moment in his adherence to the old theory. He had, in 1783,made the important discovery that 'calces' are reduced to the metallic state by heating in hydrogen, but failed to notice the water formed. In 1785, however, he made an admirable series of quantitative experiments on the oxidation of iron and the reduction of the oxide by hydrogen, with formation of water ; but, in spite of this, under the influence of Watt (Phil.Trans. 1785, pp. 279-89), he finally rejected the Lavoisierian doctrine. He concluded later that water was already contained in all gases, and that the acid formed in the Cavendish experiments was the essential product of what he viewed as the 'decomposition of dephlogisticated and inflam mable air.' In 1786 he published a series of experiments on 'various kinds of inflammable air,' under which name he included hydrogen, carbon monoxide, and various inflammable vapours; though he was aware that these had distinct properties, he often confused them. In the same year he published a further statement of his general theoretical views (Experiments on Natural Philosophy, iii. 400). In the condensed edition of his works, published in 1790, he described interesting experiments on the thermal conductibility of gases, which he found to be much the greatest in the case of hydrogen. In 1793 he published his 'Experiments on the Generation of Air from Water,' with a dedication to the Lunar Society, in which he explains the reasons for his rupture with the Royal Society, and with a reprint of the only paper contributed to their ' Philosophical Transactions ' and not included in his own works — the 'Experiments relating to the Decomposition of Inflammable and Dephlogisticated Air' (Phil. Trans. 1791, p. 213).

In 1796 Priestley published his 'Considerations on . . . Phlogiston.' This, addressed to the surviving answerers of Mr. Kirwan,' was promptly replied to by Pierre Auguste Adet, the eminent chemist, then French ambassador to the United States. Priestley rejoined in a second edition of his work, to which Berthollet and Fourcroy replied (Annales de Chimie, vol. xxvi.) The controversy, which relates chiefly to the composition of water, and to the existence of oxygen in 'finery cinder ' (magnetic oxide of iron), on which the new theories partly depended, was continued, mainly in America.

In 1798, evidently through forgetfulness (Med. Repository, ii. 254, v. 264), Priestley published, as if they were new, experiments on the combustion of the diamond, well known through numerous researches of Cadet, Lavoisier, and others, at least fifteen years previously. Priestley's objections to the explanation of certain experiments on the action of charcoal on steam and on me tallic oxidos (a stumbling-block to him since 1785) were well founded. They led William Cruickshank to discover that Priestley and his opponents alike had failed to recognise the existence of carbonic oxide as a distinct chemical species (Nicholson, Journal [1], v. 1, 1801). Priestley rejected Cruickshank's views, but asserted that if there were any discovery it was his. In 1800, when he confessed himself all but alone in his opinions, and appealed somewhat pathetically for a hearing, he published his last book, 'The Doctrine of Phlogiston established,' of which the second edition in 1803 shows no change of view. In his last papers he replied to Noah "Webster and Erasmus Darwin [q. v.], attacking the theory of spontaneous genera tion and of evolution, and defending hisformer experiments with undiminished clearness and vivacity.

Priestley's eminent discoveries in chemistry were due to an extraordinary quickness and keenness of imagination combined with no mean logical ability and manipulative skill. But, owing mainly to lack of adequate training, he failed to apprehend the full or true value of his great results. Carelessness and haste, not want of critical power, led him, at the outset, to follow the retrograde view of Stahl rather than the method of Boyle, Black, and Cavendish. The modification of the physical properties of bodies by the hypothetical electricity doubtless led him to welcome the theory of a 'phlogiston' which could similarly modify their chemical properties. Priestley was content to assign the same name to bodies with different properties, and to admit that two bodies with precisely the same properties, in other respects differed in composition (Considerations . . . on Phlogiston, 1st edit. p. 17). Though often inaccurate, he was not incapable of performing exact quantitative experiments, but he was careless of their interpretation. The idea of 'composition' in the sense of Lavoisier he hardly realised, except for a brief period between 1783 and 1785. But the enthusiasm roused in him by opposition made him keen to the last to see weak points in his opponent's theory: he failed to see its strength. Priestley is unjust to himself in attributing most of his discoveries to chance; his researches offer admirable examples of scientific induction (e.g. the researches on the action of plants on air). He has been called by Cuvier a 'father of modern chemistry . . . who would never acknowledge his daughter.' [371-75]


P.J.H. [Hartog, Philip Joseph]. “Priestley, Joseph, LL.D. (1733-1804).” The Dictionary of National Biography. Ed. Sir Leslie Stephen and Sir Sidney Lee. 22 vols. London, H. Milford, 1921-22. 16.357-76. Hathi Trust Digital Library. Web. 22 September 2020.

Last modified 22 September 2020