Joseph Priestley

CHAPTER X

Determines to leave England—His arrival in America—Settles at Northumberland—His closing days—His death.

Priestley’s position in London for some time after his arrival there was very insecure, and so apprehensive were his friends of further outrage that it was thought necessary to provide him with a disguise and to arrange a plan of escape in case the house should be attacked. At first he was not allowed to appear in the streets. Ultimately he was moved to Tottenham, where he spent a month.

In the middle of October a house was taken for him in Hackney, but it was with difficulty that the landlord, who feared his property would be demolished, was persuaded to accept him as a tenant. Here, however, he proceeded to build himself a laboratory, and in a letter to Thomas Wedgwood, of October 18, 1791, he says:—

“As soon as convenient I shall be obliged to your father if he will supply me, as usual, with such retorts as you make, viz., earthen tubes closed at the end and open, and some with two necks. Small retorts, evaporating-dishes, mortars and levigators. Perhaps your servants here can tell me the price at which I must estimate those that were destroyed by the riot. I must soon give in an account of my losses, and I fear that some person on your part must attend at Warwick to attest the value. Mr Nairn, Mr Parker and others have promised to attend. But I have prepared [proposed] a conference between my appraiser and those for the county in London, which, if they be disposed to do justice, will save much trouble and expense.

“Whether I shall be invited to succeed Dr Price is uncertain. Many apprehend public disturbance in consequence of my coming. I could not get a house let in my own name. A friend took it in his. I have, however, very handsome proposals from France, particularly the offer of a house completely furnished, two miles from Paris, and another polite invitation from Toulouse, to take up my residence in the South of France in ‘a monastery which reason has recovered from superstition.’”

Priestley’s claim for damages amounted to £3628, 8s. 9d. Hutton says his real loss was upwards of £4500 (Jewitt’s Life of Hutton, p. 255). The Court allowed £2502, 18s. In the town of Birmingham property to the value of £50,000 was destroyed, of which sum £26,961, 2s. 3d. was finally paid by a rate on the Hundred, in which Birmingham is included (Sam Timmins, Trans. Midl. Inst., 1875).

Lindsey, writing to his friend, Alexander of Yarmouth, under date October 15, 1791, mentioning Priestley, says:—

“He is very well, and with his wonted cheerfulness, which has never forsaken him. Sunday last he preached for me for the first time since he has been expelled by fire and destruction out of his own place of worship, and he does me that favour to-morrow again. He has at last, though very reluctantly, and much to the concern of his late beloved people, given up the thought of continuing the pastoral office among them, as the exercise of it would not probably be consistent with his personal safety and liberty; such is the temper of his many adversaries still, and so hostile to him.”

The managers of other Dissenting chapels had not the courage of Lindsey and begged that he would refrain from preaching to their congregations. Eventually he was invited to take the position formerly occupied by his friend Price.

The rancour of his enemies now broke out afresh, and the most persistent efforts were made to damage and disparage him in the eyes of his congregation. His friends in the neighbourhood were advised to move their effects to some place of greater safety, as it was common rumour that his house was to be attacked on the succeeding anniversary of the Birmingham riot. His servants were afraid to remain for any length of time with him, and the tradespeople hesitated to take his custom. He was several times burnt in effigy along with Tom Paine. Coloured caricatures of him, of the grossest and coarsest kind, in which he was described as “the treacherous rebel and Birmingham rioter” were scattered broadcast. Insulting letters, in some of which he was likened to Guy Fawkes or the devil himself, were sent to him from all parts of the country, even from men calling themselves ministers of the Gospel. In one of these he was threatened with being burned alive before a slow fire. The Rev. Dr Tatham, Rector of Lincoln College, Oxford, whose biographer compared him with Warburton (“There is much of the same rough, unpolished strength in his language”), thus addressed him:—

“Long have you been the Danger of this country, the Bane of its Polity, and the Canker-worm of its Happiness. Long, too long, have your Principles tended to bereave it of its Religion, its Constitution, and consequently of its King.”

Burke, to his everlasting shame, inveighed against him in the House of Commons, and many of his associates in the Royal Society shunned him.

His position in the Society became eventually so irksome that he withdrew from it, as he explains in the preface to his Observations and Experiments on the Generation of Air from Water, which he published in pamphlet form at Hackney, with a dedication to the members of the Lunar Society.

In a letter to Withering, written from Clapton, October 2, 1792, he says:—

“... One of the things that I regret the most in being expelled from Birmingham is the loss of your company and that of the rest of the Lunar Society. I feel I want the spur to constant exertion which I had with you. My philosophical friends here are cold and distant. Mr Cavendish never expressed the least concern on account of anything I had suffered, though I joined a party with which he was, and talked with them some time. I do not expect to have much intercourse with any of them.

“I have, however, nearly replaced my apparatus, and intend not to be idle. I have already made some experiments relating to the doctrine of phlogiston, and when I have made a few more shall probably write something on the subject. I am surprised at the confidence with which the French chemists write; but I cannot yet learn what they have to object to my last paper in the Philosophical Transactions....

“I was in hopes to have been able to pay my friends of Birm. a visit long before this time, but was always discouraged, so that I have now given up the thoughts of it, and must content myself with seeing as many of them as I can here.... I do not, however, think I shall continue here long. Though unwillingly, I shall some time hence follow my son to France. But as I can do nothing there I will stay here as long as I can.”

To what lengths the Government were determined to go was seen in their banishment, in 1793, of Thomas Fyshe Palmer, a gentleman of a highly respectable and opulent family in Bedfordshire, to Botany Bay for seven years, because he had been concerned in publishing a paper in favour of Parliamentary Reform; and in their treatment of Mr Winterbotham, a Calvinistic minister of Plymouth Dock, on account of his political opinions. The mock trial of Mr Winterbotham at Newgate and the four years’ imprisonment which followed it, created a wide-spread feeling of indignation and alarm, and many families were constrained to leave the country in disgust. Among them was Priestley’s friend and fellow-sufferer, the worthy Mr Russell, who on his way to Boston, New England, was captured with his family by a French privateer and thrown into prison in Brest.

Priestley, at length, also determined to follow them. It was however with the greatest reluctance that he came to that decision. It meant parting from affectionate and devoted friends to whom he was warmly attached, whose zeal to serve him and to minister to his wants far outweighed the hatred of those who sought to cover him with oblivion. It meant too the relinquishment in large measure of his philosophical pursuits since he could not hope to procure elsewhere the same facilities for inquiry that he enjoyed here. More than all it seemed to mean the relinquishment of what was still dearer to him—his active efforts in the propagation of Unitarianism. Lastly it meant in all human probability a lasting severance from the daughter to whom he was so tenderly attached. He was largely guided to his decision by consideration for his sons, since, as he says, he found that the bigotry of the country in general made it impossible for him to place them here with any advantage. His second son, William, had been some time in France, but on the breaking out of the troubles in that country he had embarked for America, where his two brothers, Joseph and Henry, met him. They had a project of founding a settlement near the head of the Susquehanna in Pennsylvania, and several of Priestley’s friends at home, among them Mr William Russell of Birmingham, a leader of the New Meeting-House, were directly interested in the scheme.

Priestley at length decided to throw in his lot with his sons, and in the preface to his Fast and Farewell Sermons, which he delivered to his Hackney congregation on the eve of his departure, he gave his reasons for leaving the country:—

“After the riots in Birmingham it was the expectation, and evidently the wish of many persons, that I should immediately fly to France or America. But I had no consciousness of guilt to induce me to fly from my country. On the contrary, I came directly to London, and instantly, by means of my friend, Mr Russell, signified to the King’s ministers that I was there and ready, if they thought proper, to be interrogated on the subject of the riots.

“Ill-treated as I thought I had been, not merely by the populace of Birmingham, for they were the mere tools of their superiors, but by the country in general, which evidently exulted in our sufferings, and afterwards by the representatives of the nation, who refused to inquire into the cause of them, I own I was not without deliberating upon the subject of emigration; and several flattering proposals were made to me, especially from France, which was then at peace within itself and with all the world; and I was at one time much inclined to go thither, on account of its nearness to England, the agreeableness of its climate, and my having many friends there.

“But I likewise considered that if I went thither I should have no employment of the kind to which I had been accustomed; and the season of active life not being, according to the course of nature, quite over, I wished to make as much use of it as I could. I therefore determined to continue in England, exposed as I was not only to unbounded obloquy and insult, but to every kind of outrage; and after my invitation to succeed my friend Dr Price I had no hesitation about it....”

He then goes on to show how insecure his position was, and how impossible it was to follow his avocations in peace, in face of the odium and insult he continually met with:—

“These facts not only show how general was the idea of my particular insecurity in this country, but what is of much more consequence, and highly interesting to the country at large, an idea of the general disposition to rioting and violence that prevails in it, and that the Dissenters are the objects of it. Mr Pitt very justly observed, in his speech on the subject of the riots at Birmingham, that it was ‘the effervescence of the public mind.’ Indeed, the effervescible matter has existed in this country ever since the civil wars in the time of Charles I., and it was particularly apparent in the reign of Queen Anne. But the power of Government under the former princes of the House of Hanover prevented its doing any mischief. The late events show that this power is no longer exerted as it used to be, but that on the contrary there prevails an idea, well or ill founded, that tumultuary proceedings against Dissenters will not receive any effectual discouragement.

“After what has taken place with respect to Birmingham, all idea of much hazard for insulting and abusing the Dissenters is entirely vanished; whereas the disposition to injure the Catholics was effectually checked by the proceedings of the year 1780. From that time they have been safe, and rejoice in it. But from the year 1791 the Dissenters have been more exposed to insult and outrage than ever.

“The necessity I was under of sending my sons out of this country was my principal inducement to send the little property that I had out of it too; so that I had nothing in England besides my library, apparatus and household goods.

“By this I felt myself greatly relieved, it being of little consequence where a man already turned sixty ends his days. Whatever good or evil I have been capable of is now chiefly done; and I trust that the same consciousness of integrity which has supported me hitherto will carry me through anything that may yet be reserved for me. Seeing, however, no great prospect of doing much good, or having much enjoyment here, I am now preparing to follow my sons; hoping to be of some use to them in their present unsettled state, and that Providence may yet, advancing in years as I am, find me some sphere of usefulness with them.”

He then goes on to deal with the charge that he was a factious, political parson who preached sedition:—

“As to the great odium that I have incurred, the charge of sedition, or my being an enemy to the constitution or peace of my country, is a mere pretence for it; though it has been so much urged that it is now generally believed, and all attempts to undeceive the public with respect to it avail nothing at all. The whole course of my studies from early life shows how little politics of any kind have been my object. Indeed, to have written so much as I have in theology, and to have done so much in experimental philosophy, and at the same time to have had my mind occupied, as it is supposed to have been, with factious politics, I must have had faculties more than human.”

It is true, he says, he wrote a pamphlet “On the State of Liberty in this Country” at the time of Wilkes’s election for Middlesex, and at the request of Franklin he wrote an address to the Dissenters on the subject of the approaching rupture with America; but he has nothing to reproach himself with on that score, and posterity agrees with him. His connection with the Marquis of Lansdowne was in no sense political. “Although,” he says, “I entered into almost all his views, as thinking them just and liberal, I never wrote a single political pamphlet, or even a paragraph in a newspaper, all the time that I was with him, which was seven years.”

He had never preached a political sermon in his life, unless such as he believed all Dissenters usually preached on the 5th of November in favour of civil and religious liberty may be said to be political. Even on those occasions he had never advanced any sentiment that would have made him until then obnoxious to the administration of this country. The doctrines he adopted when young, and which were even popular then (except with the clergy, who were at that time generally disaffected to the family on the throne), he could not now abandon merely because the times were so changed that they had become unpopular and the expression of them hazardous.

Although he did not disapprove of societies for political information, he never was a member of one, nor did he ever attend any public meeting if he could decently avoid it.

“If, then, my real crime has not been sedition, or treason, what has it been? For every effect must have some adequate cause, and therefore the odium that I have incurred must have been owing to something in my declared sentiments or conduct that has exposed me to it. In my opinion it cannot have been anything but my open hostility to the doctrines of the Established Church, and more especially to all civil establishments of religion whatever. This has brought upon me the implacable resentment of the great body of the clergy; and they have found other methods of opposing me besides argument and that use of the press which is equally open to us all. They have also found an able ally and champion in Mr Burke, who (without any provocation except that of answering his book on the French Revolution) has taken several opportunities of inveighing against me in a place where he knows I cannot reply to him, and from which he also knows that his accusation will reach every corner of the country and consequently thousands of persons who will never read any writings of mine. They have had another, and still more effectual vehicle of their abuse in what are called the treasury newpapers, and other popular publications.

. . . . . . .

“I could, if I were so disposed, give my readers many more instances of the bigotry of the clergy of the Church of England with respect to me which could not fail to excite in generous minds equal indignation and contempt: but I forbear. Had I, however, foreseen what I am now witness to, I certainly should not have made any attempt to replace my library or apparatus, and I soon repented of having done it. But this being done, I was willing to make some use of both before another interruption of my pursuits.... I hoped to have had no occasion for more than one, and that a final, remove. But the circumstances above mentioned have induced me, though with great and sincere regret, to undertake another, and to a greater distance than any that I have hitherto made.... And I trust that the same good Providence which has attended me hitherto, and made me happy in my present situation, and all my former ones, will attend and bless me in which may still be before me. In all events the will of God be done.

“I cannot refrain from repeating again that I leave my native country with real regret, never expecting to find anywhere else society so suited to my disposition and habits, such friends as I have here (whose attachment has been more than a balance to all the abuse I have met with from others), and especially to replace one particular Christian friend, in whose absence I shall, for some time at least, find all the world a blank. Still less can I expect to resume my favourite pursuits with anything like the advantages I enjoy here. In leaving this country I also abandon a source of maintenance which I can but ill bear to lose. I can, however, truly say that I leave it without any resentment or ill-will. On the contrary, I sincerely wish my countrymen all happiness; and when the time for reflection (which my absence may accelerate) shall come they will, I am confident, do me more justice. They will be convinced that every suspicion they have been led to entertain to my disadvantage has been ill founded, and that I have even some claim to their gratitude and esteem. In this case I shall look with satisfaction to the time when, if my life be prolonged, I may visit my friends in this country; and perhaps I may, notwithstanding my removal for the present, find a grave (as I believe is naturally the wish of every man) in the land that gave me birth.”

As the time of his departure drew near his friends vied with each other in their expressions of esteem and affection and many evidences of their regret were offered to him. Among these was a silver inkstand from some of his admirers in the University of Cambridge, on which was an inscription of their sorrow “that this expression of their esteem should be occasioned by the ingratitude of their country.”

On April 8, 1794, Priestley and his wife set sail from London, and arrived at New York on June 4.

On the way out he wrote some Observations on the Cause of the Present Prevalence of Infidelity, which he prefixed to a new edition of his Letters to the Philosophers and Politicians of France.

Alas! one of the most distinguished of those philosophers and politicians was even then no more. Coffinhal had pronounced his judgment, declaring “the Republic has no need of men of science,” and whilst Priestley was on the high seas his great protagonist, Lavoisier, more unfortunate even than he, met his death on the scaffold.

“Such was the treatment bestowed upon the best of their citizens by two nations which considered themselves as without exception the most civilised and enlightened in the world!”

Priestley was well received in New York, many people meeting him on landing, and he was presented with addresses of welcome from various societies. After a stay of about a fortnight he proceeded to Philadelphia and received an address from the American Philosophical Society, and by a unanimous vote of the trustees was offered the Professorship of Chemistry in the University of Philadelphia.

In the following July, in order to escape from the heat of the city, he moved to Northumberland, a town about a hundred and thirty miles north-west of Philadelphia and situated at the confluence of the north-east and west branches of the Susquehanna, near to which place his eldest son, together with certain other persons, mainly Englishmen, projected a settlement. Priestley himself had no pecuniary interest, as has been stated, in the undertaking, and he was not consulted in its formation, nor had he even decided to join it if carried into effect. We learn from his son’s account that the scheme of settlement was not to be confined to any particular class or character of men, religious or political. It was set on foot to be, as it were, a rallying point for the English, who were at that period emigrating to America in great numbers, and who, it was thought, would be more happy in society of the kind they had been accustomed to than they would be if dispersed through the whole of the States.

Owing to disagreements among the projectors, the scheme of the settlement fell through. Priestley, however, who was charmed with the beauty of its situation and the nature of its surroundings, determined to settle at Northumberland. Although at that time remote from any considerable town it was obviously destined to become a great thoroughfare. It was apparently healthy and less enervating, at least in summer time, than Philadelphia. Living was cheaper there than in that city, and he would be more free from care and more at liberty to follow his own pursuits than if burdened with the responsibilities of teaching. Lastly, his poor wife, who had never recovered from the shock of the Birmingham riots, needed rest and quiet. On these grounds, therefore, he decided to decline the offer of the Professorship at Philadelphia, as well as an invitation to take charge of an Unitarian congregation at New York, and to spend his remaining days in peace and retirement on the beautiful spot he had chosen. The year before his death he was offered the principalship of the University of Pennsylvania in succession to Dr Euen, but this office also he declined.

On his first settling at Northumberland in 1795 he was mainly occupied with his theological and metaphysical studies. During this year he published the work which had occupied him during his voyage from England, his Fast and Farewell Sermons, some tracts in defence of Unitarianism, and the third part of his Letters to a Philosophical Unbeliever, in answer to Paine’s Age of Reason, and he continued his Church History from the Fall of the Western Empire to the Reformation. In the house he had first occupied, which was barely sufficient in size to contain the family, he had little opportunity or convenience for doing experimental work.

Still, he made some observations on the analysis of air, and continued his inquiries on the generation of air from water.

Having determined to make Northumberland his home, he proceeded to build a house more suitable to his needs and pursuits, and, as his letters of the period show, its planning and arrangement gave him much thought and greatly interested him.

The house, which still exists, is similar in character to many middle-class American houses built in the country, a plain substantial erection, covered with match-boarding and fitted with jalousies, and to the front a loggia or verandah. The laboratory is a small building to the side, partially shaded by a large, wide-spreading tree.

In the autumn of this year he lost his youngest son, Henry, a bright and intelligent youth, of whom he was remarkably fond. This loss greatly affected him, for he had hopes that the young man would follow him in his theological and philosophical pursuits, to which he had shown an inclination. The death of his son was even more profoundly felt by his wife, whose health and spirits now began rapidly to decline, and she too passed away a few months later.

“Through life,” says her son, “she had been truly a helpmeet for him; supporting him under all his trials and sufferings with a constancy and perseverance truly praiseworthy, and who, as he himself, in noting the event in his diary, justly observes, ‘was of a noble and generous mind, and cared much for others and little for herself through life.’”

At about this period he preached and printed another of his defences of Unitarianism and completed his Church History, and began the compilation of his last treatise in defence of phlogiston.

He spent the spring of 1796 in Philadelphia, where he delivered a series of lectures on the evidences of revelation to crowded audiences, including most of the members of the United States Congress, at that time sitting in Philadelphia, and of the executive officers of the Government. He delivered a second series on the same subject in the spring of the following year, but with less success, partly owing, his son imagines, to the novelty of the thing having passed away, and partly from prejudices that began to be excited against him on account of his supposed political principles. In reality Priestley took even less interest in the politics of America than he had done in those of his own country. He seldom read the debates in Congress, and beyond Adams and Jefferson he knew few of the leading politicians. He never attended a political meeting or took part directly or indirectly in an election, and excepting an article in a newspaper called “Aurora,” or “Maxims of Political Arithmetic,” and signed “A Quaker in Politics,” he wrote nothing on the subject of politics. At that period political feeling ran high and politics were the one subject of conversation, and to some extent, therefore, he could not escape their discussion, but it was noticed that he always argued on the side of liberty. As regards British politics his speculations went no further than a reform in Parliament, such as that which was accomplished less than thirty years after his death. He had no desire to see changed the constitution of the kingdom as vested in King, Lords and Commons.

“He used frequently to say,” says his son, “and it was said of him, that though he was an Unitarian in religion he was in that country a Trinitarian in politics. When he came to America he found reason to change his opinions, and he became a decided friend to the general principles and practice of a completely representative Government, founded upon universal suffrage, and excluding hereditary privileges, as it exists in this country. This change was naturally produced by observing the ease and happiness with which the people lived, and the unexampled prosperity of the country.”

But in his feelings he was still an Englishman. He never was naturalised, saying that as he had been born and had lived an Englishman he would die one, let what might be the consequence.

Towards the end of 1797 his new library and laboratory were finished, his books once more arranged and much of his old apparatus installed. He found workmen in Northumberland who could repair his instruments and make such new ones as he wanted. He was thus able to resume the kind of life he led at Birmingham, spending much of the day in the laboratory or alternately in his study, sometimes engaged on experimental philosophy, at other times in the composition of the theological works which seemed to flow in an unending stream from his pen. He delighted to walk in his garden and to view the beautiful prospect it afforded him of the river and the distant landscape. He had, too, a kindly interest in the whole community, and noted with pleasure the many little improvements going forward in and about the town. There was no apparent abatement in the vigour of his mind or in the keenness and enthusiasm with which he followed the extraordinary expansion of the science he loved so well during the opening years of the nineteenth century. In a letter to Humphry Davy, then at the outset of his brilliant career, he says:—

“It gives me peculiar satisfaction that, as I am far advanced in life and cannot expect to do much more, I shall leave so able a fellow-labourer of my own country in the great fields of experimental philosophy.... I rejoice that you are so young a man; and perceiving the ardour with which you begin your career I have no doubts of your success.”

The following letter to his old friend Mrs Barbauld, with whom he kept up a correspondence to the last, gives some account of his condition at this time:—

“Dear Madam,—This will, I hope, be delivered, as it will be conveyed by my son. How happy should I think myself to wait on you and Mr Barbauld in person. Should there be a peace, I do promise myself that pleasure, but at present this great blessing seems to be at a great distance. How many melancholy changes have taken place since I left England, and among these is the death of Dr Enfield, a man at least ten years younger than me, and to appearance more healthy. I am also much alarmed at the accounts I receive of your brother [Dr John Aiken], whom I left in perfect health, but the last were rather more favourable. His life is of great value, both to his relatives, acquaintances and the world at large, few men having been more usefully employed. I am willing to hope he is yet reserved for more usefulness.

“When I compare the perturbed state of Europe with the quiet of this place I wish all my friends were here, provided they could find sufficient employment to be happy; but if they be like myself they must be content to be idle, except so far as they can make themselves employment in their closets. My library and laboratory sufficiently occupy me, and of common society I have as much as I want. A few more rational Christians to form a society would make this place a paradise to me, and this would be wanting in many parts of England.

“It is a pleasure to be in a place that is continually and visibly improving, and this is the case here to an astonishing degree. In every year we find a very sensible difference, and in all probability improvements of all kind will go on more rapidly than ever. Nature has done everything that can be done for any place. Perhaps you have seen the views of it taken by Miss Daich. They are not by any means too flattering.

“Could I have my daughter here I should be happy indeed. But this, I fear, is not likely to be accomplished, owing to the strange obstinacy and prejudice of Mr Finch. Her trials must be very great, but she is naturally cheerful, and has a strong sense of religion, which, I hope, will support her. This, sufficiently impressed, will make us equal to everything. Your kindness to her affects me much. A friend in need is a friend indeed. Something will, I hope, be done for her before my son returns, but what it can be I do not know. Her uncle has some proposal to make to my son in her favour, but the obstinacy of Mr Finch may defeat everything.

“You have obliged me very much by the exquisite little poem you sent me. I hope you will add to the obligation by the communication of the fragment on the ‘Game of Chess,’ or any other little piece you may think proper to send me. You had no copy of your first poem to my wife, or I should value that above any other, and also the little poem you wrote on the birth of Joseph.

“I shall always be very happy to hear from you; and, with my best respects to Mr Barbauld, I am, dear Madam, yours sincerely, J. Priestley.

“Northumberland, Dec. 23, 1798.

“Mrs Barbauld, Hampstead,

near London.”

His son has given us a faithful picture of his closing years and of the serenity of the evening of his life.

“For the last four years of his life he lived under an administration, the principles and practice of which he perfectly approved, and with Mr Jefferson, the head of that administration, he frequently corresponded, and they had for each other a mutual regard and esteem. He enjoyed the esteem of the wisest and best men in the country, particularly at Philadelphia, where his religion and his politics did not prevent his being kindly and cheerfully received by great numbers of opposite opinions in both, who thus paid homage to his knowledge and virtue.”

In 1800 he put together his last scientific work, and the one which he regarded as the crown of all his efforts, viz., his Doctrine of Phlogiston Established. It can never be said of Priestley that he was to one thing constant never: versatile as he was, and with an extraordinary capacity for adaptation and change in matters of philosophy and theological doctrine, he was ever constant to phlogiston.

During the spring of 1801, whilst on a visit to Philadelphia, he had an attack of fever from which he never wholly recovered. It left him predisposed to the fever and ague at that time prevalent at Northumberland and he had a succession of attacks which weakened him greatly. Nevertheless, his spirits were uniformly good and his complacency and cheerfulness of manner never left him; and although he was incapable of taking much physical exercise and had to give up working in his garden, he spent a considerable amount of time in his laboratory, experimenting with all the enthusiasm and eagerness of his most active period with the newly-discovered pile of Volta, and sending his results to Nicholson’s Journal.

In 1802 he was enabled to send his Church History to press, owing to the action of his friends in England, who, unknown to him, had set a subscription on foot sufficient to cover the expense of publication.

Although he was obviously failing in strength, owing to gastric troubles, he continued to work on either in his study or in his laboratory. He sent a couple of papers to the American Philosophical Society on scientific subjects, and he published an essay on Jesus and Socrates Compared. In the November of 1803 it was evident that his end was approaching. Still he struggled on, hoping by careful attention to his diet he might still see the spring. He told the physician who attended him that if he could but patch him up for six months longer he should be perfectly satisfied, as he should in that time be able to complete the printing of his works. So precarious did he consider his life that he took the precaution of transcribing one day in longhand what he had composed the day before in shorthand, that he might by that means leave the work complete as far as it went should he not live to finish the whole.

With the beginning of 1804 his weakness had greatly increased. In his diary for January 31 he notes:—“Ill all day—not able to speak for nearly three hours.” Still he rose, dressed and shaved himself (which he never omitted doing every morning till within two days of his death), went to his laboratory and lit his fire, but found his weakness so great that he was obliged to get back to his study. During the next and following days he was better, and was able to see to the correction of his proof-sheets, but on February 4 he took to his bed, although he was able to read and look over a sheet of proof and to check the Greek and Hebrew quotations.

“In the course of the day,” says his son, “he expressed his gratitude in being permitted to die quietly in his family, without pain, with every convenience and comfort he could wish for. He dwelt upon the peculiarly happy situation in which it had pleased the Divine Being to place him in life, and the great advantage he had enjoyed in the acquaintance and friendship of some of the best and wisest men in the age in which he lived, and the satisfaction he derived from having led a useful as well as a happy life.”

In the evening he had his grandchildren brought to his bedside, saying it gave him great pleasure to see the little things kneel. After prayers they wished him a good-night and he gave each his blessing, exhorting them all to continue to love each other.

“And you, little thing,” speaking to the youngest, “remember the hymn you learned: ‘Birds in their little nests agree.’ I am going to sleep as well as you; for death is only a good long sleep in the grave, and we shall meet again.”

He lingered through the night, and in the early morning requested his son to take down some additions and alterations he wished inserted in his proofs, dictating as clearly and distinctly as he had ever done in his life. When these were read to him he said, “That is right; I have now done.” Shortly afterwards he put his hand to his face and breathed his last so easy that those who were sitting close to him hardly perceived he had passed away.

What was mortal of him now rests in a little hill-side cemetery overlooking the beautiful river. The spot is marked with a simple headstone on which is engraven—

To
the memory of the
Revd. Dr JOSEPH PRIESTLEY,
who departed this life
on the 6th Feby. 1804.
Anno. Ætatis LXXI.

CHAPTER XI

Priestley as a man of science—His characteristics as a philosopher—Experiments and Observations on Different Kinds of Air—His discovery of the influence of vegetation on vitiated air—Atmospheric air not elementary—His researches on nitric oxide—Eudiometry—Nitrous oxide—Discovers hydrogen chloride—Prepares oxygen from nitre (1771)—Isolates ammonia gas—Discovers sulphur dioxide—Dephlogisticated air (oxygen)—Discovers silicon fluoride—Intra-diffusion of gases—Respiration—Priestley’s opinions of the value of experimental science in education—Discovers nitrosulphuric acid—Notes the constancy of composition of the atmosphere—Prepares chlorine—Sound in “air”—Experiments relating to phlogiston—The seeming conversion of water into air—Watt and the compound nature of water—Discovers sulphuretted hydrogen—Priestley’s confession of faith in phlogiston.

Priestley’s position in the history of science mainly rests on his discoveries in pneumatic chemistry. The course of inquiry which he began at Leeds was continued by him, with characteristic assiduity and conspicuous success, at Calne, and his labours added largely to the number of the aeriform bodies which were clearly recognised as distinct substances, essentially differing from each other, and not merely modifications of a common principle, modified or affected by properties more or less fortuitous and accidental. The old idea of the nature of “air” had its origin in the doctrine of the Four Elements. It is Priestley’s merit that he, more than any man of his time, contributed to the overthrow of this conception as the basis of a philosophical system of the constitution of the material universe. Although Priestley could not be unmindful that his claim to scientific fame was to be found in the succession of volumes which he called Experiments and Observations on Different Kinds of Air, the very title suggests that he, at all events in the outset, was hardly conscious of the magnitude and true significance of his work. Priestley was in no real sense a speculative philosopher: he was indeed pre-eminently the type of man whom Hobbes disparaged as an “experimentarian philosopher,” and an experimentarian philosopher he remained to the end of his days. He was aware of his limitations, and many passages from his works, and especially from his correspondence, might be quoted in proof of this fact. His simple, unaffected candour was indeed one of the charms of his character and the secret of much of his influence. It is reflected in every page of his scientific writings. His own discoveries, taken collectively, did more than those of any one of his contemporaries to uproot and destroy the only generalisation by which his immediate predecessors had sought to group and connect the phenomena of chemistry, but he was wholly unable to perceive this fact. A patient and industrious observer, absolutely truthful, and, as he hoped and believed, unbiassed and impartial, he was nevertheless entirely lacking in the higher qualities of the imagination or in that power of divination which is the characteristic of men of the type of Newton. The contrast between Priestley—the social, political and theological reformer, always in advance of his times, receptive, fearless and insistent; and Priestley the man of science—timorous and halting when he might well be bold, conservative and orthodox when almost every other active worker was heterodox and progressive—is most striking. And yet, such is the irony of circumstance, Priestley’s name mainly lives as that of a chemical philosopher. When men have desired to do him honour, and have sought to perpetuate his memory by statues in public places, he is generally represented as making a chemical experiment. In reality, great as Priestley’s merit is as an experimentarian philosopher, his greater claim on our regard and esteem rests upon his struggles and his sufferings in the cause of civil, political and religious liberty.

The years which Priestley spent at Calne constitute the most fruitful period of his scientific career. Practically all that he did in the way of solid achievement and of addition to the armoury of science was effected during that time. Although, after leaving Lord Shelburne, he continued to pursue scientific inquiry with his wonted zeal and industry, doubtless adding thereby to his fame among his contemporaries, posterity has set the true measure of appreciation to his later efforts. He doubtless made many hundreds of experiments in connection with more or less well-defined trains of inquiry; nevertheless, it cannot be maintained that during his subsequent period he added many first-rate facts to our knowledge, or indeed discovered any facts at all comparable in importance with those he ascertained during his life in Wiltshire. On the contrary, what he did observe—as for example the seeming conversion of water into air—too frequently led him astray and was the cause of error to himself and others. Thus Watt’s claim to be considered as an independent, if not the first and true, discoverer of the real chemical nature of water is based upon Priestley’s experimental blunders. Watt was undoubtedly accurate in his surmise, but the surmise was right in spite of, and not by reason of, Priestley’s experimental evidence. Priestley recorded his experiments with such fulness that it is now easy to perceive where he went wrong. He was constantly on the verge of a discovery, sometimes indeed of a discovery of cardinal importance, but as constantly it eluded his grasp. The experiments on the seeming conversion of water into air might have led him, when he got over his chagrin on the detection of the real cause of his error, to the recognition of the underlying truth in it, namely, the principle of the diffusion of gases. He was, of course, familiar with the fact that the various gases he discovered, or which were known to him, differed in relative density, and he knew perfectly well that they tended to escape from the bottles in which they were contained if these were uncovered and freely exposed to the air. But, so far as we can learn, he never seems to have pondered on these facts, or noted their connection with the phenomena he observed in the course of his many experiments with Wedgwood’s retorts, and of the interchange of the water vapour he introduced into them with the gases of the fire which heated them. And yet, had he perceived even a glimmer of the truth he had sufficient means at his disposal, and sufficient knowledge from his own work and that of his contemporaries, to make the great step which it was reserved to Graham to accomplish half a century later.

Whilst the chief importance of the Experiments and Observations on Different Kinds of Air is that it is Priestley’s magnum opus, to his biographer it has the additional interest of affording an insight into the personal character and intellectual attributes of its author. Few writers on scientific subjects have ever taken their readers so completely into their confidence as Priestley. Whatever he knows or thinks he tells: doubts, perplexities, blunders are set down with the most refreshing candour; one forgives the prolixity and occasional tediousness, even the little touches of self-satisfaction, in view of the transparent honesty of purpose, the single-minded pursuit of truth for its own sake, wholly apart from preconception or bias of dogma which shine on every page. As key-notes to character, even the dedications and prefaces to the several volumes have their peculiar value and charm, as evidence of the workings of an ingenuous mind.

The publication of the six volumes comprising the original work—the edition of greatest value to Priestley’s biographer—extended from 1775 to 1786. Although the space at our disposal precludes any attempt at a full account of the contents, it is necessary to set these out in such detail as may serve to afford a just idea of their value, and with such comment as may be necessary to elucidate their significance.

In the preface to the first volume, which made its appearance in 1775, with a dedication to Lord Shelburne, Priestley thinks it necessary to explain why he has decided, contrary to his original intention, but with the approbation of the President and of his friends in the Royal Society, not to send them any more papers on the subject of “Air” at present but to make immediate publication of all he has done with respect to it. In view, he says, of the rapid progress that has been made and may be expected to be made in this branch of knowledge, “unnecessary delays in the publication of experiments relating to it are peculiarly unjustifiable.”

“When, for the sake of a little more reputation, men can keep brooding over a new fact, in the discovery of which they might possibly have very little real merit, till they think they can astonish the world with a system as complete as it is new, and give mankind a prodigious idea of their judgment and penetration, they are justly punished for their ingratitude to the fountain of all knowledge, and for the want of a genuine love of science and of mankind in finding their boasted discoveries anticipated and the field of honest fame pre-occupied by men who, from a natural ardour of mind, engage in philosophical pursuits, and with an ingenuous simplicity immediately communicate to others whatever occurs to them in their inquiries.”

Priestley’s productions, from the very nature of the case make no pretensions to completeness.

“In completing one discovery we never fail to get an imperfect knowledge of others of which we could have no idea before, so that we cannot solve one doubt without creating several new ones.”

He farther observes that a person who means to serve the cause of science effectually must hazard his own reputation so far as to risk even mistakes in things of less moment.

“Among a multiplicity of new objects and new relations some will necessarily pass without sufficient attention; but if a man be not mistaken in the principal objects of his pursuits he has no occasion to distress himself about lesser things.

“In the progress of his inquiries he will generally be able to rectify his own mistakes; or if little and envious souls should take a malignant pleasure in detecting them for him and endeavouring to expose him, he is not worthy of the name of a philosopher if he has not strength of mind sufficient to enable him not to be disturbed at it. He who does not foolishly affect to be above the failings of humanity will not be mortified when it is proved that he is but a man.”

He made it a rule to disclose the real views with which he made his experiments. Although, he says, by following a contrary maxim he might have acquired a character of greater sagacity, he thought that two good ends were secured by his method—one as tending to make his narrative more interesting, and the other as encouraging other adventurers in experimental philosophy by showing them that by pursuing even false lights real and important truths may be discovered, and that in seeking one thing we often find another. He believes, however, that he writes more concisely than is usual with those who publish accounts of their experiments, and in thus refraining from swelling his book “to a pompous and respectable size” he trusts he will earn the gratitude of those philosophers who, having but little time to spare for reading, which is always the case with those who do much themselves, will thereby be kept not too long from their own pursuits. He then comments on what he justly considers the amazing improvements in natural knowledge which have been made within the century, and contrasts these with the comparative poverty as regards scientific results of the many preceding ages, which yet abounded with men who had no other object but study; and he rejoices to think that this rapid progress of knowledge, extending itself not this way or that way only, but in all directions, will be the means of extirpating all error and prejudice and of putting an end to all undue and usurped authority in the business of religion as well as of science.

“It was ill policy in Leo the Tenth to patronise polite literature. He was cherishing an enemy in disguise. And the English hierarchy (if there be anything unsound in its constitution) has equal reason to tremble even at an air-pump or an electrical machine.”

He regrets that the rich and great in this country, unmindful of the example of Bacon, give less attention to these matters than do men of rank and fortune in other countries: he contrasts the pleasure of the pursuit of science with the pains and penalties of the pursuit of politics.

“If extensive and lasting fame be at all an object, literary, and especially scientifical, pursuits are preferable to political ones in a variety of respects.... If extensive usefulness be the object, science has the same advantage over politics. The greatest success in the latter seldom extends farther than one particular country and one particular age, whereas a successful pursuit of science makes a man the benefactor of all mankind and of every age. How trifling is the fame of any statesman that this country has ever produced to that of Lord Bacon, of Newton, or of Boyle; and how much greater are our obligations to such men as these than to any other in the whole Biographia Britannica.”

It would be interesting to know the sentiments of Lord Shelburne, then in the cold shade of retirement, as he perused these passages, and whether he realised the truth of the little homily from his “tame philosopher.”

The preface is followed by an introduction, in which Priestley gives a rapid and confessedly imperfect survey of the state of knowledge concerning “air” prior to 1774. He gives to Boyle the credit of first clearly recognising that elastic fluids exist differing essentially from the air of the atmosphere, but agreeing with it in the properties of weight, elasticity and transparency. But he also points out that two remarkable kinds of factitious air had long been known to miners, viz., choke damp, which is heavier than air, which lies at the bottom of pits, extinguishes flame and kills animals; and the other, called fire damp, which is lighter than common air, is found, therefore, near the roofs of subterraneous places and is liable to take fire and explode like gunpowder. “The word damp signifies vapour or exhalation in the German and Saxon languages.”

“Air of the former kind, besides having been discovered in various caverns, particularly the Grotta del Cane in Italy, had also been observed on the surface of fermenting liquors, and had been called gas (which is the same with geist, or spirit) by Van Helmont and other German chemists; but afterwards it obtained the name of fixed air, especially after it had been discovered by Dr Black of Edinburgh to exist, in a fixed state, in alkaline salts, chalk, and other calcareous substances.”

Black’s work is dealt with in half a dozen lines, and a passing reference is made to Macbride and Brownrigg. A very imperfect account is given of the work of Hales, although it is stated that “his experiments are so numerous and various that they are justly esteemed to be the solid foundation of all our knowledge of this subject.” This section concludes with the mention of Cavendish’s determinations of the relative weights of fixed air (carbon dioxide), and inflammable air from metals (hydrogen), and of Lane’s observations that water charged with carbonic acid will dissolve iron, “and thereby become a strong chalybeate.”

Priestley was the last man in the world to seek to disparage the work of his predecessors or to minimise what was due to them. In reality he had the intention, as he distinctly states, to write at his leisure the history and present state of discoveries relating to air, in a manner similar to his History of Electricity, and of the Discoveries Relating to Vision, Light and Colours, when no doubt he would have done full justice to all concerned. In the meantime he gives only such particulars as are necessary, in his judgment, to the understanding of his own work.

The remaining section of the introduction deals with his method of experimenting and with the apparatus he employed. It is of historical interest as containing a description of that most useful article of chemical furniture, his well-known pneumatic trough. He explains its use and gives details of his modes of manipulation. What an advance these were in simplicity, ingenuity and convenience can only be fully realised by comparing his methods with those of Hales. Not the least of Priestley’s services to science were the improvements he effected in that section of operative chemistry which is concerned with the preparation, collection and storage of gaseous substances.

The main body of the volume is divided into two parts—the first dealing with observations made in and before 1772, the second with observations made in the year 1773 and in the beginning of 1774. In the outset Priestley finds himself at a disadvantage in regard to the only terms at that time in vogue for the factitious airs, viz., fixed, mephitic and inflammable, which, he rightly says, are not sufficiently characteristic and distinct. Strictly speaking, any two of these terms might be applied to any one of the “airs” then known. The inflammable air from metals, as well as choke damp, is noxious, and therefore mephitic, as is fixed air, and since the inflammable airs are, apparently, capable of being imbibed by certain substances they may equally be considered fixable. The term fixed air had, however, acquired a distinctive meaning, and rather than introduce a new term or change the signification of an old one, he would, with his contemporaries, restrict the term to the air which had been made the subject of Black’s memorable investigation. The first paper in this section deals with fixed air; it is practically a reprint of that in the Phil. Trans. and which has already been described in sufficient detail. In the course of his experiments he says he once thought that the readiest method of procuring fixed air, and in sufficient purity, would be to heat pounded lime-stone in a gun barrel, “making it pass through the stem of a tobacco pipe or a glass tube carefully luted to the orifice of it.”

“In this manner I found that air is produced in great plenty; but, upon examining it, I found to my great surprise that little more than one half of it was fixed air, capable of being absorbed by water; and that the rest was inflammable, sometimes very weakly, but sometimes pretty highly so.”

He surmised that this “air” must come from the iron, and yet, he noted, it differed from the ordinary inflammable air from iron by the remarkable blue colour of its flame, and he concludes that “this inflammable principle may come from some remains of the animals from which it is thought that all calcareous matter proceeds.” Priestley, we now know, had incidentally converted some of the fixed air into the only other oxide of carbon, but he failed to appreciate the significance of his observation, and the credit of the discovery of carbon monoxide belongs to Cruikshank.

In his next paper on “Air in which Candles have burned,” Priestley made a discovery of the very highest importance. He had attempted to verify without success the allegation by the Count de Saluce, made in the memoirs of the Philosophical Society of Turin, that air vitiated by the combustion of candles could be restored by exposure to cold.

“Though this experiment failed,” he says, “I have been so happy as by accident to have hit upon a method of restoring air which has been injured by the burning of candles, and to have discovered at least one of the restoratives which Nature employs for this purpose. It is vegetation. This restoration of vitiated air, I conjecture, is effected by plants imbibing the phlogistic matter with which it is overloaded by the burning of inflammable bodies. But whether there be any foundation for this conjecture or not, the fact is, I think, indisputable.”

He then proceeds to give an account of his observations on the growing of plants in confined air which led to his discovery.

“One might have imagined,” he says, “that since common air is necessary to vegetable as well as to animal life, both plants and animals had affected it in the same manner; and I own I had that expectation when I first put a sprig of mint into a glass jar standing inverted in a vessel of water: but when it had continued there for some months I found the air would neither extinguish the candle, nor was it at all inconvenient to a mouse, which I put into it.... Finding that candles would burn very well in air in which plants had grown a long time, and having had some reason to think that there was something attending vegetation which restored air that had been injured by respiration, I thought it was possible that the same process might also restore the air that had been injured by the burning of candles.

“Accordingly, on the 17th of August 1771, I put a sprig of mint into a quantity of air in which a wax candle had burned out, and found that on the 27th of the same month another candle burned perfectly well in it. This experiment I repeated, without the least variation in the event, not less than eight or ten times in the remainder of the summer.

“Several times I divided the quantity of air in which the candle had burned out into two parts, and putting the plant into one of them left the other in the same exposure, contained also in a glass vessel immersed in water, but without any plant, and never failed to find that a candle would burn in the former but not in the latter.... This remarkable effect does not depend upon anything peculiar to mint, which was the plant that I always made use of till July 1772; for on the 16th of that month I found a quantity of this kind of air to be perfectly restored by sprigs of balm, which had grown in it from the 7th of the same month.

“That this restoration of air was not owing to any aromatic effluvia of these two plants not only appeared by the essential oil of mint having no sensible effect of this kind, but from the equally complete restoration of this vitiated air by the plant called groundsel, which is usually ranked among the weeds and has an offensive smell. Besides, the plant which I have found to be the most effectual of any that I have tried for this purpose is spinach, which is of quick growth, but will seldom thrive long in water.”

The next paper on “Inflammable Air” is of slight importance, and indeed is full of errors. Priestley made no distinction between the inflammable air obtained by the action of acids on metals (hydrogen) and that formed by the destructive distillation of coal and other organic substances (marsh gas or carbonic oxide, or mixtures of the two), and his inability to distinguish these different gases accounts for many of the phenomena he observed and which he confesses himself unable to explain. The most sagacious observation in the memoir has reference to the colour of the electric spark in the different gases which he accurately describes.

The paper on “Air Infected with Animal Respiration or Putrefection” may be considered as the complement of that on “Air in which a Candle has burned out,” and is no less valuable.

“That candles will burn only a certain time in a given quantity of air is a fact not better known than it is that animals can live only a certain time in it; but the cause of the death of the animal is not better known than that of the extinction of flame in the same circumstances; and when once any quantity of air has been rendered noxious by animals breathing in it as long as they could, I do not know that any methods have been discovered of rendering it fit for breathing again. It is evident, however, that there must be some provision in Nature for this purpose, as well as for that of rendering the air fit for sustaining flame; for without it the whole mass of the atmosphere would, in time, become unfit for the purpose of animal life; and yet there is no reason to think that it is, at present, at all less fit for respiration than it has ever been. I flatter myself, however, that I have hit upon two of the methods employed by Nature for this great purpose. How many others there may be I cannot tell.”

One of these methods he eventually finds to be, as in the first case, the action of vegetation, and he proves by a number of decisive experiments

“that plants, instead of affecting the air in the same manner with animal respiration, reverse the effects of breathing and tend to keep the atmosphere sweet and wholesome when it is become noxious in consequence of animals either living and breathing, or dying and putrefying in it.”

The other method he conceived to be the action of water, since he found that by vigorous agitation with water, air which breathing had rendered noxious could again be breathed for a further period.

“I do not think it improbable but that the agitation of the sea and large lakes may be of some use for the purification of the atmosphere, and the putrid matter contained in water may be imbibed by aquatic plants, or be deposited in some other manner.”

When a confined volume of common air is placed in contact with a mixture of iron filings and sulphur made into a paste with water, a certain portion of the air is imbibed by the paste. This fact was first observed by Hales. Priestley repeated the observation and found that about a fifth or rather more of the volume of the air was thus absorbed. He noted that the residual “air” was rather lighter than common air, it had no action on lime-water and was exceedingly noxious to animals, by which is meant that it could not be breathed by them. Priestley had thus prepared nitrogen, but he failed to recognise the individuality of this gas.

In his Statical Essays Hales makes mention of an experiment in which common air and air generated from pyrites by spirit of nitre made a turbid red mixture, and in which part of the common air was absorbed. This phenomenon “particularly struck” Priestley, who, acting upon Cavendish’s hint that the red appearance was probably dependent “upon the spirit of nitre only” and that the metals might answer as well as pyrites, proceeded to investigate the action of nitric acid upon a number of the metals, and as the result of his inquiries he succeeded in isolating the gas we now know as nitric oxide, but which he termed nitrous air.

“Though,” he says, “I cannot say that I altogether like the term, neither myself nor any of my friends, to whom I have applied for the purpose, have been able to hit upon a better.”

This paper exhibits Priestley at his best. In it he describes all the main properties of nitric oxide.

“One of the most conspicuous properties of this kind of air,” he says, “is the great diminution of any quantity of common air with which it is mixed, attended with a turbid red, or deep orange colour, and a considerable heat.... The diminution of a mixture of this and common air is not an equal diminution of both the kinds, which is all that Dr Hales could observe, but of about one fifth of the common air, and as much of the nitrous air as is necessary to produce that effect; which, as I have found by many trials, is about one half as much as the original quantity of common air.

“I hardly know any experiment that is more adapted to amaze and surprise than this is, which exhibits a quantity of air which, as it were, devours a quantity of another kind of air half as large as itself, and yet is so far from gaining any addition to its bulk that it is considerably diminished by it....

“It is exceedingly remarkable that this effervescence and diminution, occasioned by the mixture of nitrous air, is peculiar to common air, or air fit for respiration, and, as far as I can judge from a great number of observations, is at least very nearly, if not exactly, in proportion to its fitness for this purpose; so that by this means the goodness of air may be distinguished much more accurately than it can be done by putting mice or any other animals to breathe in it.

“This was a most agreeable discovery to me, as I hope it may be a useful one to the public; especially as from this time I had no occasion for so large a stock of mice as I had been used to keep for the purpose of these experiments.”

Priestley here suggests the basis of a method of Eudiometry, or method of measuring the goodness of air, which in his hands, but more especially in those of Cavendish, led to most important results. The quantitative analysis of the air may be said to have taken its rise from the publication of Priestley’s paper.

In the course of subsequent work on nitrous air Priestley had occasion to study its action on iron, whereby he says:—

“A most remarkable and most unexpected change was made in the nitrous air,” the iron “makes it not only to admit a candle to burn in it, but enables it to burn with an enlarged flame.... Sometimes I have perceived the flame of the candle, in these circumstances, to be twice as large as it is naturally, and sometimes not less than five or six times larger; and yet without anything like an explosion, as in the firing of the weakest inflammable air.”

Priestley in this manner obtained nitrous oxide, the properties of which he subsequently studied in some detail.

In the paper which follows, viz., “On Air infected with the Fumes of Burning Charcoal,” he incidentally gains further insight into the nature of atmospheric air. By what he called throwing the focus of a burning mirror on charcoal suspended in air contained in a glass tube standing over water or mercury—a favourite method of his when he had occasion to heat a substance in a gas—he could observe the phenomena with great precision. He noticed the formation of the fixed air and determined the degree of diminution when the burning took place over water or over lime-water.

“In this manner,” he says, “I diminished a given quantity of air one-fifth. Air thus diminished by the fumes of burning charcoal not only extinguishes flame, but is in the highest degree noxious to animals; it makes no effervescence with nitrous air, and is incapable of being diminished any farther by the fumes of more charcoal.... All my observations show that air which has once been fully diminished ... is not only incapable of any further diminution ... but that it has likewise acquired new properties, most remarkably different from those which it had before....”

By heating pieces of lead and tin in air by means of a burning glass he observed the formation of a metallic calx, the volume of air was diminished, and it also “was in the highest degree noxious and made no effervescence with nitrous air.”

The real significance of these phenomena was, however, wholly unperceived by Priestley, and phlogiston, as usual, led him astray. He had, of course, in all these experiments prepared nitrogen, and in a state of sensible purity. He imagined, however, that he had simply “phlogisticated” the air, the phlogiston coming from the charcoal and the metals, and that this phlogisticated air was imbibed by the water.

An experiment described by Cavendish led Priestley to study the action of “Spirit of Salt” (hydrochloric acid) upon copper. As Cavendish had already stated, the gas so evolved “lost its electricity by coming into contact with water.” By collecting the gas over mercury Priestley was able to study its properties more exactly. From certain anomalies in the experiments he says:—

“I concluded that this subtle air did not arise from the copper, but from the spirit of salt; and presently making the experiment with the acid only, without any copper, or metal of any kind, this air was immediately produced in as great plenty as before; so that this remarkable kind of air is, in fact, nothing more than the vapour, or fumes of spirit of salt, which appear to be of such a nature that they are not liable to be condensed by cold, like the vapour of water and other fluids, and therefore may be very properly called an acid air, or more restrictively the marine acid air.”

The new gas discovered by Priestley we now call hydrogen chloride. Ordinary hydrochloric acid is simply an aqueous solution of it.

“Water impregnated with it makes the strongest spirit of salt that I have seen, dissolving iron with the most rapidity.... Iron filings, being admitted to this air, were dissolved by it pretty fast, half of the air disappearing and the other half becoming inflammable air, not absorbed by water. Putting chalk to it, fixed air was produced.”

He subsequently found that the marine acid air was more conveniently made by the action of oil of vitriol upon common salt.

From the “miscellaneous observations” with which this section of the volume concludes, there can be little doubt that Priestley, without knowing it, had prepared oxygen gas from nitre as far back as 1771. The accounts he gives of the behaviour of the gas obtained by heating nitre in a gun-barrel plainly indicate this fact.

“A candle,” he says, “not only burned but the flame was increased, and something was heard like a hissing similar to the decrepitation of nitre in an open fire.” He also noted the effect of nitrous air upon it and concludes that “this series of facts relating to air extracted from nitre appear to me to be very extraordinary and important, and in able hands may lead to considerable discoveries.”