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    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. 168 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 169 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 170 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 171 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, 173 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 174 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 175 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 176 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 177 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 178 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 179 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 180 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 181 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 182 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 183 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) 184 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.”

The second section of the volume deals with experiments and observations made in 1773 and the beginning of 1774, and opens with an account of the discovery of ammonia gas.

    “After I had made the discovery of the marine acid air, which the vapour of spirit of salt may properly enough be called ... it occurred to me that by a process similar to that by which this acid air is expelled from the spirit of salt an alkaline air might be expelled from substances containing volatile alkali.

    “Accordingly I procured some volatile spirit of sal ammoniac, and having put it into a thin phial, and heated it with the flame of a candle, I presently found that a great quantity of vapour was discharged from it; and being received in a vessel of quicksilver, standing in a basin of quicksilver, it continued in the form of a transparent and permanent air, not at all condensed by cold; so that I had the same opportunity of making experiments upon it as I had before on the acid air, being in the same favourable circumstances.... Wanting, however, to procure this air in greater quantities, and this method being rather expensive, it occurred to me that alkaline air might probably be procured, with the most ease and convenience, from the original materials, mixed in the same proportions that chemists had found by experience to answer the best for the production of the volatile spirit of sal ammoniac. Accordingly I mixed one-fourth of pounded sal ammoniac with three-fourths of slaked lime; and filling a phial with the mixture, I presently found it completely answered my purpose. The heat of a candle expelled from this mixture a prodigious quantity of alkaline air; and the same materials ... would serve me a considerable time without changing....”


He next studied the properties of the alkaline air. He found, of course, it was readily soluble in water.

    “Having satisfied myself with respect to the relation that alkaline air bears to water, I was impatient to find what would be the consequence of mixing this new air with the other kinds with which I was acquainted before, and especially with acid air; having a notion that these two airs, being of opposite natures, might compose a neutral air, and perhaps the very same thing with common air. But the moment that these two kinds of air came into contact a beautiful white cloud was formed, and presently filled the whole vessel in which they were contained.... When the cloud was subsided there appeared to be formed a solid white salt, which was found to be the common sal ammoniac, or the marine acid united to the volatile alkali....

    “Fixed air admitted to alkaline air formed oblong and slender crystals.... These crystals must be the same thing with the volatile alkalis which chemists get in a solid form by the distillation of sal ammoniac with fixed alkaline salts....

    “Alkaline air, I was surprised to find, is slightly inflammable....

    “That alkaline air is lighter than acid air is evident from the appearances that attend the mixture, which are indeed very beautiful. When acid air is introduced into a vessel containing alkaline air, the white cloud which they form appears at the bottom only and ascends gradually. But when the alkaline air is put to the acid the whole becomes immediately cloudy quite to the top of the vessel.”

Up to now Priestley had mainly confined himself to the narration of the new facts which he had discovered, barely mentioning any hypotheses that occurred to him.

    “The reason why I was so much upon my guard in this respect was lest, in consequence of attaching myself to any hypothesis too soon, the success of my future inquiries might be obstructed. But subsequent experiments having thrown great light upon the preceding ones, and having confirmed the few conjectures I then advanced, I may now venture to speak of my hypotheses with a little less diffidence. Still, however, 187 I shall be ready to relinquish any notions I may now entertain if new facts should hereafter appear not to favour them.”

In a paper on “Common Air Diminished and made Noxious by Various Processes” he attempts to apply the current doctrine of phlogiston to account for the various phenomena he has observed, and with what success may be inferred from his conclusion

    “that in the precipitation of lime by breathing into lime-water, the fixed air, which incorporates with lime, comes not from the lungs but from the common air, decomposed by the phlogiston exhaled from them, and discharged, after having been taken in with the aliment, and having performed its function in the animal system.”

Priestley’s attempts at theorising brought little satisfaction to him or to his readers. Indeed he says:—

    “I begin to be apprehensive lest, after being considered as a dry experimenter, I should pass into the opposite character of a visionary theorist.... In extenuation of my offence let it, however, be considered that theory and experiments necessarily go hand-in-hand, every process being intended to ascertain some particular hypothesis, which, in fact, is only a conjecture concerning the circumstances or the cause of some natural operation; consequently that the boldest and most original experimenters are those who, giving free scope to their imaginations, admit the combination of the most distant ideas; and that though many of these associations of ideas will be wild and chimerical, yet that others will have the chance of giving rise to the greatest and most capital discoveries, such as very cautious, timid, sober and slow-thinking people would never have come at.

    “Sir Isaac Newton himself, notwithstanding the great advantage which he derived from a habit of patient thinking, indulged bold and eccentric thoughts, of which his queries at the end of his book of Optics are a sufficient evidence. And a quick conception of distant analogies, which is the great key to unlock the secrets of Nature, is by no means incompatible 188 with the spirit of perseverance in investigations calculated to ascertain and pursue those analogies.”

After this apologia, Priestley gives the reins to his imagination, or rather he allows phlogiston to drive the halting, ambling thing for him, with the result that he utterly loses his way and is eventually landed into an impassable quagmire. It is not too much to say that not one of the “Queries, Speculations and Hints” with which the volume closes has stood the test of time.

The second volume, which made its appearance towards the end of 1775, is dedicated to Sir John Pringle, at that time President of the Royal Society. It opens, as usual, with a somewhat prolix but characteristic preface. But to his biographer Priestley’s prefaces are not the least interesting or valuable of his literary productions.

    “In a preface,” he says, “authors have always claimed a right of saying whatever they pleased concerning themselves, and not to lose this right it must now and then be exercised.”

In this respect Priestley has championed the prerogatives of authors for all time. This particular preface begins with an expression of self-laudation for the little delay the writer made in putting the first volume to the press.

    “In consequence of this considerable discoveries have been made by people of distant nations; and this branch of science, of which nothing, in a manner, was known till very lately, indeed now bids fair to be farther advanced than any other in the whole compass of natural philosophy.... And it will not now be thought very assuming to say that by working in a tub of water or a basin of quicksilver we may perhaps discover principles of more extensive influence than even that of gravity itself, the discovery of which, in its full extent, contributed so much to immortalise the name of Newton.


    “Having been the means of bringing so many champions into the field, I shall, with peculiar pleasure, attend to all their achievements, in order to prepare myself, as I promised in the preface to my last volume, for writing the history of the campaign.”

After a delightfully na?ve compliment to his own ability as an accumulator of facts, and to his merits as an “instrument in the hands of Divine Providence ... concerning which I threw out some further hints in my former preface, which the excellent French translator was not permitted to insert in his version,” he advances this testimony to his impartiality as an historian:—

    “I even think that I may flatter myself so much, if it be any flattery, as to say that there is not, in the whole compass of philosophical writing, a history of experiments so truly ingenuous as mine, and especially the section on the discovery of dephlogisticated air, which I will venture to exhibit as a model of the kind. I am not conscious to myself of having concealed the least hint that was suggested to me by any person whatever, any kind of assistance that has been given me, or any views or hypotheses by which the experiments were directed, whether they were verified by the result or not.”

There is much else in the preface that might be quoted as illustrative of the character and mental attributes of its author. Priestley, the natural philosopher, never forgot that he was a minister of religion, and that to him theology was the greatest and most important of all the sciences, and he cannot forbear even, in what he intended to be a scientific disquisition on purely natural phenomena, from inculcating his belief in the divine origin of Christianity and his opinion concerning the doctrine of purgatory and the worship of the dead.

The first chapter is concerned with the discovery of what its author called Vitriolic Acid Air, but which we now know as sulphur dioxide.

Priestley imagined that as the liquid marine acid—that is hydrochloric acid—readily yielded an “air” on heating it might be that vitriolic acid, or oil of vitriol, would also afford a characteristic “air” when treated in a similar manner. Acting upon a suggestion of Mr Lane he heated oil of vitriol with olive oil, when he readily obtained a new species of air, which he collected over mercury as he “had been used to do it with the marine acid air; and the whole process was as pleasing and as elegant.” Priestley at once surmised that the olive oil worked by transferring its phlogiston to the vitriolic acid, and he naturally concluded that any substance rich in phlogiston would bring about the same result. He next tried charcoal.

    “I put some bits of charcoal into my phial instead of the oil or other inflammable matter which I had used before, and applying the flame of a candle I presently found that the vitriolic acid air was produced as well as in the former process, and in several respects more conveniently, the production of air being equable, whereby the disagreeable effect of a sudden explosion is avoided.... Finding that a great variety of substances containing phlogiston enabled the oil of vitriol to throw out a permanent acid air, I had some suspicion that mere heat might do the same, but I did not find that there was any foundation for that suspicion.... But though I got no air from the oil of vitriol by this process, air was produced at the same time in a manner that I little expected, and I paid pretty dearly for the discovery it occasioned. Despairing to get any air from the longer application of my candles, I withdrew them, but before I could disengage the phial from the vessel of quicksilver a little of it passed through the tube into the hot acid, when instantly it was all filled with dense white fumes, a prodigious quantity of air was generated, the tube through which it was transmitted was broken into many pieces, and part of the hot acid being spilled upon my hand burned it terribly, so that the effect of it is visible to this day. The inside of the phial was coated 191 with a white saline substance, and the smell that issued from it was extremely suffocating.

    “This accident taught me what I am surprised I should not have suspected before, viz., that some metals will part with their phlogiston to hot oil of vitriol, and thereby convert it into a permanent elastic air, producing the very same effect with oil, charcoal, or any other inflammable substance.

    “Not discouraged by the disagreeable accident above mentioned, the next day I put a little quicksilver into the phial with the ground stopple and tube, along with the oil of vitriol, when, long before it was boiling hot, air issued plentifully from it, and being received in a vessel of quicksilver appeared to be genuine vitriolic acid air, exactly like that which I had procured before, being readily imbibed by water and extinguishing a candle in the same manner as the other had done....

    “After this I repeated the experiment with several other metals.... Copper treated in the same manner yielded air very freely, with about the same degree of heat that quicksilver had required, and the air continued to be generated with very little application of more heat.”

The theory apart, this paper is as important as these on ammonia and the marine acid air, and exhibits Priestley at his best. The observations he makes concerning the main properties of the new gas and its solubility in water, its inability to burn and to support flame, its heaviness, its power to unite with ammonia, to be absorbed by charcoal and to liquefy camphor, are all accurate.

    “Having hit upon a method of exhibiting some of the acids in the form of air, nothing could be easier than to extend this process to the rest.”

Accordingly he attempted to procure what he called the vegetable acid air by heating “exceedingly strong concentrated acid of vinegar,” and states that he succeeded in obtaining an air which extinguished the flame of a candle and was soluble in water. The paper 192 is very short and is full of contradictions. In reality, as he subsequently found, he was dealing with vinegar largely adulterated with oil of vitriol. The “vegetable acid air” had no real existence.

The next paper in the series is the most important of the whole, and the one of all others that has contributed most largely to Priestley’s reputation. It is entitled “Of Dephlogisticated Air, and of the Constitution of the Atmosphere,” and deals with the discovery of oxygen. It begins in the following characteristic fashion:—

    “The contents of this section will furnish a very striking illustration of the truth of a remark which I have more than once made in my philosophical writings, and which can hardly be too often repeated, as it tends greatly to encourage philosophical investigations, viz., that more is owing to what we call chance—that is, philosophically speaking, to the observation of events arising from unknown causes than to any proper design or preconceived theory in this business. This does not appear in the works of those who write synthetically upon these subjects, but would, I doubt not, appear very strikingly in those who are the most celebrated for their philosophical acumen did they write analytically and ingenuously.

    “For my own part, I will frankly acknowledge that at the commencement of the experiments recited in this section I was so far from having formed any hypothesis that led to the discoveries I made in pursuing them that they would have appeared very improbable to me had I been told of them; and when the decisive facts did at length obtrude themselves upon my notice it was very slowly, and with great hesitation, that I yielded to the evidence of my senses. And yet, when I reconsider the matter, and compare my last discoveries relating to the constitution of the atmosphere with the first, I see the closest and the easiest connection in the world between them, so as to wonder that I should not have been led immediately from the one to the other. That this was not the case I attribute to the force of prejudice which, unknown to ourselves, biases not only our 193 judgments, properly so called, but even the perceptions of our senses; for 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.”

He then points out that there are few maxims in philosophy that have laid firmer hold upon the mind than that air, meaning atmospherical air ... is a simple elementary substance, indestructible and unalterable, at least as much so as water was supposed to be. Priestley, in the course of his inquiries, was soon satisfied that atmospherical air was not an unalterable thing; that bodies burning in it, and animals breathing it and various other chemical processes, so far alter and deprive it as to render it altogether unfit for the purposes to which it is subservient; and he had discovered methods, particularly the process of vegetation, which tended to restore it to its original purity.

    “But,” he says, “I own I had no idea of the possibility of going any further in this way and thereby procuring air purer than the best common air.”

As this paper is one of the classics of chemistry, as well as the chief corner-stone in the monument which Priestley erected to himself, it is necessary to examine it, as well as certain other papers which grew immediately out of it, in some degree of detail.

After a reference to a hypothesis of the origin and constitution of the atmosphere which occurs among the “Queries, Speculations and Hints” above referred to, and which is on a par with much in Priestley’s speculations, he proceeds to relate the circumstances which more immediately led to the most important of all his discoveries. 194 It was the accident of possessing a burning lens of “considerable force,” for want of which he could not possibly make many of the experiments that he had projected.

    “But having afterwards procured a lens of twelve inches diameter and twenty inches focal distance, I proceeded with great alacrity to examine, by the help of it, what kind of air a great variety of substances, natural and factitious, would yield, putting them into vessels [short, wide, round-bottomed phials], which I filled with quicksilver and kept inverted in a basin of the same. Mr Warltire, a good chemist, and lecturer in Natural Philosophy, happening to be at that time in Calne, I explained my views to him, and was furnished by him with many substances, which I could not otherwise have procured.

    “With this apparatus, after a variety of other experiments, an account of which will be found in its proper place on the 1st August 1774, I endeavoured to extract air from mercurius calcinatus per se;[17] and I presently found that, by means of this lens, air was expelled from it very readily. Having got about three or four times as much as the bulk of my materials, I admitted water to it, and found that it was not imbibed by it. But what surprised me more than I can well express was that a candle burned in this air with a remarkably vigorous flame, very much like that enlarged flame with which a candle burns in nitrous air exposed to iron or liver of sulphur,[18] but as I had got nothing like this remarkable appearance from any kind of air besides this particular modification of nitrous air, and I knew no nitrous acid was used in the preparation of mercurius calcinatus, I was utterly at a loss how to account for it.

    “In this case also, though I did not give sufficient attention to the circumstance at that time, the flame of the candle, besides being larger, burned with more splendour and heat than in that species of nitrous air; and a piece of red-hot wood sparkled in it, exactly like paper dipped in a solution of nitre, and it consumed very fast; an experiment which I had never thought of trying with nitrous air.

    “At the same time that I made the above-mentioned experiment I extracted a quantity of air with the very same property from the common red precipitate[19] which, being produced by a solution of mercury in spirit of nitre (nitric acid), made me conclude that this peculiar property, being similar to that of the modification of nitrous air above mentioned, depended upon something being communicated to it by the nitrous acid; and since the mercurius calcinatus is produced by exposing mercury to a certain degree of heat, where common air has access to it, I likewise concluded that this substance had collected something of nitre, in that state of heat, from the atmosphere.

    “This, however, appearing to me much more extraordinary than it ought to have done, I entertained some suspicion that the mercurius calcinatus on which I had made my experiments, being bought at a common apothecary’s, might, in fact, be nothing more than red precipitate; though, had I been anything of a practical chemist, I could not have entertained any such suspicion. However, mentioning this suspicion to Mr Warltire, he furnished me with some that he had kept for a specimen of the preparation, and which, he told me, he could warrant to be genuine. This being treated in the same manner as the former, only by a longer continuance of heat, I extracted much more air from it than from the other.

    “This experiment might have satisfied any moderate sceptic; but, however, being at Paris in the October following, and knowing that there were several very eminent chemists in that place, I did not omit the opportunity, by means of my friend Mr Magellan, to get an ounce of mercurius calcinatus prepared by Mr Cadet, of the genuineness of which there could not possibly be any suspicion; and at the same time I frequently mentioned my surprise at the kind of air which I had got from this preparation to Mr Lavoisier, Mr le Roy, and several other philosophers, who honoured me with their notice in that city, and who, I daresay, cannot fail to recollect the circumstance.”

This last remark is significant in reference to a claim which was subsequently put forward that the real 196 discoverer of oxygen was Lavoisier, and that he obtained it by heating mercuric oxide.[20]

Priestley also obtained the same air from red lead, which, he says,

    “confirmed me more in my suspicion that the mercurius calcinatus must get the property of yielding this kind of air from the atmosphere, the process by which that preparation and this of red lead is made being similar. As I never make the least secret of anything that I observe, I mentioned this experiment also, as well as those with the mercurius calcinatus and the red precipitate, to all my philosophical acquaintance at Paris and elsewhere, having no idea, at that time, to what these remarkable facts would lead.” [Nitrous oxide.]

Priestley, on his return to England, made an experiment with Cadet’s preparation, which he found to behave precisely like that he had procured from Warltire. He observed that the new gas was only sparingly soluble in water and that its power of causing a candle to burn with a strong flame was in nowise diminished by agitation with water—facts which he said convinced him

    “that there must be a very material difference between the constitution of the air from mercurius calcinatus and that of phlogisticated nitrous air, [nitrous oxide] notwithstanding their resemblance in some particulars.”

It was not, however, until the following March (1775) (he having meanwhile been intent upon his experiments on the vitriolic air [sulphur dioxide]), that he ascertained the real nature of the new air, and was led “though very gradually ... to the complete discovery of the constitution of the air we breathe.” By trials with the nitrous air and with mice he found that the new gas was eminently fit for respiration: nitrous air 197 reduced its volume to a greater extent than in the case of common air, and a mouse lived longer in it than it would in the same volume of common air.

    “Thinking of this extraordinary fact upon my pillow, the next morning I put another measure of nitrous air to the same mixture, and to my utter astonishment found that it was farther diminished to almost one-half of its original quantity.”

Priestley now utterly missed his way for a time. He sought to get the new air from the various oxides of lead, but the fetish of phlogiston again led him wrong, and eventually by a train of reasoning which is fully set forth in the paper, but which need not here be repeated, there remained, he says, no doubt in his mind

    “but that atmospherical air, or the thing that we breathe, consists of the nitrous acid and earth, with so much phlogiston as is necessary to its elasticity; and likewise so much more as is required to bring it from its state of perfect purity to the mean condition in which we find it.”

Priestley’s “complete discovery of the constitution of the air we breathe” was thus wholly erroneous: he was very far indeed from having a clear conception of its real nature.

Priestley’s description of the main properties of oxygen is however accurate, and lecturers in chemistry are indebted to him for some striking experimental illustrations of them.

    “I easily conjectured,” he says, “that inflammable air would explode with more violence and a louder report by the help of dephlogisticated than of common air; but the effect far exceeded my expectations, and it has never failed to surprise every person before whom I have made the experiment.... The dipping of a lighted candle into a jar filled with dephlogisticated air is alone a very beautiful experiment. The strength and vivacity of the flame is striking, and the heat 198 produced by the flame in these circumstances is also remarkably great.... Nothing would be easier than to augment the force of fire to a prodigious degree by blowing it with dephlogisticated air instead of common air.... Possibly platina might be melted by means of it.

    “From the greater strength and vivacity of the flame of a candle, in this pure air, it may be conjectured that it might be peculiarly salutary to the lungs in certain morbid cases.... But perhaps we may also infer from these experiments that though pure dephlogisticated air might be very useful as a medicine, it might not be so proper for us in the usual healthy state of the body: for, as a candle burns out much faster in dephlogisticated than in common air, so we might, as may be said, live out too fast, and the animal powers be too soon exhausted in this pure kind of air. A moralist, at least, may say that the air which Nature has provided for us is as good as we deserve.... Who can tell but that, in time, this pure air may become a fashionable article in luxury. Hitherto only two mice and myself have had the privilege of breathing it.”

An experiment which Priestley says “I had the pleasure to see at Paris, in the laboratory of Mr Lavoisier, my excellent fellow-labourer in these inquiries, and to whom, in a variety of respects, the philosophical part of the world has very great obligations,” led him into a train of inquiry upon the action of nitric acid upon a wide range of organic substances, from which however no general results followed, in spite of much experimenting. He had at one time the idea that a fundamental difference existed in the behaviour of animal and vegetable matter with respect to nitric acid, but the observations were contradictory, and although it is readily possible to interpret the phenomena in the light of our present knowledge, they led Priestley to no definite conclusions.

Of more importance is the work on the “Fluor Acid Air”—a substance discovered by “Mr Scheele, a Swede; 199 from which circumstance the acid is often distinguished by the name of the Swedish acid.” Priestley sought to make the air by heating Derbyshire spar (fluor spar) with oil of vitriol in glass vessels,

    “as in the process of making spirit of nitre from saltpetre; and the most remarkable facts that have been observed concerning it are, that the vessels in which the distillation is made are apt to be corroded; so that holes will be made quite through them; and that when there is water in the recipient, the surface of it will be covered with a crust of a friable stony matter.”

What Priestley actually produced by this method of experimenting was more or less pure silicon fluoride, which he proceeded to collect, in his usual fashion, over quicksilver.

    “I had no sooner produced this new kind of air but I was eager to see the effect it would have on water, and to produce the stony crust formed by their union, as described by Mr Scheele; and I was not disappointed in my expectations. The moment the water came into contact with this air the surface of it became white and opaque by a stony film.... Few philosophical experiments exhibit a more pleasing appearance than this, which can only be made by first producing the air confined by quicksilver, and then admitting a large body of water to it. Most persons to whom I have shown the experiment have been exceedingly struck with it.... The union of this acid air and water may also be exhibited in another manner, which to some persons makes a still more striking experiment, viz., by admitting the air, as fast as it is generated, to a large body of water resting on quicksilver.... It is, then, very pleasing to observe that the moment any bubble of air, after passing through the quicksilver, reaches the water, it is instantly, as it were, converted into a stone; but continuing hollow for a short space of time, generally rises to the top of the water.... I have met with few persons who are soon weary of looking at it; and some could sit by it almost a whole hour, and be agreeably amused all the time.”


Priestley’s attempts to explain the real nature of the fluor acid air were, as may be expected, not very happy.

    “These appearances I explain by supposing that the vitriolic acid, in uniting with the spar, is in part volatilised by means of some phlogiston contained in it, so as to form a vitriolic acid air; and there is also combined with this air a portion of the solid earthy part of the spar, which continues in a state of solution till, coming into contact with the water, the fluid unites with the acid, and the earth is precipitated.”

The third volume of the work was published in the early part of 1777, with a dedication to Lord Stanhope. It opens, as usual, with the characteristically discursive preface, extending to thirty pages, in which the author apologises for the character of much in the volume. He is constrained to admit that numerous as his facts are, “few of them will appear so brilliant in the eye of the general scholar” as in either of the two former volumes, although he trusts they will “be thought no less valuable by philosophers and chemists.” Priestley, it would seem, was conscious that he was beginning, as the phrase goes, “to write himself out.”

    “Lest my readers should be alarmed at this addition of one volume after another on the same subject, I do assure them that I shall now certainly give them and myself some respite, and deliver the torch to anyone who may be disposed to carry it, foreseeing that my attention will be sufficiently engaged by speculations of a very different nature.... It will be a great satisfaction to me, after the part that I have taken in this business, to be a spectator of its future progress, when I see the work in so many and so good hands, and everything in so rapid and so promising a way.

    “On taking leave of this subject I would entreat the candour and indulgence of my readers for any oversights they may discover in me as a philosopher, or imperfections as a writer. I am far from pretending to infallibility; but I have the satisfaction to reflect that, imperfect as my works may be found 201 to be, they are each as perfect as I was able to make them....

    “Upon this, as upon other occasions, I can only repeat that it is not my opinions on which I would be understood to lay any stress. Let the new facts, from which I deduce them, be considered as my discoveries, and let other persons draw better inferences from them if they can. This is a new and a wide field of experiment and speculation, and a premature attachment to hypothesis is the greatest obstruction we are likely to meet with in our progress through it; and as I think I have been pretty much upon my guard myself, I would caution others to be upon their guard too.”

These passages evidently were written under the influence of the feeling of resentment with which he viewed the criticism to which his speculations were subjected abroad. Fontana, Lavoisier and others were, indeed, zealously engaged in using Priestley’s own facts to destroy the conception by which he explained them. An appeal to the balance was felt to be necessary, and Priestley, as a logician, could not resist it. But he was no quantitative chemist: the habits of a Cavendish were quite foreign to his genius: patient, scrupulous attention to numerical accuracy was not one of his characteristics: he was one of the most industrious of experimenters—delighting, indeed, in manipulation for the mere sake of it, but withal hasty and superficial. It is nowhere evident in his writings that his problems were attacked according to any carefully-thought-out plan. He confesses indeed, on more than one occasion, he tested the inflammability of one of his numerous “airs” because he had a lighted candle near him: had the candle not been lighted it would not have occurred to him to do it. Priestley was, in fact, a pioneer: he showed the existence of a new world for science, and he 202 himself roamed over a portion of it, like a second Joshua; but he had not the experience or the aptitude to accurately map out even that fraction.

There is little in the third volume of permanent value. It is largely an account of a series of disconnected observations on the action of nitric acid upon a variety of substances, which, however, led to no general conclusions. It is, however, certain that if Priestley could have induced himself to follow up certain of his observations he would have arrived at facts of far greater importance than those he actually narrates. “Speculation,” he said, by way of rejoinder to Lavoisier, “is a cheap commodity. New and important facts are most wanted, and therefore of most value,” and the new and important facts were within his grasp if he had only reached out for them.

Another portion of the work is concerned with supplementary observations on the gases treated of in the preceding volumes, partly by way of correction and partly additional. Here and there we have a suggestive passage, as in the paper on “Experiments on the Mixture of Different Kinds of Air that have no Mutual Action,” in which he thus clearly indicates the principle of the intra-diffusion of gases.

    “The result of my trials has been this general conclusion: that when two kinds of air have been mixed it is not possible to separate them again by any method of decanting or pouring them off, though the greatest possible care be taken in doing it. They may not properly incorporate, so as to form a third species of air, possessed of new properties; but they will remain equally diffused through the mass of each other; and whether it be the upper or the lower part of the air that is taken out of the vessel, without disturbing the rest, it will contain an equal mixture of them both.”


Another suggestive paper is on “Respiration and the Use of the Blood,” which was read to the Royal Society on January 25, 1776, and appears in the Phil. Trans., vol. lxvi. Priestley, of course, regarded respiration as a phlogistic process, and “that the use of the lungs is to carry off a putrid effluvium, or to discharge that phlogiston, which had been taken into the system with the aliment, and has become, as it were, effete, the air that is respired serving as a menstruum for that purpose.” This he thinks he has “proved to be effected by means of the blood, in consequence of its coming so nearly into contact with the air in the lungs, the blood appearing to be a fluid wonderfully formed to imbibe and part with that principle which the chemists call phlogiston, and changing its colour in consequence of being charged with it or being freed from it.” The facts in this paper are for the most part correctly stated, but the discoverer of oxygen led the world woefully astray as to the part played by that gas in the phenomena of respiration.

The fourth volume made its appearance in March 1779, with a dedication to Sir George Savile, who had rendered Priestley the service of introducing him and his invention of soda-water to the notice of the Admiralty. In the preface, which is commendably short, he makes some reference to the respite which he had promised himself and his readers, but trusts, by way of extenuation, “it may be sufficient to allege the instability of human purposes and pursuits.” He had intended to devote himself to metaphysics.

    “But that kind of writing,” he says, “is a thing of a very different nature from this. I can truly say ... that single sections in this work have cost me more than whole volumes of the 204 other; so great is the difference between writing from the head only and writing, as it may be called, from the hands.”

The fact was Priestley could not keep away from his laboratory.

    “Having acquired a fondness for experiments, even slighter inducements than I have had would have been sufficient to determine my conduct.”

The preface is noteworthy for its plea for the position of experimental science in the scheme of general education.

    “If we wish to lay a good foundation for a philosophical taste, and philosophical pursuits, persons should be accustomed to the sight of experiments and processes in early life. They should, more especially, be early initiated in the theory and practice of investigation, by which many of the old discoveries may be made to be really their own; on which account they will be much more valued by them. And, in a great variety of articles, very young persons may be made so far acquainted with everything necessary to be previously known as to engage (which they will do with peculiar alacrity) in pursuits truly original.”

In the course of some observations on the effect “of impregnating oil of vitriol with nitrous acid vapour” he discovered nitrosulphuric acid, the so-called “Leaden Chamber Crystals,” whose properties and behaviour with water he describes with accuracy and even eloquence. Of these crystals he says: “A more beautiful appearance can hardly be imagined, and I am afraid I shall never see the like again.” He also noticed the formation of the dark brown compound which nitric oxide forms with a solution of green vitriol, and adds:—

    “To determine whether the phenomena attending the impregnation of the solution of green vitriol with nitrous air depended in any measure upon the seeming astringency of that solution ... I impregnated a quantity of green tea, which is also 205 said to be astringent, with nitrous air, but no sensible change of colour was produced in it.”

He several times noticed the deep blue liquid which nitrogen peroxide forms with cold water. He made many attempts to use nitric oxide as an antiseptic, especially for culinary purposes. But the gastronomic results with fowls and pigeons were not to his liking, although he says, “my friend Mr Magellan ... had not so bad an opinion of this piece of cookery as I had.” One cannot read Priestley’s description of his multifarious experiments without being struck with the number of occasions in which he just missed making discoveries of first-rate importance. It is obvious that he had obtained chlorine without recognising it, even before the news of Scheele’s discovery reached this country. He had also prepared, without knowing it, phosphoretted hydrogen and phosphorous acid. At times, however, he can follow a clue with remarkable perspicacity; as in his observation of the cause of the “flouring” of mercury, and in his discovery of a method of removing lead and tin from that metal.

The subject of “dephlogisticated air” naturally continued to interest him, and he again returns to it in this volume, for he says:—

    “As it sometimes amuses myself it may perhaps amuse others to look back with me to the several steps in the actual progress of this investigation, some of which I overlooked in my last account of it.”

He points out, as already stated, that he must have had the new gas in his hands as far back as November 1771, having obtained it from nitre. He admits that he had no particular view in making his crucial experiment of August 1, 1774,

    “excepting that of extracting air from a variety of substances 206 by means of a burning lens in quicksilver, which was then a new process with me, and which I was very fond of.”

He explains how he was led to his speculation that “this kind of air, and consequently of atmospherical air, which is the same thing but in a state of inferior purity,” consists “of earth and spirit of nitre.”

    “But,” he adds, “I have since seen reason to suspect that hypothesis, plausible as it appears. Indeed, some of my late experiments would lead me to conclude that there is no acid at all in pure air.”

He then experiments with manganese, which Scheele, who independently discovered oxygen, had already employed, and finds that it yields the new air both when heated alone or with oil of vitriol. The production of oxygen from manganese was contrary to his expectations as the substances he had hitherto used, the precipitate per se and the red lead and the nitre, had all been subjected to “the influence of the atmosphere,” whereas “here was pure air from a substance which for anything that appeared had always been in the bowels of the earth, and never had had any communication with the external air.” This led to the surmise that possibly the expulsion of dephlogisticated air from such mineral substances

    “might assist in sustaining subterraneous fires.... The solution of the phenomena of subterraneous fires would certainly be much easier on the supposition of their supplying their own pabulum, by means of dephlogisticated air contained in substances exposed to their heat. I therefore desired Mr Landriani, who being in Italy had a good opportunity of making inquiries on the subject, to inform me whether any of those substances, and particularly manganese be found in their volcanoes; and his answer makes it rather probable that those fires are, in part, sustained by this means.”

The ease with which nitre parts with its oxygen on 207 heating furnished Priestley with the true explanation of its so-called “detonation,” “concerning which,” he says, “the most improbable conjectures have been advanced by the most eminent philosophers and chemists.” After a reference to the hypothesis of Macquer, who assumes that what he calls “a nitrous sulphur” is produced, Priestley points out that

    “the doctrine of dephlogisticated air supplies the easiest solution imaginable of this very difficult phenomenon. Let any person but attend to the phenomena of the detonation of charcoal in nitre, and that of dipping a piece of hot charcoal into a jar of dephlogisticated air, and I think it will be impossible for him not to conclude that the appearances are the very same and must have the same cause.”

Of all the quantitative exercises performed by Priestley, by far the most numerous depended upon his application of nitric oxide to measure the “goodness” of air.

    “When,” he says, “I first discovered the property of nitrous air as a test of the wholesomeness of common air, I flattered myself that it might be of considerable practical use, and particularly that the air of distant places and countries might be brought and examined together with great ease and satisfaction; but I own that hitherto I have rather been disappointed in my expectations from it.... I gave several of my friends the trouble to send me air from distant places, especially from manufacturing towns, and the worst they could find to be actually breathed by the manufacturers, such as is known to be exceedingly offensive to those who visit them; but when I examined those specimens of air in Wiltshire, the difference between them and the very best air in this county, which is esteemed to be very good, as also the difference between them and specimens of the best air in the counties in which these manufacturing towns are situated, was very trifling.... I have frequently taken the open air in the most exposed places in this country at different times of the year, and in different states of the weather, etc., but never found the difference so great as the 208 inaccuracy arising from the method of making the trial might easily amount to or exceed.”

Other observers, less careful or more sanguine than Priestley, were, however, successful in detecting the differences which prejudice led them to anticipate. Thus Signor Marsilio Landriani of Milan, whose name has already been mentioned in connection with the theory of subterraneous fires, in the course of a tour through Italy had the satisfaction of convincing himself

    “that the air of all those places, which from the long experience of the inhabitants has been reputed unwholesome, is found to be so to a very great degree of exactness by the eudiometer.... The air of the Pontine lakes, that of the Sciroccho at Rome (so very unwholesome), that of the Campagna Romana, of the Grotto del Cane, of the Zolfatara at Naples, of the baths of Nero at Baja, of the seacoast of Tuscany, were all examined by me and found to be in such a state as daily experience led me to expect.”

Modern eudiometry, making use of methods of far greater precision than were possible to Priestley, has confirmed his supposition that atmospheric air is remarkably constant in composition, and that its wholesomeness depends upon other causes than the relative amount of the dephlogisticated air contained in it.

Perhaps the most important of the many papers contained in this volume are those which relate to the “Melioration of Air by the Growth of Plants,” a subject to which Priestley gave attention, even whilst at Leeds, in 1771. In these papers he clearly proves that this “melioration” is connected with the green matter of leaves and that it is dependent upon sunlight. This observation is of fundamental importance and attracted much attention.

In the fifth volume, which was published in the 209 spring of 1781, with a dedication to Dr Heberden, when Priestley had moved to Birmingham, he again returns to this subject. Practically all the experimental work to which it relates was done whilst he was with Lord Shelburne, and mainly at Calne. During the former parts of the summer of 1780 he suffered from an illness which greatly interfered with his work, although he thinks that during his incapacity for making experiments his “hints for the farther prosecution of them are greatly accumulated.” It cannot be said that the five papers on the relations of vegetation to air, with which the volume opens, added very materially to the fundamental fact which Priestley had discovered. They furnished, however, additional evidence of it and no doubt stimulated further inquiry. If his facts could not be controverted, his explanations and surmises were at least open to attack, and a number of observers, both here and abroad, busied themselves with the problems of physiological botany thereby suggested.

As regards the subject of “air” in general, although a large number of isolated observations are recorded in somewhat tedious detail, no new fact of first-rate importance is apparent. The experiments are largely supplementary to those in the preceding volumes and are for the most explanatory or corroborative of them. Perhaps the most important are those dealing with “the production of nitrous air in which a candle will burn,” by which is signified the gas we now know as nitrous oxide, but which Priestley eventually termed dephlogisticated nitrous air. The process he employed is no longer used in the production of this gas, but it sufficed in his hands to determine its individuality without doubt.

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 212 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 214 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 215 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 216 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 217 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 219 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, 221 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 222 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 223 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.

The End


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