Joseph Priestley

Priestley’s methods of experiment with his various “airs” were very uniform. He tried their solubility in water, their power of supporting or extinguishing flame, whether they were respirable, how they behaved with acid and alkaline air, and with nitric oxide and inflammable air, and lastly how they were affected by the electric spark. He occasionally made attempts to weigh them, but his determinations of their relative density were altogether untrustworthy. Indeed, it is evident from the terms in which he speaks of these efforts that he was conscious of their inadequacy. The result of submitting alkaline air (ammonia) to the electric spark, whereby it is resolved into nitrogen and hydrogen, surprised him not a little.

“There are few experiments the rationale of which I less pretend to understand than the production of genuine and permanent inflammable air from alkaline air by means of the electric spark.... One query on this subject is, whence comes the phlogiston, which is certainly a principal ingredient in the constitution of inflammable air. Alkaline air, indeed, contains phlogiston, because in the manner in which I have generally produced it, it is itself partially inflammable; but it is not nearly so much so as the inflammable air which is produced by means of it. Besides, it will appear by the following experiments that the quantity of the inflammable air far exceeds that of the alkaline.”

Although Priestley clearly recognised the production of the inflammable air, “in no respect to be distinguished from that which is extracted from metals by acids,” and inferred it must come from the alkaline air (“the production having its limits”), he failed to detect the other constituent of ammonia. His determination of the actual increase in volume was inaccurate, and his attempt to explain the phenomenon wholly fallacious.

At the instigation of Mr Woulfe, whose name mainly lives in connection with a useful piece of chemical apparatus, Priestley was encouraged to hope that he would

“find something remarkable in the solution of manganese in spirit of salt. Mr Woulfe, however, in a very friendly manner, at the same time, cautioned me with respect of the vapour that would issue from it, as from his own experience he apprehended it was of a very dangerous nature.... I cannot say that it was the apprehension of danger, but rather having other things in view, that prevented my giving much attention to the subject.”

Priestley’s experiments led to no decisive result: he of course recognised the

“peculiar smell, exactly resembling that which is procured by dissolving red lead in the same acids.... On the application of heat it was easy to perceive that air, or vapour, was expelled; but it was instantly seized by the quicksilver.... This is a new field that is yet before me.”

Priestley never occupied that field. It is tolerably certain that both Woulfe and he had unknowingly prepared chlorine gas, but the glory of its discovery belongs to Scheele.

The paper “Of Sound in Different Kinds of Air” is worth quoting as showing Priestley at his best:—

“Almost all the experiments that have hitherto been made relating to sound have been made in common air, of which it is known to be a vibration, though it is likewise known to be capable of being transmitted by other substances. There could be little doubt, however, of the possibility of sound originating in any other kind of air, as well as being transmitted by them; but the trial had not been actually made, and I had an easy opportunity of making it.

“Besides, the experiments promised to ascertain whether the intensity of sound was affected by any other property of the air in which it was made than the mere density of it. For the different kinds of air in which I was able to make the same sound, besides differing in specific gravity, have likewise other remarkable chemical differences, the influence of which with respect to sound would, at the same time, be submitted to examination.

“Being provided with a piece of clock-work, in which was a bell, and a hammer to strike upon it (which I could cover with a receiver, and which, when it was properly covered up, I could set in motion by the pressure of a brass rod going through a collar of leather), I placed it on some soft paper on a transfer. Then taking a receiver, the top of which was closed with a plate of brass, through which the brass rod and collar of leathers was inserted, I placed the whole on the plate of an air-pump, and exhausted the receiver of all the air that it contained. Then removing this exhausted receiver, containing the piece of clock-work, I filled it with some of those kinds of air that are capable of being confined by water.... Then by forcing down the brass rod through the collar of leathers I made the hammer strike the bell, which it would do more than a dozen times after each pressure. And the instrument was contrived to do the same thing many times successively after being once wound up.

“Everything being thus prepared, I had nothing to do, after filling the same receiver with each of the kinds of air in its turn, but receding from the apparatus, while an assistant produced the sound, to observe at what distance I could distinctly hear it. The result of all my observations, as far as I could judge, was that the intensity of sound depends solely upon the density of the air in which it is made, and not at all upon any chemical principle in its constitution.

“In inflammable air the sound of the bell was hardly to be distinguished from the same in a pretty good vacuum; and this air is ten times rarer than common air.

“In fixed air the sound was much louder than in common air, so as to be heard about half as far again; and this air is in about the same proportion denser than common air.

“In dephlogisticated air the sound was also sensibly louder than in common air, and, as I thought, rather more than in the proportion of its superior density; but of this I cannot pretend to be quite sure.

“In all these experiments the common standard was the sound of the same bell in the same receiver, every other circumstance also being the same; the air only being changed by removing the receiver from the transfer and blowing through it, etc.”

The sixth and last volume appeared in 1786 with a dedication to William Constable, Esq., of Barton Constable.

In the preface Priestley is concerned to defend himself against the charge that he occupies himself too much with Theology to the detriment of Natural Philosophy. Theology, he pleads, is his original and proper province, and for which, therefore, he may be allowed to have a justifiable predilection. But as with Metaphysics, so with Theology. Neither subject engrossed so much of his time as some persons imagined.

“I am particularly complained of at present as having thrown away so much time on the composition of my History of the Corruptions of Christianity, and of the Opinions Concerning Christ. But I can assure them, and the nature of the thing, if they consider it, may satisfy them, that the time I must necessarily have bestowed upon the experiments, of which an account is contained in this single volume, is much more than I have given to the six, of which the above-mentioned works consist, and to all the controversial pieces that I have written in defence of the former of them. The labour and attention necessary to enable me to write single paragraphs in this work have been more than was requisite to compose whole sections or chapters of the former.... Besides, these different studies so relieve one another that I believe I do more in each of them, by applying to them alternately, than I should do if I gave my whole attention to one of them only.”

But Priestley’s main defence rests “on the superior dignity and importance of theological studies to any other whatever.” The whole preface must be read in the light of Priestley’s altered circumstances and of his relations to the theological world, which, since his removal to Birmingham, had greatly increased in weight and importance. As already stated, he regarded himself as ordained to champion the cause of religion among the persons to whom his writings as a natural philosopher specially appealed. The author of the Institutes of Natural and Revealed Religion was the writer of the Letters to a Philosophical Unbeliever and, in an age of unbelief, the doughty antagonist of Gibbon. Otherwise the incongruous mixture of Theology and Natural Philosophy, of which the preface is made up, seems inexplicable.

To the historian of chemistry the last volume of the series is hardly less interesting than any one of its predecessors, not so much as affording knowledge of new “airs” as by reason of Priestley’s relation to the waning doctrine of phlogiston, and on account of the part that his own work was playing, in spite of himself, in completing its overthrow. The volume indeed significantly opens with “Experiments relating to Phlogiston,” a reprint with notes of his paper in the 73rd volume of the Philosophical Transactions. Priestley truly says:—

“There are few subjects, perhaps none, that have occasioned more perplexity to chemists than that of phlogiston, or, as it is sometimes called, the principle of inflammability. It was the great discovery of Stahl that this principle, whatever it be, is transferable from one substance to another, how different soever in their other properties, such as sulphur, wood, and all the metals, and therefore is the same thing in them all. But what has given an air of mystery to this subject has been that it was imagined that this principle, or substance, could not be exhibited except in combination with other substances, and could not be made to assume separately either a fluid or solid form. It was also asserted by some that phlogiston was so far from adding to the weight of bodies that the addition of it made them really lighter than they were before; on which account they chose to call it the principle of levity. This opinion had great patrons.

“Of late it has been the opinion of many celebrated chemists, Mr Lavoisier among others, that the whole doctrine of phlogiston has been founded on mistake, and that in all cases in which it was thought that bodies parted with the principle of phlogiston, they in fact lost nothing, but on the contrary acquired something; and in most cases an addition of some kind of air; that a metal, for instance, was not a combination of two things, viz., an earth and phlogiston, but was probably a simple substance in its metallic state; and that the calx is produced not by the loss of phlogiston, or of anything else, but by the acquisition of air.”

He then goes on to say that the arguments in favour of this opinion, especially those which were drawn from the experiments of Lavoisier on mercury, were “so specious” that he owns he was much inclined to adopt it. But he was evidently loth to part company with a conception which had hitherto been the central idea of his chemical creed, the very key-stone of the structure which he was pleased to regard as his philosophy. As an abstract conception, as the principle of levity, as something which was the negation of mass and which gravity repelled, phlogiston was eminently unsatisfactory. But what if phlogiston were an entity? A ponderable substance, no matter how light? In that case Stahl’s generalisation might still afford salvation. “My friend, Mr Kirwan”—a clever, ingenious Irishman, with a nimble wit and a facile pen—supplied the hint—“Phlogiston was inflammable air”—and Priestley by a series of experiments, faultless as to execution but utterly fallacious as to interpretation, persuades himself that Kirwan is right and that Mr Lavoisier’s opinion and his “specious arguments” are therefore to be discountenanced. The paper, in certain respects, is one of the most noteworthy of Priestley’s productions. The experiments are original, ingenious and striking, but as an example of his inductive capacity, or as an indication of its author’s logical power, or of his ability to try judicially the very issue he has raised, it is significant only of the profound truth of his own words that

“we may take a maxim so strongly for granted that the plainest evidence of sense will not entirely change, and often hardly modify, our persuasions; and the more ingenious a man is, the more effectually he is entangled in his errors, his ingenuity only helping him to deceive himself by evading the force of truth.”

The next paper in the volume, on “The Seeming Conversion of Water into Air,” is a record of experiments which cost Priestley much labour and the Lunar Society, for a time, much mystification. Priestley eventually detected the fallacy in the observation which originally induced him to believe that it was possible to transmute water into a permanently elastic fluid, but he got no further in his explanation than that air has a faculty of passing through the pores of an earthern vessel “by means of a power very different from that of pressure.”

This and the third paper in the series are classical, and this partly by reason of, and partly in spite of, their blunders, for they are the record of the work upon which James Watt largely based his conjectures concerning the real chemical nature of water, whereby his name has been associated with that of Cavendish and Lavoisier as the true discoverer of its composition. In the course of his inquiry Priestley studied the action of steam upon red-hot iron by an arrangement generally similar to that employed by Lavoisier, but his explanation of the phenomena is essentially different from that of the French chemist, as may be seen from the following quotation:—

“Since iron gains the same addition of weight by melting it in dephlogisticated air, and also by the addition of water when red-hot, and becomes, as I have already observed, in all respects the same substance, it is evident that this air or water, as existing in the iron, is the very same thing; and this can hardly be explained but upon the supposition that water consists of two kinds of air, viz., inflammable and dephlogisticated.”

This, however, is how Priestley actually does explain it:—

“When iron is melted in dephlogisticated air we may suppose that, though part of its phlogiston escapes to enter into the composition of the small quantity of fixed air which is then procured, yet enough remains to form water with the addition of the dephlogisticated air which it has imbibed, so that this calx of iron consists of the intimate union of the pure earth of iron and of water; and therefore when the same calx, thus saturated with water, is exposed to heat in inflammable air, this air enters into it, destroys the attraction between the water and the earth, and revives the iron while the water is expelled in its proper form.

“Consequently, in the process with steam, nothing is necessary to be supposed but the entrance of the water and the expulsion of the phlogiston belonging to the iron, no more phlogiston remaining in it than what the water brought along with it, and which is retained as a constituent part of the water or of the new compound.”

No more striking illustration of how a man’s ingenuity may help him to deceive himself could be given than is afforded by this passage. Priestley to the end of his days never got a just conception of the real chemical constitution of water.

The remaining papers call for little comment. In the course of some further inquiries Priestley discovered sulphuretted hydrogen, termed by him sulphurated inflammable air, and which he prepared by the action of oil of vitriol upon ferrous sulphide. This gas must of course have been frequently obtained or perceived by him, and possibly by others, as it is produced by a number of processes. Its characteristic smell was associated with sulphur: it was thought to be nothing but inflammable air modified or polluted by the accidental presence of sulphur. It cannot be held that Priestley drew the same sharp distinctions between the various kinds of inflammable air that we draw to-day. To us they are essentially different substances. Priestley, however, regarded them as in the main phlogiston combined or associated with other substances which affected the character of their flames or gave them different properties. In his opinion they were essentially the same. This fact serves to explain what is otherwise incomprehensible, and accounts for many of his mistakes.

The last paper in the volume, excluding the “Supplementary Observations,” has a special interest. It is entitled “Observations relating to Theory,” and is in fact Priestley’s Confession of Faith in the doctrine which enslaved and misled him throughout the whole of his scientific career. But he makes it so hesitatingly and with so many reservations that one wonders why he is constrained to make it at all. He appears to think, however, that it is expected of him.

“It is always our endeavour, after making experiments, to generalise the conclusions we draw from them, and by this means to form a theory, or system of principles, to which all the facts may be reduced, and by means of which we may be able to foretell the results of future experiments.... In my former publications I have frequently promised to give such a general theory of the experiments in which the different kinds of air are concerned, as the present state of our knowledge of them will enable me to do. But, like Simonides with respect to the question that was proposed to him concerning God, I have deferred it from time to time; and indeed I am more than ever disposed to defer it still longer, as I own that I am at present even less able to give such a theory as shall satisfy myself than I was some years ago; new difficulties having arisen, which unhinge former theories, and more experiments being necessary to establish new ones.

“Fluctuating, however, as the present state of this branch of knowledge is, I do not think that I can, on this occasion, entirely decline giving some observations of a theoretical nature, and though I cannot pretend to perform the whole of my promise, I shall give a summary view of what appears to me to be the constituent parts of all the kinds of air with which we are acquainted, and a more particular account of the hypothesis concerning phlogiston, which is at present more an object of discussion than anything else of a theoretical nature.”

Priestley then passes in review all the “airs” of which the chemistry of his time had any knowledge, giving the elements or constituent principles of which he imagined them to be composed.

The only kind of air that he thinks to be properly elementary, and to consist of a simple substance, is dephlogisticated air, with possibly the addition of the principle of heat, which, as it is not probable that it adds to the weight of bodies, can hardly be called an element in their composition.

“Dephlogisticated air appears to be one of the elements of water, of fixed air, of all the acids, and of many other substances which, till lately, have been thought to be simple. The air of the atmosphere, exclusive of a great variety of foreign impregnations, appears to consist of dephlogisticated and phlogisticated air.”

As regards phlogisticated air—the mephitic air of Rutherford, the azote of Lavoisier, the nitrogen of Chaptal—Priestley, reasoning from Cavendish’s work, concluded that it was probably not elementary, but “that it consists of nitrous acid and phlogiston; this acid having always been produced by decomposing it with ... dephlogisticated air.”

He is conscious, however, of the insufficiency of this hypothesis, and suggests

“that the acid principle is supplied by the dephlogisticated air, while the nitrous air gives the base of the nitrous acid and phlogiston; and then this [phlogisticated] air may perhaps be considered as phlogiston combined not with all the necessary elements of nitrous acid, but only what may be called the base of it, viz., the dephlogisticated nitrous vapour, or something which when united to dephlogisticated air will constitute nitrous acid.”

“Fixed air (carbonic acid) seems to be a compound of phlogiston and dephlogisticated air.” In other words, carbonic acid and water have, according to Priestley, “the same elementary composition.” “It is something remarkable that two substances so different from each other as fixed air and water should be analysed into the same principles. But there is this difference between them, that water is the union not of pure phlogiston but of inflammable air and dephlogisticated air.”

Of the true nature of inflammable air, Priestley, as we have more than once had occasion to point out, had only the vaguest notions.

“Inflammable air,” he says, “seems now to consist of water and inflammable air, which however seems extraordinary, as the two substances are hereby made to involve each other, one of the constituent parts of water being inflammable air, and one of the constituent parts of inflammable air being water; and therefore, if the experiments would favour it (but I do not see that they do so) it would be more natural to suppose that water, like fixed air, consists of phlogiston and dephlogisticated air in some different mode of combination.”

That Priestley to the last imagined that the various kinds of inflammable air known to him were at bottom one and the same substance, modified or affected by other substances, accidental and unessential, might be proved by a number of passages. He says with respect to inflammable air generally:—

“There is an astonishing variety in the different kinds of inflammable air, the cause of which is very imperfectly known. The lightest, and therefore, probably, the purest kind seems to consist of phlogiston and water only. But it is probable that oil, and that of different kinds, may be held in solution in several of them, and be the reason of their burning with a lambent flame, and also of their being so readily resolved into fixed air when they are decomposed with dephlogisticated air; though why this should be the case I cannot imagine.”

Nitrous air (nitric oxide) he conceives to be a combination of a dephlogisticated nitrous air and phlogiston, and that by adding to it dephlogisticated air and water it is converted into nitrous acid.

Dephlogisticated nitrous air (nitrous oxide) he conceives may, like dephlogisticated air, be an elementary substance and to be formed by depriving nitrous air of its phlogiston.

The various acid airs (e.g., marine acid air, vitriolic acid air, etc.) consist of the peculiar acids as vapours combined with phlogiston.

The Alkaline air (ammonia) he thought to consist of inflammable air and phlogisticated air (nitrogen),

“or of something capable of being converted into phlogisticated air.... That water enters into the composition of alkaline air seems necessary to be admitted, because it is decomposed into inflammable air, which I cannot help thinking necessarily requires water. It seems, however, clearly to be inferred ... that there is no occasion to admit the alkaline principle into the number of elements; the alkalinity, as I may say, some way or other, arising from phlogiston, or phlogisticated air, as acidity arises from dephlogisticated air.”

After these theoretical speculations, “in which,” he says, “I fear I have not communicated much light, though it is as much as I have been able to get,” Priestley proceeds to make some observations relating to phlogiston, “the existence of which is at present a great subject of discussion with philosophers; some maintaining that there is no such thing, and others holding the doctrine of Stahl on the subject.”

“According to Stahl, phlogiston is a real substance, capable of being transferred from one body to another; its presence or absence making a remarkable difference in the properties of bodies, whether it add to their weight or not. Thus he concluded that oil of vitriol deprived of water, and united to phlogiston, becomes sulphur; and that the calces of metals, by the addition of the same substance, become metals.... What is now contended for is that in the oil of vitriol changing into sulphur something is lost and nothing gained, and also that a calx becomes a metal by the loss of air only. And did facts correspond to this theory it would certainly be preferable to that of Stahl, as being more simple; there being one principle less to take into our account in explaining the changes of bodies. But I do not know of any case in which phlogiston has been supposed to enter into a body, but there is room to suppose that something does enter into it....

“What has been insisted upon, as most favourable to the exclusion of phlogiston, is the revival of mercury without the addition of any other substance from the precipitate per se. In this case it is evident that mere heat ... is sufficient to revive the metal. And as what is expelled from this calx is the purest dephlogisticated air, it has been said that mercury is changed into this calx by imbibing pure air, and therefore becomes a metal again, merely in consequence of parting with that air.”

The dexterous Mr Kirwan, not long before he himself embraced the French doctrine, furnished Priestley with an argument which satisfied him that this cardinal fact can be accounted for without excluding phlogiston. “Since therefore the supposition is exceedingly convenient, if not absolutely necessary, to the explanation of many other facts in chemistry, it is at least advisable not to abandon it.”

“That calces do not become metals merely by parting with the air they contain, is evident from my experiments on heating them in contact with inflammable air, in which the inflammable air, or some necessary part of it, is undoubtedly absorbed; and though a little moisture be deposited in the process, it may well be supposed to be that which in conjunction with phlogiston constituted the inflammable air. And what can the other principle that is absorbed by the calx be but the same thing which, when united to water, is recovered again from the metal and found to be inflammable air having all the same properties with that which was employed in the revival of it. Metals therefore are not simple substances, but consist of their calces, and something else which they take from inflammable air. And as the same may also be taken from any combustible substance, it corresponds exactly to Stahl’s phlogiston, and therefore the doctrine of it is confirmed by these experiments; that is, we must still say that in all combustible substances there is a principle capable of being transferred to other substances, which when united to the calces of metals makes them to be metals, and which, united to oil of vitriol (deprived of its water) makes it to be sulphur.”

Thus was the ingenious man effectually entangled in his errors, his ingenuity helping him to deceive himself by evading the force of truth. To err is human. If Priestley saw through a glass darkly, and but dimly discerned the truth, he at least strove, so far as in him lay, to reach the light. Posterity forgives, and may well forget, his errors in grateful recognition of the many noble services he rendered to our common humanity, and in humbling recollection of the suffering and sacrifice with which those services were requited.

FOOTNOTES

INDEX

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Aikin, Anna Lætitia, 33, 43;

her poem Corsica, 70, 75.

Aikin, John, 32, 33, 36, 121.

Aikin, Lucy, 38.

Air, Experiments and Observations on Different Kinds of, Priestley’s treatise on, 77, 103, 167 et seq.

Air, Fixed, 175, 176;

Priestley’s view of its nature, 220.

Air, Inflammable, 179, 220.

Alkaline Air (Ammonia), Priestley’s isolation of, 185;

its properties, 186;

decomposition of by electricity, 210;

nature of, 221.

Ammonia Gas, Priestley’s discovery of, 185;

its properties, 186.

Ammonium Sesquicarbonate, Synthesis of, by Priestley, 186.

B

Barbauld, Mrs (Anna Lætitia Aikin), 33, 160.

Belloc, Madame, account of the Priestleys, 2;

of the times of Priestley, 5;

her account of Mrs Priestley, 49.

Bewley, Richard, his mephitic julep, 79.

Birmingham Riots of 1791, 120 et seq.

Bolton, H. Carrington, his account of the Lunar Society, 96.

Boulton, Matthew, founds the Lunar Society of Birmingham, 94.

Boyle, Robert, his recognition that different elastic fluids exist, 174.

Bright, Henry A., his account of the Warrington Academy, 34.

Burke, Edmund, his denunciation of Priestley, 147.

C

Canton, John, his school, 9, 62.

“Conversion of Water into Air, Seeming,” Priestley’s work on, 216.

Corruptions of Christianity, Priestley’s, 106.

D

Darwin, Erasmus, his connection with the Birmingham Lunar Society, 94, 95.

Daventry Academy, 18.

Dephlogisticated Air (Oxygen), Priestley’s discovery of, 184, 192 et seq.

Doctrine of Phlogiston Established, Priestley’s last scientific work, 162.

E

Edgeworth, Richard Lovell, his account of the Lunar Society, 96.

Electricity, History of, Priestley’s, 63.

Enfield, William, 33, 43.

Eudiometry, 182, 207, 208.

Eyres, William, of Warrington, 38.

F

Faujar St Fond, his visits to Priestley, 103.

Fieldhead, 3.

Fluor Acid Air, 198.

Forster, John Reinhold, 33, 81.

Franklin Benjamin, 63, 91, 92;

his opinion of Priestley, 93.

G

Galton, Samuel, 94, 96.

Gases, diffusion of, 202.

“General History of the Christian Church to the Fall of the Western Empire,” 109, 163.

Gordon, Rev. Alexander, his account of Priestley as a theologian, 109.

H

Harrison, Frederic, on Priestley, 1.

Horner, Leonard, his account of the Lunar Society, 102.

Hutton, Miss, her account of Priestley as a preacher, 105.

I

India-rubber, its use for erasing lead-pencil marks, 72.