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Joseph Priestley, LL.D. (1733-1804)

This article was written by Alexander Gordon and Philip Joseph Hartog

Joseph Priestley, theologian and man of science, eldest of six children of Jonas Priestley (1700-1779), a cloth-dresser, by his first wife, Mary (d. 1739), only child of Joseph Swift of Shafton, near Wakefield, was born at Fieldhead, a wayside farmhouse in the parish of Birstall, West Riding of Yorkshire, on 13 March 1733. A lithograph of his birthplace (removed in 1858) was executed by Hanhart in 1864. His father became bankrupt in 1777. Timothy Priestley was a younger brother. His parents were members of the congregational church at Upper Chapel, Heckmondwike; but his grandfather, Joseph Priestley (1661-1745), a woollen manufacturer, attended the parish church at Birstall. Joseph was taught by his mother the Westminster catechism, which he could repeat at four years of age. From 1742 he was adopted by his father's eldest sister, Sarah (d. 1764), who had married John Keighley (d. 1745) of the Old Hall, Heckmondwike. Keighley was a man of substance. In early life a strong opponent of dissent, he was brought round by a sermon he had attended with a view to a prosecution. His wife entertained all dissenting ministers in the neighbourhood, and though a strong Calvinist made honest heretics very welcome. Priestley described her in 1777 as ‘in all respects as perfect a human character as I have yet been acquainted with’.

At Batley grammar school (from 1745) he was well grounded in Latin; began Greek, learned the shorthand invented by Peter Annet, wrote to Annet suggesting improvements, and sent some commendatory verses, which Annet prefixed to a new edition. Subsequently he became a pupil of John Kirkby (1677-1754), congregational minister of Upper Chapel, Heckmondwike, who had previously taught him Hebrew ‘on holidays.’ He had no taste for lighter reading, but early showed a turn for experiment. At the age of eleven, his brother tells us, he bottled up spiders to see how long they would live without fresh air.

His aunt wished to make him a minister, and he ‘readily entered into her views;’ but his health stood in the way; there were symptoms of consumption, and in 1749 (when Kirkby closed his school) it seemed unadvisable to proceed further with his education. He had some thoughts of medicine. A mercantile uncle proposed to put him into a counting-house at Lisbon. With this view he began to teach himself French, German, and Italian, and was able to reply to some of his uncle's foreign correspondents. He sought instruction in algebra and mathematics from George Haggerston (d. 1792), congregational minister at Hopton. All was ready for his voyage, when his health improved, and it was decided that he should study at a dissenting academy. For two years he had been teaching Hebrew to John Tommas, baptist minister at Gildersome, and had acquired the rudiments of Chaldee, Syriac, and Arabic. Before he was twenty he had read the Hebrew Bible twice through, once with points and once without. His aunt would have sent him to Plasterers' Hall Academy, London, under Zephaniah Marryat, D.D. (1685-1754), but he ‘resolutely opposed’ the condition of subscribing every six months to ‘ten printed articles of the strictest Calvinistic faith’. He was accordingly entered at Daventry Academy, at its opening, near the end of 1751, and was the first student who began his theological training under Caleb Ashworth, a connection of his family. In consequence of his proficiency he was exempted from all the studies of the first, and most of those of the second, year.

He was already drifting away from orthodox opinion. Haggerston, who inclined to the Baxterian compromise between Calvinism and Arminianism, had given his views a liberal tone. He owed more to the conversation of John Walker (1719-1805), who preached as a candidate at Heckmondwike in 1751. Walker, originally a churchman, was connected with the liberal dissenters of Dukinfield, Cheshire, and became ‘an avowed Baxterian.’ His reasoning made Priestley an Arminian. ‘Ah, Walker,’ said Priestley, when they met again in 1794, ‘it was you that first led me astray from the paths of orthodoxy’. Before going up to Daventry he was anxious to communicate at Heckmondwike. Kirkby would have admitted him, but on examination by the ‘elders’ (Timothy Armitage and Joseph Hodgson) he was rejected as ‘not quite orthodox.’ He was ‘distressed’ that he could not ‘feel a proper repentance for the sin of Adam.’

Ashworth was assisted in the Daventry Academy by Samuel Clark (1727-1769), eldest son of Samuel Clarke (properly Clark), (1684-1750). In 1751 Clark spoke of the new student as one ‘who seems to be a good, sensible young fellow, though he has unfortunately got a bad name, Priestley; those who gave him it I hope were no prophets’. Doddridge's lectures formed the textbook of theological study, and free discussion was admitted, ‘Ashworth taking the orthodox side of every question,’ and Clark ‘that of heresy.’ Priestley was a favourite with Ashworth, but was more influenced by Clark. Thus he became an Arian, still retaining a ‘qualified’ belief in the atonement. Clark revised a draft which Priestley made at the academy in 1755 of his ‘Institutes of Natural and Revealed Religion,’ which was not published till 1772-3. Neither tutor was strong in scholarship.

Before entering the academy Priestley had corresponded with Annet on the subject of freewill, maintaining the position of ‘philosophical liberty’ against Annet's ‘necessarian’ doctrine. Annet ‘importuned’ him for leave to publish the correspondence; this Priestley withheld, though from no doubt of his own arguments. He was moved by the ‘Enquiry’ (1715; reprinted by Priestley in 1790) of Anthony Collins, but remained unconvinced for several years. ‘I gave up my liberty,’ he says, ‘with great reluctance’; and it would appear that the instances of Annet and Collins had led him to connect determinism with ‘unbelievers’. From a reference in Doddridge's divinity lectures he became acquainted with the ‘Observations on Man’ (1749) by David Hartley (1705-1757), a book which exercised a decisive and permanent influence on his speculations. He ranked it next to the bible. Hartley's theory of association he embraced at once, and it carried the ‘necessarian’ doctrine as its consequence. His conversion to determinism probably dates from 1754. In 1757 he entered into a correspondence with Hartley, which was cut short by Hartley's death.

On Ashworth's recommendation Priestley was engaged in September 1755 as assistant and successor to John Meadows, presbyterian minister at Needham Market, Suffolk. Meadows, who had held this charge for fifty-four years, was superannuated, and the congregation decayed. Priestley was promised £40 a year; he got less than £30, declining the customary subsidy from the London congregational fund, as he ‘did not choose to have anything to do with the independents.’ The London presbyterians helped him by the usual subsidy from their fund, and by occasional benefactions through George Benson and Andrew Kippis. Though his preaching was uncontroversial, he made no secret of his Arianism, which alienated some hearers. Popularity was impossible for him, owing to an hereditary stammer. His aunt's last benefaction was a sum of twenty guineas, the fee of a London quack, one Angier, who undertook ‘to cure all defects of speech’ under an oath of secrecy. This business took Priestley to London for the first time, with the result that his impediment was ‘worse than ever.’

To provide means for his support, Priestley issued ‘proposals’ for a boarding-school, but no pupils came; this he attributes to his heterodox repute, ignoring, perhaps, the disadvantages of his bachelor situation. He gave a dozen lectures on the use of the globes to a class of adults. Meanwhile he was pursuing his theological studies. He managed to afford the luxury of subscribing for Tayler's Hebrew concordance, and set about comparing the Septuagint with the original. Soon he rejected the atonement, the inspiration of the sacred text, and all idea of direct divine action on the human soul. He wrote on the ‘Doctrine of Remission,’ and entrusted the manuscript to Caleb Fleming and Nathaniel Lardner, who published it, with an important omission, in 1761. Lardner, who accepted Priestley's views on atonement, strongly disapproved his criticism of St. Paul's dialectics. Priestley worked the excluded section into a separate essay. Kippis advised him to publish it ‘under the character of an unbeliever.’ This Priestley declined. While it was at press the printing was stopped at Kippis's urgent remonstrance; the essay did not see the light till 1770 in the ‘Theological Repository.’

Rejected by the Sheffield dissenters as ‘too gay and airy’, in September 1758 Priestley became minister at Nantwich, Cheshire. The congregation was very small, chiefly consisting of ‘travelling Scotchmen,’ and ‘not one of them was at all Calvinistical.’ He wrote few sermons, but established a flourishing school, never giving ‘a holiday on any consideration.’ His school and private tuition occupied him from seven in the morning till seven at night. Yet he learned to play the flute, ‘as the easiest instrument,’ and congratulated himself on having no ear, being thus ‘more easily pleased.’ He formed a friendship with Edward Harwood, and was intimate with Joseph Brereton (d. 1787), vicar of Acton, near Nantwich, who gave him a telescope ‘made with his own hands’.

Aikin's promotion to the divinity tutorship at Warrington Academy was followed by Priestley's appointment (September 1761) to the tutorship there in languages and belles-lettres. He would have preferred the chair of natural philosophy, held by John Holt. In his own department he introduced public exercises in English and Latin, and gave three courses of historical lectures, dealing especially with constitutional history, for students designed for ‘civil and active life.’ These lectures, published in 1788, were recommended at Cambridge by John Symonds, professor of modern history. His ‘Essay on Government,’ written at Warrington, and published in 1768, contains the sentence to which Jeremy Bentham considered himself indebted for the phrase ‘the greatest happiness of the greatest number.’ Edinburgh University conferred on him the diploma of LL.D. (4 Dec. 1764).

Priestley had been ordained on 18 May 1762 at Warrington. On 23 June in the same year he married, at Wrexham, Mary, only daughter of Isaac Wilkinson, of Plas Grono, ironmaster at Bersham, near Wrexham, afterwards of Bristol; her age was eighteen. She was a woman of sound culture and strong sense. Before his marriage Priestley described her to his brother as ‘very orthodox,’ but Timothy, on making her acquaintance, decided that she was ‘no dox.’ At the wedding the bride was given away by Priestley's pupil, Thomas Threlkeld, an absent-minded scholar, who, finding a Welsh bible in a pew of the parish church, forgot his duty in its perusal. His marriage led Priestley to project a ‘widows' fund’ for protestant dissenters of Lancashire and Cheshire. The scheme was launched on 16 May 1764, and produced a valuable benefit society, since become wealthy.

Priestley spent a month of every year in London, where he met Franklin. His life at Warrington was ‘singularly happy.’ The tutors worked harmoniously, and had their Saturday club for graver converse; for lighter recreation there was a coterie of anonymous verse writers, whose pieces were dropped into Mrs. Priestley's workbag. Some of Priestley's own verses first roused the poetic gift in Aikin's only daughter (afterwards known as Anna Lætitia Barbauld). But the academy did not flourish; Priestley was cramped for means (his salary was £100 with a house, in which he took a few boarders at £15 apiece), and his wife's health failed. Accordingly he welcomed a call to the ministry of Mill Hill Chapel, Leeds, and removed thither in September 1767. His salary, though exceeding that of most dissenting ministers at that date, was only a hundred guineas and a house, but his time was at his own disposal.

He devoted his weekdays to his studies, and wrote few discourses, making no secret of his habit of exchanging sermons with his friends; but he carefully instructed his flock in graduated classes for systematic catechising, a practice neglected by the liberal dissenters of that day. For ten years his theology had remained stationary. He now read Lardner ‘On the Logos,’ published in 1759, and became ‘what is called a Socinian,’ a development which much stimulated his controversial activity. As an organ of critical inquiry he projected (1768) and set on foot (1769) the ‘Theological Repository,’ which was published at irregular intervals till 1788. He offended public opinion by inviting, without success, the co-operation of deists; he aspired to make his magazine an open platform for the discussion of all subjects relating to biblical science. His first polemical piece (1769) was in reply to an attack by Henry Venn. His propagandist publications began with his ‘Appeal’ (1770), the most successful of his tracts, written in view of the progress of methodism among dissenters.

Priestley's ecclesiastical views retained the impress of his early training among independents. The decay of church organisation and the neglect of the sacraments among liberal dissenters concerned him; he proposed remedies in his address (1770) on church discipline, and his discourse (1782) on the constitution of a Christian church. He upheld the autonomy of the particular congregation, and was ‘for increasing the number of sects rather than diminishing them;’ hence his spirited ‘Remarks’ (1769) on Blackstone, who had classed nonconformity among crimes. He stood alone among his friends in advocating complete toleration for ‘papists,’ against the opinion of Lardner and Kippis. With the idea of a national church he had no sympathy, though admitting the utility of existing establishments, and desiring, not their dissolution, but their reform. He advocated the withdrawal of the ‘regium donum,’ then given to English as well as to Irish dissenters. It was with difficulty that he was persuaded to add his name to the petition (1772) for modifying the Toleration Act, which resulted in the amended act of 1779. ‘You have hitherto,’ he writes in a pamphlet of 1773, ‘preferred your prayer as Christians; stand forth now in the character of men, and ask at once for the repeal of all the penal laws which respect matters of opinion.’ He never qualified under either act, but thought liberty less menaced by the old subscription, practically a dead letter, than by the new and easier subscription, which might be enforced. In the same spirit he advised Theophilus Lindsey not to resign his benefice, but to make his own alterations in the prayer-book (as several clergymen did), and wait till he was ejected. But when Lindsey resigned (1773), Priestley acknowledged his friend's ‘better judgment,’ and entered heartily into his plans for a new religious movement under the unitarian name.

Till a minister's house was ready for him, he resided in Meadow Lane in the suburbs of Leeds, next door to a brewery. In 1770 he founded the Leeds circulating library. In December 1771 his study of science, to which he had long devoted his leisure, had brought him sufficient reputation to lead Sir Joseph Banks to offer him the appointment of ‘astronomer’ to the second expedition of James Cook (1728-79). The Mill Hill congregation agreed to provide an assistant during his absence; but clerical influence intervened, and Priestley's place was filled by Johann Reinhold Forster, who had succeeded him at Warrington. A curious story belonging to this period is told of a woman, who imagined herself possessed, applying to him as ‘a great philosopher who could perform miracles;’ he exorcised the demon by help of an electrical machine.

In December 1772 William Fitzmaurice-Petty, second earl of Shelburne, afterwards first marquis of Lansdowne, on the recommendation of Price, appointed Priestley his librarian or ‘literary companion.’ He was to furnish Shelburne with information on topics arising in parliament, and to superintend the education of Shelburne's sons, with Thomas Jervis under him as tutor. For this he was to have a salary of £250 with a house at Calne, Wiltshire (near to Bowood), and rooms in Shelburne's London house in Berkeley Square; if the agreement ended by mutual consent, Priestley was to receive an annuity of £150. He was to preach when he pleased, and pursue his own studies. He resigned Mill Hill on 20 December 1772, preached his farewell sermon on 16 May 1773, and removed to Calne in June. For some years the arrangement worked smoothly. Priestley catalogued Shelburne's books and manuscripts, and indexed his private papers. Shelburne gave him an addition of £40 a year towards his scientific experiments; a similar sum was contributed annually (from 1777) by scientific friends through John Fothergill, M.D.. In 1774 he spent three months (August-October) abroad with his patron, visiting Brussels (where a ‘popish priest’ tried to convert him), Holland, with which he was ‘much disgusted,’ the Rhine, and Paris, where he exhibited some of his experiments on air. Just before starting he had made his capital discovery (1 August 1774) of ‘dephlogisticated air’. His winters were spent in London, where he frequented the Whig Club at the London coffee-house, Ludgate Hill, of which Franklin and Canton were members.

By 1778, for some reason unknown to Priestley, but probably owing to his adoption of ‘materialism,’ his patron's feeling towards him had cooled, and in May 1780 he proposed to transfer him to an establishment on his Irish estate. Priestley at once offered to retire from Shelburne's service. The separation was amicable, and the annuity was punctually paid. Some years later (apparently in 1784) Shelburne made overtures for a renewal of the connection, which Priestley wisely declined.

During Priestley's engagement with Shelburne appeared his ‘Examination’ (1774) of the Scottish philosophy, written in a tone which he afterwards regretted. It was his first effort in psychology. Up to 1774 he maintained the ordinary distinction of soul and body, as having no common properties; though he had held, with Edmund Law, that the soul acts only through an organism. His first hint of the doctrine of the homogeneity of man was given in an essay (1775) introductory to a selection from Hartley. It brought upon him the imputation of atheism. A copy of the work, at the sale of the Abbé Needham's library at Brussels in 1782, was seized by the licensers, and burned along with a copy of Cudworth's ‘Intellectual System.’ Further study resulted in his ‘Disquisitions relating to Matter and Spirit’ (December 1777), which Shelburne's friends (but not Shelburne) tried to dissuade him from publishing. It led to correspondence with John Henderson (1757-1788) and Augustus Montague Toplady, and to an amicable discussion (1778) with Price. A supplemental volume on ‘philosophical necessity’ was the occasion of his first controversial encounter with Samuel Horsley. Priestley called his system by the name of ‘materialism,’ but by 1772 he had adopted from Ruggiero Giuseppe Boscowich (1711-1787) the theory that matter consists only of points of force; the doctrine of the penetrability of matter had independently suggested itself (before 1772) to his friend Michell. Rutt supposes that Boscowich was the ‘priest of the catholic communion,’ having ‘a taste for science,’ who met Priestley in Paris (1774), and embraced him ‘with tears’ as the first philosopher among his acquaintance who made profession of Christianity.

A more strictly professional work of his Shelburne period was his Greek ‘Harmony’ of the Gospels, projected in 1774, and published in 1777. It shows no appreciation of the real difficulties of the problem, and is chiefly remarkable as adopting the theory of Nicholas Mann, who limited the ministry of our Lord to little more than a single year. On this topic Priestley had a friendly controversy (1779-81) with William Newcome, then bishop of Waterford. During its progress he began his ‘Letters to a Philosophical Unbeliever’ (1780-2), directed primarily against Hume.

After quitting Shelburne's service he remained at Calne till Michaelmas 1780, and then removed to Birmingham, partly to be nearer his brother-in-law, John Wilkinson (d. 14 July 1808) of Castle Head in the parish of Cartmel, Lancashire, who provided him with a house. A wealthy widow, Elizabeth Rayner (d. 11 July 1800, aged 86), of Sunbury, Middlesex, gave him one hundred guineas towards his removal, the first instalment of many benefactions from the same quarter. A handsome addition to his income was made by the annual subscriptions of his friends. William Heberden the elder contributed largely in aid of his theological as well as his scientific research. On Fothergill's death his contribution was continued by Samuel Galton, a Birmingham quaker, who was disowned (1795) ‘for fabricating and selling instruments of war.’ Josiah Wedgwood, the potter, besides an annual benefaction, furnished him with apparatus made to his instructions. Samuel Parker (d. 1817), a London optician (a Calvinistic dissenter), supplied him with every instrument he required in glass, including his burning lenses, twelve and sixteen inches in diameter. Soon after 1772 he was elected one of the eight associates of the French Academy of Sciences. In December 1780 he was made a member of the Imperial Academy of Sciences at St. Petersburg. Similar honours reached him from Turin, Haarlem, and elsewhere.

Before Christmas 1780 William Hawkes (1732-1796) resigned his office as junior minister of the New Meeting, Birmingham. Priestley was at once elected colleague with Samuel Blyth (1719-1796), and began his duties on 31 December. He was without pastoral charge, being engaged only for Sunday duty. He pursued the plan of catechetical instruction which he had introduced at Leeds, adding the practice of expounding the scripture lessons. His salary was £100; but his congregation, led by his friend William Russell (1740-1818), was liberal in gifts. A donation of £200, in acknowledgment of his catechetical work, he insisted on dividing with Blyth. Early in 1781 he declined a call to George's Meeting, Exeter. Twice he was sounded in vain about accepting a government pension; by Lee when solicitor-general (1782), and again (1784) ‘by a bishop,’ probably Edmund Law, a member with Priestley of a ‘society for promoting the knowledge of the Scriptures’ (1783). He preferred the aid of ‘lovers of science and also lovers of liberty.’ Brougham remarks that ‘different men entertain different notions of independence.’ Huxley, with more reason, refers to ‘the generous and tender warmth with which his many friends vied with one another in rendering him substantial help.’ Edmund Burke, who visited him at Birmingham at the close of 1782, ‘reported him to all his friends as the most happy of men, and most to be envied’.

Early in his Birmingham ministry his social relations, even with the established clergy, were pleasant enough. Once a month he dined with the ‘Lunar Society,’ meeting Matthew Boulton, James Keir, James Watt, William Withering, M.D., the botanist, and, for a time, Erasmus Darwin. Every fortnight he discussed theology at tea with his clerical comrades. He continued his periodic visits to London. It has been said that Dr. Johnson refused to meet Priestley, the fact being that it was Priestley who repeatedly declined an introduction to Johnson, till at length John Paradise at Johnson's request, brought them together at dinner. Johnson promised to call on him the next time he was at Birmingham.

In 1772 he had appended to a reprint of his Leeds ‘Appeal’ a ‘concise history’ of certain established doctrines. He began to amplify it for a fourth part of his ‘Institutes.’ It took shape as a ‘History of the Corruptions of Christianity’ (December 1782), the best known, though not the best, of his theological writings (in 1785 it was burned by the common hangman at Dort). In this work he challenged a discussion with Gibbon, who, in a short correspondence, advised him (28 January 1783) to stick to ‘those sciences in which real and useful improvements can be made,’ and contemptuously declined the challenge. Criticism on the first section of the work, relating to the person of Christ, led him to prepare a more elaborate treatise on this head. John Hawkins, rector of Hinton-Ampner, Hampshire, procured him books from the cathedral library at Worcester. He began to question the received accounts of our Lord's nativity, and in articles in the ‘Theological Repository’ (1784) rejected the doctrine of the virgin birth as without historical basis. His opinion that our Lord was born at Nazareth has been revived by modern critics. In this connection he startled his friend Lindsey by maintaining that our Lord was neither naturally impeccable nor intellectually infallible, was under delusion respecting demoniacal possession, and had misconceived the purport of some of the prophecies. His labours culminated in the ‘History of Early Opinions concerning Jesus Christ’ (1786). Writing as a sectary, he damaged at the outset his claim to scrutinise in a scientific spirit the course of thought in Christian antiquity; but he was one of the first to open the way to the study of doctrinal development, and while proclaiming his own bias with rare frankness, he submitted his historical judgments to the arbitrament of further research. His account of the origin of Arianism, as a novel system, has stood this test. What was special in his method was the endeavour, discarding the speculations of the fathers, to penetrate to the mind of the common Christian people. He broke entirely with the old application of the principle of private judgment, maintaining that a purely modern interpretation of Scripture is, ipso facto, discredited, and the meaning attached to it by the earliest age, if ascertainable, must be decisive. A good summary of his position is in his ‘Letters’ (1787) to Alexander Geddes, the Roman catholic scholar, who had addressed him as his ‘fellow-disciple in Jesus.’

He was criticised by Samuel Badcock, a contributor to his ‘Theological Repository,’ with whom he had been on terms of very close literary correspondence, by Francis Howes, James Barnard, and Thomas Knowles. The attack was led by Horsley, who, refusing to enter on ‘the main question,’ set himself ‘to destroy the writer's credit and the authority of his name’. He adopted, with masterly effect, Bentley's line against Collins. In showing that Priestley failed to understand Platonism, Horsley did real service. His brilliant exposure of Priestley's slips was less in point. Priestley, while not a finished scholar, had competent learning, though he wrote in haste. The charge of borrowing from Daniel Zwicker (1612-1678) was the less reasonable, as neither Priestley nor Horsley had seen Zwicker's tracts, which Horsley only knew from the animadversions of George Bull. That he abstained from reading Priestley's riper treatise illustrates his controversial skill rather than his fairness.

The controversy with Horsley lasted from 1783 to 1790. From 1786 Priestley issued an annual defence of unitarianism, in review of all opponents. In 1787 he resisted the resolution of Charles Cooke (carried 12 December) to exclude controversial divinity from the Birmingham Public Library, which he had reorganised in 1782. In 1789 he projected a new version of the Scriptures, in conjunction with Michael Dodson, William Frend and Lindsey. Priestley was to be answerable for the hagiographa of the Old Testament, getting what assistance he could (Martineau errs in supposing that he undertook to translate the Hebrew Bible singlehanded). The first instalment of his ‘General History of the Christian Church,’ a work of some merit, was published in 1790. In July 1790 he met Samuel Parr at the ordination of William Field. Being at Buxton in the following autumn, he preached by special request in the assembly room (19 September). Grattan was present, and John Hely-Hutchinson, provost of Trinity College, Dublin. The sermon (afterwards published) was a powerful argument for the resurrection of our Lord. In October he asked his Roman catholic neighbour, Joseph Berington, to preach the Sunday-school sermon at the New Meeting. Berington hoped at some future time that it might be prudent to do so. Early in 1791 Priestley concurred in the formation of the ‘Unitarian Society.’ The preamble, drawn by Thomas Belsham, was meant to exclude Arians; nevertheless Price joined it. Meanwhile he was pursuing his experiments in science and publishing the results.

In politics he had taken little part. He had written in 1769 and 1774 two anonymous pamphlets on the relations of Great Britain with the colonies. The second of these (against war) was revised by Franklin, with whom he was on the most confidential terms. His intimacy with Burke lasted till 1783. He states that he was never a member of any political club, though it appears that he had attended the Birmingham dinner (4 November 1788) in celebration of the landing of William III, from which the toast of ‘church and constitution’ was excluded; and he had a hand in the framing of the Birmingham Constitutional Society (June 1791) on the model of that at Manchester. The measures of reform in the advocacy of which he co-operated were the abolition of the slave trade, and the repeal of the test and corporation acts. On the latter topic he wrote his ‘Letter to Pitt’ (1787) and a Fifth of November sermon (1789). The defeat of Fox's motion for repeal (2 March 1790) was largely caused by the preface (17 February) of Priestley's ‘Letters’ addressed to Edward Burn. Extracts were furnished to all members of the House of Commons. He had called on the clergy to avert revolution by reform, and, with more imagination than usual, described his own theological efforts as ‘grains of gunpowder’ for which his opponents were ‘providing the match’. The nickname ‘Gunpowder Priestley’ was adopted in songs and caricatures. Popular feeling against him was increased by his ‘Letters to Burke’ (1 January 1791), in which he vindicated the principles of the French revolution. These ran through three editions, and were followed in June by his anonymous ‘Dialogue on the General Principles of Government.’

On Thursday, 14 July 1791, the ‘Constitutional Society’ of Birmingham held a dinner in Thomas Dadley's Hotel, Temple Row, to commemorate the fall of the Bastille. Priestley had ‘little to do’ with it, but he meant to be present, and on 6 July he asked William Hutton (1723-1815) and Berington to join the party; they both declined. The promoters invited, by public advertisement (7 July), ‘any friend to freedom.’ An inflammatory handbill of republican tendency was disowned by the promoters, who publicly advertised their ‘firm attachment to the constitution.’ On the morning of the 14th his friend Russell sent Priestley a note from town, advising him not to attend the dinner; hence he did not go. An angry crowd hung about the door as the company (numbering eighty-one) assembled at three o'clock, but the dinner, during which some extravagant toasts were honoured, ended quietly before six. The chairman, James Keir, was a churchman. It appears there was a dinner, not public, ‘of the opposite party,’ at the Swan in Bull Street, which kept up till a later hour.

About eight o'clock in the evening the crowd broke the windows of Dadley's Hotel. Finding that the guests had left, the mob directed their attention to the residences of the organisers, among whom they wrongly assumed Priestley was the chief. After wrecking and burning the New Meeting and the Old Meeting, they attacked Priestley's house at Fairhill, a mile from Birmingham, and destroyed nearly all his books, papers, and apparatus. He and his family managed to escape before the incendiaries arrived. Rioting continued on Friday and Saturday; the town was in the hands of the mob, the gaols were opened, seven residences were burned, and many others wrecked; the meeting-house at Kingswood, seven miles from Birmingham, was also destroyed. The magistrates were powerless; great exertions to restore order were made by Heneage Finch, fourth earl of Aylesford (a pupil of Horsley), without avail. At length dragoons arrived from Nottingham on Saturday night, and the disorder ceased.

Much mutual recrimination filled the pamphlets of the time. The Riot Act was not read at the beginning of the disorder, as it was next year (May 1792) to stop a raid on the brothels of Birmingham. Priestley's friends charged the authorities, including the clergy, with culpable dereliction of duty. This view was shared by Sir Samuel Romilly, who was in Birmingham in the latter part of July, and it was emphasised in the well-known lines in Coleridge's ‘Religious Musings written on Christmas Eve,’ 1794. Priestley's friends, however, hardly made allowance for their own miscalculation of the current of popular feeling to which they ran counter. George III, writing to Dundas, expressed himself as ‘pleased that Priestley is the sufferer,’ though disapproving the ‘atrocious means’ employed. For Priestley it was a rude awakening. He had passed the day in the company of Adam Walker, a lecturer on physics from London, who had dined at Fairhill. Late in the evening, while playing backgammon with his wife, he was warned of his danger, and, though incredulous, he allowed himself to be driven in a chaise to his friend Russell's, at Showell Green, a mile further from town. After watching the fires from the meeting-houses, he proceeded to Thomas Hawkes's, at Moseley Wake Green, half a mile further. Here he was within earshot of the shouts of the wreckers of his own house. It seems they tried to get fire from his electrical machine, to burn the building, ‘with that love for the practical application of science which is the source of the greatness of Birmingham’. At four o'clock in the morning he was retiring to bed at Showell Green, when the mob approached, and he drove to the house of William Finch, his son-in-law, at Heath Forge, five miles beyond Dudley.

He made up his mind, if it were a fine Sunday, to preach in the ruins of his meeting-house, and chose his text. On Friday night he was roused from sleep, and rode to Bridgnorth, Shropshire, driving back thence to Kidderminster. Thinking all was safe, he rode back to Heath Forge on Saturday evening, but was persuaded at once to retrace his steps. From Kidderminster he made his way to Worcester, and, catching the London coach, reached Lindsey's house in Essex Street at five o'clock on Monday morning. Next day he wrote an expostulatory letter to the inhabitants of Birmingham, and at once began his discourse on the duty of forgiveness of injuries. This sermon did not convert his spirited wife. ‘I do not think,’ she writes (26 August) to Mrs. Barbauld, ‘that God can require it of us as a duty, after they have smote one cheek, to turn the other. … They will scarcely find so many respectable characters a second time to make a bonfire of. So much for King and Church for ever.’ Four or five of the rioters were tried at Worcester; one was executed on 19 August, and another subsequently. Twelve were tried at Warwick on 22 and 23 August by Sir Richard Perryn; four were convicted; of these, two were executed on 8 September A moderate compensation was awarded to the sufferers. Priestley's compensation (paid in 1793) fell short of his losses by some £2,000. Some of his private papers, which fell into the hands of Curtis, were sent by him to Henry Dundas, afterwards first viscount Melville, then home secretary, and not returned. Addresses of sympathy reached him from the French Academy of Sciences and many other public bodies.

For a few months Priestley was the guest of William Vaughan at Missenden, Buckinghamshire. He preached for the first time after the riots on 26 September in a Calvinistic baptist chapel at the neighbouring town of Amersham, by the unanimous request of minister and people. This was probably through the influence of Robert Hall (1764-1831). Two other congregations of orthodox dissenters requested his services. Even among methodists he had sympathisers. ‘The curse of God,’ said Samuel Bradburn in a sermon (1793) at Birmingham, ‘hangs over your town for the infamous treatment Dr. Priestley experienced among you.’ He was invited to Paris and Toulouse, but resolved to settle in London; a house was taken for him at Clapton in a friend's name. ‘He has taken,’ writes Hutton, ‘a house near London for twenty-one years, provided he lives and the house stands so long.’ He wished, however, to return to Birmingham and continue his ministry till Christmas; his congregation begged him not to run the risk, and asked him to nominate his successor. His ‘forgiveness’ sermon was delivered at Birmingham by John Coates (d. 2 April 1826, aged 73), of the Old Meeting. The first part of his ‘Appeal’ on the subject of the riots is dated 1 November. On 7 November, by fifty-one votes to nineteen, he was elected to succeed Price as morning preacher at the Gravel Pit Chapel, Hackney, and entered on his pastoral duties on 4 December. No fixed salary was guaranteed, but his receipts were at the rate of a hundred and fifty guineas a year. A section of Price's friends left, but there was a large accession of newcomers.

At Hackney his life went on ‘even more happily’ than at Birmingham. His pecuniary losses were more than made up by his friends. Wilkinson, his brother-in-law, gave him £500, transferred to him a nominal sum of £10,000 in the French funds, and, as this was unproductive, paid him £200 a year. His catechetical classes, contrary to expectation, attracted many outsiders. Lindsey and Belsham were near neighbours; he had superior advantages for his scientific pursuits; he gave lectures at Hackney College on history and chemistry. In September 1792 he was made a citizen of France, and elected a member for the department of Orne in the National Convention. Other departments followed suit, but, while he accepted citizenship, he declined election. The majority of members of the Royal Society fought shy of him. Finding that they were rejecting eligible candidates on political grounds, he withdrew from attendance (1793), and ceased to publish in the ‘Philosophical Transactions.’

As early as 1772 he had contemplated a removal to America for the sake of his children. His wife's first thought after the riots was ‘for trying a new soil.’ His three sons emigrated to America in August 1793, and he expected to follow them. His wife was ‘more bent on’ it than himself. He resigned his charge on 21 February 1794, preached a farewell sermon on 30 March, and embarked in the Sansom, off Gravesend, on 7 April. On 4 June he landed at New York, where Mrs. Priestley ‘never felt herself more at home in her life.’ He received a number of addresses. His answer to a blatant address of the ‘Democratic Society’ of New York ‘pleased everybody except the society itself.’ In reply to one from ‘republican natives of Great Britain,’ he declared his preference for a republic, and his hope of the abolition of slavery. He was disappointed at having no invitation to preach.

His sons and his friend Thomas Cooper, M.D. were interested in a proposed settlement in Pennsylvania on the Susquehanna. To be near them he left New York on 18 June, stayed a fortnight at Philadelphia, and on 11 July reached Northumberland, Pennsylvania. The settlement scheme was abandoned, but finding Northumberland a ‘delightful situation’ he made it his home, and built a house. He once preached in the presbyterian meeting-house, but the invitation was not repeated. Accordingly he held public services in his own house, and from about 1799 in a wooden building adjoining. A projected college came to nothing, though a building was begun. He had declined (November 1794) a chemistry chair at Philadelphia, than which he ‘never saw a town’ he liked less. But he resolved to spend two months there every winter, in hope of founding a unitarian congregation. His discourses on the evidences, delivered there (February-May 1796) in Elhanan Winchester's universalist meeting-house, drew distinguished congregations, and a small unitarian society was formed. On subsequent visits he attracted less attention; his voice was very weak, and his teeth were gone.

The deaths of his youngest son Henry (1795) and of his wife (1796) left him lonely, and the unfilial conduct of his second son, which his biographers pass in silence, affected him deeply. To his friend Lindsey he writes, on 29 October 1796, ‘Could I pay you one visit in England, I should sing my nunc dimittis.’ Henceforth he lived in the family of his eldest son.

In America his theology advanced to its final point by his adoption of a doctrine of ‘universal restitution,’ which he reached more slowly and with greater hesitation than was his wont. With the old universalist opinion, limiting retribution to this life, he had no sympathy; he looked for a moral progression to succeed the sleep of death. Thus on the death of his youngest son (1795) in his nineteenth year, he hopes that he ‘had the foundation of something in his character on which a good superstructure may be raised hereafter.’ Before 1803 this theory had established itself in his mind as a ‘firm faith.’ With this exception his American period shows industry in old directions rather than fresh activity of mind. To the American Philosophical Society at Philadelphia he communicated the results of new experiments. He wrote against Paine and Volney and a number of French freethinkers, upheld the biblical institutions in comparison with those of oriental antiquity, completed his church history, contrasted Socrates with our Lord, and annotated the whole Bible. His friends continued to contribute to his resources; Mrs. Rayner sent him £50 a year and left him £2,000; the Duke of Grafton sent him £40 a year.He was never naturalised as an American citizen. In American politics he sided with the democrats against the federalists, which exposed him to the attacks of William Cobbett. He corresponded occasionally with Adams, more with Jefferson.

Throughout 1800 he had serious thoughts of returning to Europe; by 13 November he had made up his mind to sail for France (where he had property) as soon as there was ‘free and safe communication.’ But on 8 March 1801, while visiting Philadelphia, he was attacked by a bilious fever and pleurisy, which nearly cost him his life, and left him permanently enfeebled. He ceased to dig his garden, and was less in his laboratory, living much among his books. He was sounded (1803) about accepting the principalship of the university of Pennsylvania, but declined the overture. In May 1803 his left leg was lamed by a fall; soon after this his digestive powers failed. Till the close of that year he was the first to rise in the morning, always lighting his own fire. At the end of January 1804 news reached London that he had suffered a loss of £200 a year by the withdrawal of Wilkinson's aid. His English friends met on 6 February (the day of his death) and raised an annual subscription of nearly £400 On 2 February he made the last entry in his diary. Less than an hour before his death he dictated, with great precision, some emendations for a posthumous publication, adding, ‘I have now done.’

He died at Northumberland on 6 February 1804, and was buried in the quakers' burial-ground there on 9 February, William Christie giving a funeral address. His wife had died at Northumberland on 17 September 1796, aged 52. His children were:

1. Sarah (d. 1803), married to William Finch.
2. Joseph, born at Leeds on 24 July 1768; he left Northumberland in January 1812, settled at Cradley, Staffordshire, and died at Exeter on 2 Sept. 1833; he married (1792) Elizabeth (d. 8 May 1816, aged 46), elder daughter of Samuel Ryland, Birmingham; secondly (1825), Mrs. Barton, daughter of Joshua Toulmin; his daughter Eliza married Joseph Parkes.
3. William, who was naturalised as a French citizen on 8 June 1792, and admitted to the bar in Paris; he married Bettie Foulke, and died a planter in Louisiana before 1835.
4. Henry, who died at Northumberland on 11 December 1795, aged 18.

Priestley spoke and moved rapidly; in private converse he was vivacious and fond of anecdote, ‘often smiled, but seldom laughed’; he would walk twenty miles before breakfast, carrying a long cane, and was a good horseman. Of his preaching Catherine Hutton writes (1781): ‘He uses no action, no declamation, but his voice and manner are those of one friend speaking to another.’ His experiments imply great deftness of delicate manipulation with rude apparatus, but he had no mechanical readiness; his brother says ‘he could scarcely handle any tool.’ From 1783, being troubled with gall-stones, he used chiefly a vegetable diet, with ‘one glass of wine at dinner.’ He found it easy to be very methodical in his habits, working with his watch before him, and turning immediately to another task when the allotted time was up. Hence he could say (31 August 1789), ‘I am far from being a close student; I never fatigue myself in the least.’ He thought his main talent was a facility in arrangement, but affirms that he could do nothing in a hurry. Edward Burn reports him as saying, in reference to his theological controversies, ‘I set apart an hour in the morning and an hour in the evening, just to tease you a little’. His literary work was often done at his fireside, amid conversation. He composed in shorthand; his rapid pen never left his meaning doubtful; a turn for epigram is the chief ornament of his style. He had little humour, but enjoyed a remarkable faculty for making the best of things. His home affections were strong. He provided a maintenance for his younger brother Joshua at Birstall. Domestic management he left to his wife, speaking of himself as a lodger in her house. To the faults of his memory he often alludes; it is curious that he never learned the American currency, and would say to a shopkeeper, ‘You will give me the proper change, for I do not know it’.

Toplady said of Priestley's character, ‘I love a man whom I can hold up as a piece of crystal, and look through him.’ He ‘charmed away the bitterest prejudices in personal intercourse’. Nor was this merely a triumph of amiability; it illustrates the variety of his human interests, as well as his constitutional straight-forwardness. The history of his religious mind exhibits a continuous renunciation of prepossessions. He scouted ambiguity, the refuge of earlier heretics. The fearlessness and frankness of his propaganda were entirely new; for Whiston, whom he resembled in temperament, wrote only for the learned. Like Whiston's, his nature was essentially devout, and he had a conservatism of his own which he identified with primitive Christianity, holding tenaciously to the miraculously attested mission of Moses and messiahship of Christ, whose second coming he expected by 1814 at latest. His crusade against Arians was more successful in detaching them from liberal dissent than in converting them; his influence among unitarians soon paled before that of Channing. It was as a pioneer of religious reform that he wished to be judged; to his theological aims his philosophy was subsidiary; his chemistry was the recreation of his leisure time. Dr. Martineau, in an able estimate, published in 1833, does justice to his ‘extraordinary versatility,’ his ‘passion for simplicity,’ and ‘eager rather than patient’ attention, but goes too far in claiming that ‘his conclusions’ were ‘drawn by the absolutely solitary exercise of his own mind.’ Martineau specifies his ‘Analogy of the Divine Dispensations’ as his finest piece. Brougham wrote rather grudgingly of his career. Mr. Leslie Stephen (English Thought in the Eighteenth Century, 1876.) construes his many-sided activity as restlessness, and criticises his partial retention of the supernatural. More sympathetic is the Birmingham address, by Professor Huxley, in whose judgment ‘his philosophical treatises are still well worth reading.’

In person Priestley was slim but large-boned; his stature about five feet nine, and very erect. His countenance is best seen in profile, and the right and left profiles differ remarkably; the front face is heavy. He wore a wig till he settled in Northumberland, which did not boast of a hairdresser.

Priestley's Scientific Work.— 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. 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’, 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), Sir William Watson, M.D, John Canton 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.R.S. 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; 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, 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 November 1767 that electrification may be only a modification of the body electrified; but he afterwards identified ‘the electric matter’ with phlogiston. In his ‘History’ he anticipated Henry Cavendish 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. In February 1770 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. 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.

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. 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. 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, of Joseph Black 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 gas 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), he 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’. Certain experiments on this part of his work were made for Priestley by William Hey. 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 simultaneously and independently obtained a like result, which he first described in ‘De Aere fixo’, dated 12 September 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 November 1773), and was at once abstracted at length by Lavoisier and criticised by him. Henceforward 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 November 1771), ‘If I do work for nothing, it shall be on theological subjects.’ In the ‘History of Light’ he announced his adherence to Boscowich's theory of points of force. 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 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 influence of Richard Kirwan and Cavendish. At various periods he identified phlogiston with electricity and with hydrogen. But his whole scientific energies from this time forward were devoted to the upholding of the phlogistic 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 August 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. 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. Fifteen years later — in the 1790 edition of ‘Experiments on Air’ — Priestley declared specifically that he told Lavoisier of his experiments during this visit to Paris. There is no doubt that immediately after that date Lavoisier made oxygen for himself, and in the May following published the first of a long series of memoirs, in which he used his experiments to explain the constitution of the air, combustion and respiration, and to give an experimental interpretation of the Greek 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 Northumberland, Pennsylvania, on 1 August 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 possession of’ the gas ‘before November 1771’ , it is admitted that Karl Wilhelm Scheele, the great Swedish chemist, working quite independently, first recognised it as a distinct species ‘before 1773’, 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.

In November 1774 Priestley discovered vitriolic acid air (sulphur dioxide), and before November 1775, continuing an investigation by Scheele, fluor acid air (silicon tetrafluoride). 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, 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, 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 conferva in water, but was doubtful as to the nature of the conferva until the following winter, when, with the help of William Bewley 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 had published the main facts in 1779. Priestley accused him of plagiarism in 1800, after exonerating him from all suspicion in 1787. Priestley showed that the oxygen given off is due to the presence of gas in the water, and, also with the help of Bewley, and in opposition to Ingenhousz, that the ‘seeds’ (spores) of the conferva come from the air, or pre-exist in the water, 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. 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, 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 inflammable 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. 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’.

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. 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, 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 metallic oxides (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. 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, attacking the theory of spontaneous generation and of evolution, and defending his former 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. 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.’

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