Joseph BlackJOSEPH BLACK.
JOSEPH BLACK.
He began his medical studies in Edinburgh in the year 1751, and in 1755 he published, as his thesis for the degree of M.D., the work which has rendered his name famous. It appears that as early as 1752 he had been occupied with investigations on quicklime, which was then attracting attention as a remedy for urinary calculi. Opinion was divided regarding its virtue. In a manuscript copy of notes of Black’s lectures, which the author is so fortunate as to possess, he mentions that his attention was directed to the subject through the rival views of Drs. Alston and Whytt. It was not long before he proved that, in opposition to the commonly received notion, quicklime had gained nothing from the fire in which it was made, but that the limestone used for its preparation had lost nearly half its weight in becoming caustic. He also attempted successfully to trap the escaping gas, and again placed it in presence of lime, confining it over water. Instead of any escape of material when the lime became mild, “nothing escapes—the cup rises considerably by absorbing air.” And in his notes, a few pages farther on, he compares the loss of weight undergone by limestone on being calcined, with its loss on being dissolved inmuriatic acid. These experiments appear from his journal to have been made before November 1752.
His thesis was not published, however, until 1755. Immediately after, in 1756, he succeeded Dr. Cullen as Professor in Glasgow, where he remained until 1766. During these ten years he began and made great progress with his well-known researches on the heat of fusion of ice, and the heat of vaporisation of water, or, as he termed them, the “latent heats” of water and of steam. In 1766, Dr. Cullen was appointed Professor of Medicine in the University of Edinburgh, and Black again succeeded him as Professor of Chemistry. There he lectured until 1797, when he retired from public life; he died as peacefully as he had lived, in 1799, in the seventy-first year of his age. Thomson, who relates these particulars, was one of his last students; he writes:—“I never listened to any lectures with so much pleasure as to his; and it was the elegant simplicity of his manner, the perfect clearness of his statements, and the vast quantity of information which he contrived in this way to communicate, that delighted me.... His illustrations were just sufficient to answer completely the object in view, and no more.”
Black’s original thesis for his degree was entitledExperiments upon Magnesia Alba, Quicklime, and other Alcaline Substances. It was published in 1755, and several times reprinted. It is now to be had in a convenient form as one of the “Alembic Club Reprints.”
It was the custom in those days to administer alkalies as a remedy for urinary calculi; and about the year 1750 lime-water was tried as a substitute. Opinion was divided as regarded its efficacy; and it was with the view of preparing a better remedy that Black undertook researches on magnesia alba. Black prepared magnesia from “bittern,” which remains in the pans after the crystallisation of salt from sea-water, and also from Epsom salts, “which is evidently composed ofmagnesiaand the vitriolic acid.” The magnesia is thrown down from the sulphate as carbonate, by the addition of pearl ashes, at the temperature of ebullition, the soluble product being “vitriolated tartar,” or potassium sulphate. He describes how “magnesia is quickly dissolved with effervescence or explosion of air, by the acids of vitriol, nitre, and of common salt, and by distilled vinegar,” and gives an account of the properties of the sulphate, nitrate, chloride,and acetate. He subsequently heated this magnesia, and found that it lost “a remarkable proportion of its weight in the fire,” and his “attempts were directed to the investigation of this volatile part.” The residue in the retort did not effervesce on the addition of acids; hence the volatile part had been driven away by the heat. “Chemists have often observed, in their distillations, that part of the body has vanished from their senses, notwithstanding the utmost care to retain it; and they have always found, upon further inquiry, that subtile part to be air, which, having been imprisoned in the body, under a solid form, was set free, and rendered fluid and elastic by the fire. We may safely conclude that the volatile matter lost in the calcination ofmagnesiais mostly air; and hence the calcinedmagnesiadoes not emit air, or make an effervescence when mixed with acids.”
Magnesia, thus freed from “air” by ignition, was dissolved in “spirit of vitriol” and thrown down with an alkali. Its weight was nearly equal to that which it possessed before calcination, and it again effervesced with acids. “The air seems to have been furnished by the alkali, from which it was separated by the acid; for Dr. Hales has clearly provedthat alkaline salts contain a large quantity of fixed air, which they emit in great abundance when joined to a pure acid. In the present case, the alkali is really joined to an acid, but without any visible emission of air: and yet the air is not retained in it; for the neutral salt, into which it is converted, is the same in quantity, and in every other respect, as if the acid employed had not been previously saturated with magnesia, but offered to the alkali in its pure state, and had driven the air out of it in their conflict. It seems, therefore, evident that the air was forced from the alkali by the acid, and lodged itself in themagnesia.”
After an account of some experiments showing that magnesia is not identical with lime or with alumina, he proceeds:—“It is sufficiently clear that the calcareous earths in their native state, and that the alkalis andmagnesiain their ordinary condition, contain a large quantity of fixed air; and this air certainly adheres to them with considerable force, since a strong fire is necessary to separate it frommagnesia, and the strongest is not sufficient to expel it entirely from fixed alkalis, or take away their power of effervescing with acid salts.
“These considerations led me to conclude that the relation between fixed air and alkaline substances was somewhat similar to the relation between these and acids: that as the calcareous earths and alkalis attract acids strongly, and can be saturated with them, so they also attract fixed air, and are, in their ordinary state, saturated with it; and when we mix an acid with an alkali, or with an absorbent earth, that the air is then set at liberty, and breaks out with violence; because the alkaline body attracts it more weakly than it does the acid, and because the acid and air cannot both be joined to the same body at the same time.... Crude lime was therefore considered as a peculiar acrid earth, rendered mild by its union with fixed air; and quicklime as the same earth, in which, by having separated the air, we discover that acrimony or attraction for water, for animal, vegetable, and for inflammable substances.”
The solubility of slaked lime in water is next discussed. If a solution of lime “be exposed to the open air, the particles of quicklime which are nearest the surface gradually attract the particles of fixed air which float in the atmosphere.”
Black next points out that, on mixingmagnesia albawith lime-water, the air leaves the magnesia and joins itself to the lime; and as both magnesia and calcium carbonate are insoluble in water, the water is left pure. Similarly quicklime deprives alkalies of their air and renders them caustic. And it follows that if caustic alkali be added to a salt of magnesia or of lime, it will separate the magnesia or the calcareous earth from the acid, in a condition free from “air” but combined with water.
In order to show that the “air” which exists in combination with lime or alkalies is not the air which is contained in solution in water, lime-water was placed under an air-pump, along with an equal quantity of pure water; on making a vacuum, an approximately equal amount of air was evolved from each. “Quicklime, therefore, does not attract air when in its most ordinary form, but is capable of being joined to one particular species only, which is dispersed through the atmosphere, either in the shape of an exceedingly subtile powder, or more probably in that of an elastic fluid. To this I have given the name offixed air, and perhaps very improperly; but I thought it better to use a word already familiar in philosophy than to invent a new name, before we be more fully acquainted with the nature and properties of this substance, which will probably be the subject of my further inquiry.”
The next proceeding was to render “mild alkali” caustic by means of lime, and to determine that nearly the same amount of acid is required to saturate the caustic alkali as to saturate the mild alkali from which the caustic alkali had been prepared. On exposure to air for a fortnight, the caustic alkali again became mild, owing to its absorption of fixed air. Careful experiments were made to prove that such caustic alkali contains no lime, and does not therefore owe its causticity and corrosive properties to the presence of that ingredient. The volatile alkali (ammonium carbonate) was also rendered caustic, and Black “obtained an exceedingly volatile and acrid spirit, which neither effervesced with acids nor altered in the least the transparency of lime-water; and although very strong was lighter than water, and floated upon it like spirit of wine.”
After a description of some unsuccessful attempts to render mild alkalies caustic by heat alone (i.e.to expel carbon dioxide from potassium carbonate), Black examines the action of the “sedative salt” or boracic acid on mild alkalies, by rubbing them together in presence of some water. At first there is no effervescence, but on adding successive quantities of boracic acid, brisk effervescence finally takes place, borax being formed. “This phenomenon may be explained by considering the fixed alkalis as not perfectly saturated with air ... if they expel a small quantity of air from some of the salt, this air is at the same time absorbed by such of the contiguous particles as are destitute of it.” And on “exposing a small quantity of a pure vegetable fixed alkali (carbonate of soda) to the air, in a broad and shallow vessel, for the space of two months,” crystals were obtained, which possessed a milder taste than that of ordinary salt of tartar, which effervesced with acids more violently than usual, and which could not be mixed with the smallest portion of boracic acid without emitting a sensible quantity of air (hydrogen sodium carbonate). It therefore follows that such alkaline substances have an attraction for fixed air; and this was proved by mixingmagnesia albain fine powder with caustic alkali, and shaking for some time. The magnesia was converted into the variety which did not effervesce with acids, and the alkaliwas rendered mild, like a solution of salt of tartar. These are the principal results of Black’s researches, and he concludes with a table of affinity of acids for fixed alkali, calcareous earth, volatile alkali, and magnesia, contrasting it with the affinity possessed by fixed air for the same bases.
It was the habit of the Scottish students to pass down notes taken during the lectures of their professors from one generation to another. As the lectures were generally read, and not deliveredextempore, the process resulted in an almost verbatim report of the actual words of the lecturer. One of these copies of lectures, bearing the date 1778, gives an account of the experiments which have been described, in words almost identical with those used in the thesis of 1755. Black appears to have shown his class this air, made, however, according to Hales’ plan, by heating magnesium carbonate in a bent gun-barrel, and collected over water in the usual way. He demonstrated its weight by pouring it from one vessel to another, and showed that it extinguished the flame of a candle. He mentions also that in 1752 he discovered that this air is the same as choke-damp, and that it is fatal to animal life. He speaks of the Grotto del Cane, and observes that fixed airis produced by fermentation, and by the burning of charcoal, and showed to his class experiments in which air from each source is shaken with lime-water, giving a turbidity of carbonate. The well-known experiment of inspiring air through lime-water, which, owing to the small amount of carbonic anhydride it contains, does not produce a turbidity, and expiring through lime-water, showing the formation of carbon dioxide in the lungs, is described and performed. He next describes Cavendish’s experiments on the solubility of fixed air and its density, and researches by Dr. Brownrigg and Dr. Gahn of Sweden on its occurrence in mineral waters. He also explains how calcareous petrifactions are produced by the escape of fixed air from water, which then deposits its dissolved calcium carbonate, present in solution as bicarbonate. The deposit of iron from chalybeate waters is ascribed to the same cause, and the explanation is attributed to Mr. Lane.
“Upon the whole,” these manuscript notes relate, “this sort of air is quite distinct from common air, though it is commonly mixed with it in small quantity.” “With regard to its origin, when treating ofinflammable substances and metals I shall consider this more completely. I shall now only hint that it is a vital air, changed by some matter, seemingly the principle of inflammability. This appears from several phenomena when an animal or burning body is enclosed with a certain quantity of this air, until it is changed as much as possible.” The air is diminished in volume by the breathing of the animal or by the burning of the candle. And Dr. Priestley has found that “growing vegetables had the power of restoring this sort of air to common or vital air again, which must be by their taking away some matter which it had received from the burning body or animal.”
Black’s account of fixed air and its properties is the first example we possess of a clear and well-reasoned series of experimental researches, where nothing was taken on trust, but everything was made the subject of careful quantitative measurement. It was not long since Hales had pronounced air to be a chaotic mixture of effluvia. Black showed that common air contains a small amount of fixed air, and that fixed air must be considered as a fluid differing in many of its properties from common air, especially in its being absorbed by quicklime and byalkalies. It must be remembered that at that time carbon was not recognised as an element; and hence, though Black knew that fixed air was a product of the combustion of charcoal, he did not attribute it to the union of carbon with oxygen, although the sentence quoted above closely approaches to the truth.
The discovery of nitrogen was next in the order of time. It was made by Daniel Rutherford, a pupil of Black’s, and at his instigation, and its description formed a thesis for his degree of Doctor of Medicine.
Daniel Rutherford was born at Edinburgh on November 3rd, 1749. He was the son of a medical man, Dr. John Rutherford, one of the founders of the Medical School in that city. He was educated at Edinburgh University, and after graduating in Arts, became a medical student, taking his degree of M.D. in 1772. His diploma was obtained on 12th September. He then travelled for three years in England, France, and Italy, and in 1775 he returned to his native town, where he practised his profession. In 1786 he succeeded Dr. John Hope in the Chair of Botany in his University, but he did not on that account resign hispractice. He was president of the Royal College of Physicians of Edinburgh from 1796 to 1798. During the greater part of his life he suffered from gout; he died in 1819, at the age of seventy.
Rutherford does not seem to have pursued the study of chemistry further: his duties led him into other fields. His genial, pleasant face, seen in the portrait by Raeburn, shows him to have possessed a happy disposition; and he is said to have maintained until his death his friendship with Black, and his interest in the progress which science was then rapidly making.
The title of Rutherford’s dissertation, of which I have been able to find a copy only in the British Museum, isDissertatio Inauguralis de aere fixo dicto, aut mephitico. It was published at Edinburgh in 1772, seventeen years after Black’s memorable dissertation on Fixed Air. As will be seen shortly, it precedes Priestley’s and Scheele’s writings by a year or two. Evidently Black had noticed that a residue was left after the combustion of carbonaceous bodies in air, and absorption of the fixed air produced by the combustion, and had suggested to Rutherford, then a student of his, the advantage of further investigating the matter, and ascertaining the properties of the residual gas.
Daniel RutherfordDANIEL RUTHERFORD.
DANIEL RUTHERFORD.
Rutherford begins his essay with an apt quotation from Lucretius:—
Denique res omnes debent in corpore habereAëra, quandoquidem rara sunt corpora et aërOmnibus est rebus circumdatus appositusque.
Denique res omnes debent in corpore habereAëra, quandoquidem rara sunt corpora et aërOmnibus est rebus circumdatus appositusque.
He next proceeds to define the atmosphere as a pellucid thin fluid, in which clouds float and vapours rise. Its necessity for animal and vegetable life is acknowledged by all. It possesses weight and elasticity. It can be fixed by other bodies; but the air obtained from them by distillation differs from ordinary vital, salubrious air, and is often termed mephitic or poisonous.
After acknowledging his debt to his illustrious preceptor Black, he proceeds to quote from the latter to the effect that mephitic or fixed air is the air which proves fatal to animals and extinguishes fire; which is easily absorbed by quicklime and by alkaline salts; which occurs in the Grotto del Cane, and in mineral waters; and which is produced during exhalation from the lungs, by combustion, and during certain kinds of fermentation. Its density, compared with that of ordinary air, is as 15½ or 16 to 9; hence it can be kept for sometime in an open glass, and a candle lowered into it is extinguished. It has an agreeable taste and smell; and it changes the colour of syrup of violets from blue to purple. It prevents putrefaction, but putrefied bodies are not made fresh by it. It possesses the power of combining with lime, which acquires new properties as the result of its action. Rutherford then recalls Black’s experiments on lime and on magnesia, pointing out how these bases absorb fixed air, and how it can be recovered from them and from its compounds with alkalies, sometimes by heat, and always by the action of acids.
Rutherford next describes experiments which show that a mouse, placed in atmospheric air, and left till dead, diminishes the volume of the air by one-tenth; and that the residual air, on treatment with alkali, loses one-eleventh of its volume. The residue extinguishes the flame of a candle; but tinder continues to smoulder in it for a short time. It is thus proved that after the whole of the fixed air has been withdrawn by alkalies, the residue is still incapable of supporting life and combustion.
Some burning bodies deprive air of its “salubrity” more easily than others. The phosphorus of urine continues to glow in air in which a candle has ceased to burn, or in which charcoal has burned until it is extinguished. Even after the absorption of all fixed air by alkalies, phosphorus burns, emitting clouds of the dry acid of phosphorus, which can be absorbed by lime-water.
“It therefore appears that pure air is not converted into mephitic air by force of combustion, but that this air rather takes its rise or is thrown out from the body thus resolved. And from this it is permissible to draw the conclusion that that unwholesome air is composed of atmospheric air in union with, and, so to say, saturated with, phlogiston. And this conjecture is confirmed by the fact that air which has served for the calcination of metals is similar, and has clearly taken away from them their phlogiston.” Such air differs from the air evolved from metals by the action of acids, which is more thoroughly impregnated with phlogiston; and also from that from decaying flesh, which is a mixture of mephitic air and combustible air.
He proceeds:—“I had intended to add something regarding the composition of mephitic air, and to seek for a reason for itsunwholesome effects, but I have not been able to find out anything with certainty. Certain experiments appear to show, however, that it consists of atmospheric air in union with phlogistic material; for it is never produced except from bodies which abound in inflammable parts: the phlogiston appears to escape from such bodies when they become converted into the calces of metals. I say from phlogistic material, because, as already mentioned, pure phlogiston, in combination with common air, can be seen to yield another kind of air [viz. hydrogen].... I have lately heard that Priestley believes that vegetables growing in mephitic air dispel its noxious ingredients, or, as it were, extract them, and restore its original wholesomeness; and that mephitic air, added to air from putrid flesh, partly mitigates its unwholesome character. But I have been unable to try such experiments.”
We see, then, that Rutherford’s claims to the discovery of nitrogen amount to this:—he removed the oxygen from ordinary air by combustibles such as charcoal, phosphorus, or a candle; and having got rid of the carbon dioxide, in those cases when it was formed, by alkalior lime, he obtained a residue, now known as nitrogen. His view of the nature of this gas, in the phlogistic language of the time, was that the burning bodies had given up some of their “phlogistic material” to the air, which was thus altered. Nitrogen was “phlogisticated air,” even though incombustible; hydrogen, too, was phlogisticated air, but air produced by the union of pure phlogiston with atmospheric air. The step taken by Rutherford, under Black’s guidance, was an advance, though not a great one, in the development of the theory of the true nature of air; and he may be well credited with the discovery of nitrogen.
THE DISCOVERY OF “DEPHLOGISTICATED AIR”BY PRIESTLEY AND BY SCHEELE— THE OVERTHROWOF THE PHLOGISTIC THEORY BY LAVOISIER
We have seen that Stephen Hales must have prepared oxygen, among the numerous gases and mixtures of gases which he extracted from various substances; for, among the many materials which he heated, one was red-lead. The red-lead of that day, however, must have contained carbonate, because, as we shall see, Priestley always obtained a mixture of oxygen and carbon dioxide from that source. In the account of his researches, Hales only incidentally mentions the collection of gas from minium; and he appears to have made no experiments with the object of ascertaining its properties.
The discovery of oxygen was made nearly simultaneously by Priestley and Scheele, though it appears from the recent publication of Scheele’slaboratory notes by Baron Nordenskjöld that Scheele had in reality anticipated Priestley by about two years. His researches, however, were not published until a year after Priestley had given to the world an account of his experiments. Priestley had no theory to defend; his experiments were undertaken in an almost haphazard manner, probably as a relaxation. “For my own part,” he says,[4]“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 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.” On the other hand, Scheele was engaged in forming a theory of the nature of fire. He writes:[5]— “I perceived the necessity of a knowledge of fire, because without this it is impossible to make any experiment; and without fire or heat, itis impossible to utilise the action of any solvent. I began, therefore, to dismiss from my mind all explanations of fire, and undertook a series of experiments in order to gain as full knowledge as possible of these lovely phenomena. I ere long found, however, that it was not possible to form any correct opinion concerning the appearances which fire exhibits, without a knowledge of the air. After a series of experiments, I saw that air really is concerned in the mixture termed fire, and that it is a constituent of flame and sparks. I learned, moreover, that such a treatise on fire as this could not be compiled with thoroughness without also taking air into consideration.”
Scheele’s views concerning fire need not be mentioned here; but his researches on air are so methodical and so complete as to command our entire admiration. They remind us of those of Mayow, and had the latter lived a little longer, they would not improbably have been carried out by him. Since, however, Priestley had the advantage of priority of publication, we shall commence with an account of his researches.
Joseph Priestley was born in 1733 at Fieldheads, about six miles from Leeds. His father, a maker and dresser of woollen cloth, lost his wife when his son Joseph was about six years of age; and being poor, his sister, Mrs. Keighley, offered to bring up the boy. The early associations of the lad were closely connected with dissent; and after some time spent at a public school in the neighbourhood, he was sent, in 1752, to the Academy at Daventry, in which he was trained for the ministry. There he gained some knowledge of mechanics and metaphysics, and also acquired some acquaintance with Chaldee, Syriac, and Arabic, besides being a competent French and German scholar. After leaving the Academy, he settled at Needham in Suffolk, as assistant in a small meeting-house, where his income was not over £30 a year. His views were, however, too liberal for his hearers; and after some years he moved to Nantwich in Cheshire, where he preached and also taught a school. Here his income was improved, though still miserably small; yet he managed to buy some books, a small air-pump, and an electrical machine. He subsequently removed to Warrington, being employed there in teaching and in literary work; among his writings was aHistory of Electricity, which first brought him into notice, and which procuredfor him the degree of LL.D. of Edinburgh, thus giving him a right to the title of Doctor, by which he was always afterwards known. At Warrington, too, he married. We next find him being asked in 1767 to take the pastorship of Millhill Chapel at Leeds, a call which he accepted. The chapel was next door to a brewery, and this circumstance first induced him to take up the subject of the chemistry of gases, which has made his name famous. Here too he published hisHistory of Discoveries relative to Light and Colours. After six years spent at Leeds, he became librarian to the Earl of Shelburne (afterwards Marquis of Lansdowne) and travelled with him on the Continent. While with Lord Shelburne he published the first three volumes ofExperiments on Air, and carried out investigations which were recorded in a fourth volume, published after his removal to Birmingham. After some years spent in this way, he was pensioned off, and settled as minister of a meeting-house in Birmingham, where he employed his time partly in theological controversy, and partly in prosecuting researches in chemistry. He published during this period another three volumes giving a description of his experiments on air, and communicated severalpapers to thePhilosophical Transactionsof the Royal Society, of which he had been made a Fellow. Towards the year 1790 he was so unfortunate as to attack Burke’s book on the French Revolution; and this had the effect of rousing popular opinion against him, more especially that of the local clergy, whose political views he had frequently opposed. During the riots which took place at Birmingham in 1791, his house was burned, and he was obliged to escape to London under an assumed name. After some years spent in the charge of a meeting-house at Hackney, he left England for America. His opinions, though by no means uncommon at the present day, were so antagonistic to those of his English contemporaries that he was cut by his Fellows of the Royal Society, and he therefore resigned his Fellowship. And this feeling was in no way lessened by the action of the French Government of the time, which made him a Citizen of the Republic, and even chose him as a member of their Legislative Assembly. Arriving in America in 1795, he was well received, and settled at Northumberland, not far from Philadelphia. There he died in 1804.
Joseph PriestlyJOSEPH PRIESTLEY.
JOSEPH PRIESTLEY.
In Priestley’s work on gases he employed the form of apparatus which had been used by Mayow a century before. Such apparatus is indeed generally used now: the flasks with bent delivery-tubes, the Wolff’s bottles with two necks, and the pneumatic trough filled with water or mercury were his chief utensils. By means of such apparatus, gases can be collected in a state of comparative purity: they can be easily transferred from one vessel to another, and substances which it is desired to submit to their action can be readily introduced. Scheele, on the other hand, employed less convenient methods: his gases were generally collected in bladders, and their transference to bottles must have been attended with the introduction of atmospheric air. Scheele’s method was to allow a certain amount of gas to escape from the generating flask in order to expel air; an empty bladder was then tied over the neck, and the gas entered the bladder. When he wished to transfer the gas to a bottle, the bladder was tied at some distance from the neck, and its loose open end was secured by a string round the neck of a bottle full of water. The string confining the gas was then untied, and the bottle was inverted; the water ran into the bladder and was replaced by gas. A cork was also enclosed in the bladder, and it waspossible to push this cork into the neck of the bottle and re-tie the string which confined the gas; and then, by loosing the string which secured the bottle to the bladder, the full bottle could be conveyed away. This process is obviously a clumsy one, although in Scheele’s hands it yielded splendid results; and the methods which Priestley had borrowed from Mayow have attested their superiority by their survival.
The first gas which Priestley investigated was “nitrous gas,” or, as it is now named, nitric oxide. It had previously been prepared by Mayow (see p. 25) by the action of nitric acid on iron; and Mayow had made the important observation that when it was introduced into ordinary air confined over water, the volume of the air was decreased, and a rise of temperature occurred. But Mayow did not apply his discovery to the analysis of air, though he rightly conjectured that the reason of the decrease in volume of the latter was due to combination between the nitric oxide and his “fire-air particles.” It was left for Priestley to rediscover this fact, and to apply it to the analysis of air, or, as he expressed it, to the determination of its “goodness.”
Priestley’s use of a mercurial trough enabled him to collect and investigate various kinds of airs, among others “marine acid air” or gaseous hydrogen chloride, a gas differing entirely in properties from ordinary air. This made his mind familiar with the thought that different kinds of air exist, not necessarily modifications of atmospheric air. He had previously from his experiments come to the conclusion that “atmospheric air is not an unalterable thing, for that the phlogiston with which it becomes loaded from bodies burning in it, and animals breathing it, and various other chemical processes, so far alters and depraves it, as to render it altogether unfit for inflammation, respiration, and other purposes to which it is subservient; and I had discovered that agitation in water, the process of vegetation, and probably other natural processes, by taking out the superfluous phlogiston, restore it to its natural purity. But I own I had no idea of the possibility of going any farther in this way, and thereby procuring air purer than the best common air.”
On the 1st of August 1774, Priestley heated by means of a burning-glass red oxide of mercury. This was produced by heating mercury until itoxidised, and therefore had been untouched by acids, or by any substance which could have “imparted phlogiston” to atmospheric air. The resulting air was insoluble in water, and supported combustion better than common air, for a candle burned more brightly, and a piece of red-hot wood sparkled in it. This air he also produced from “red precipitate,” the product of heating nitrate of mercury; and at the same time from red-lead, or minium. It differed from “modified nitrous air,” in which a candle also burns brightly, inasmuch as shaking with water the gases produced after a candle had burned for some time in it did not deprive it of its power of supporting combustion; nor did it diminish the bulk of common air, as the nitrous air does in some degree. Priestley here refers to a mixture obtained by distilling nitrates, which is essentially a mixture of nitric peroxide with oxygen. A candle burns in such a mixture, depriving the nitric peroxide of part of its oxygen, and converting it into nitric oxide mixed with nitrogen. Nitric oxide, deprived of the excess of peroxide by shaking with water, with which the peroxide reacts and is absorbed, is no longer capable of supporting the combustion of a candle; and when added to ordinary air it combines with its oxygen, again forming nitric peroxide, which in its turn is absorbed by water.
Priestley’s experiments were performed at intervals from August 1774 till March 1775, and at that date it occurred to him to mix with his dephlogisticated air some nitric oxide over water; absorption took place, and he concluded that he might assume his new air to be respirable. And what surprised him especially was, that even after addition of nitric oxide and agitation with water, the residue still supported the combustion of a candle. A mouse, too, lived half an hour in the new air, and revived after being removed; whereas similar experiments with an equal volume of common air had shown that, after respiring it for a quarter of an hour, a mouse was indisputably dead. Even after the mouse had breathed it for so long a time, it was still capable of supporting the combustion of a candle; and this induced him to add more nitric oxide to the respired air, when he found that a further contraction occurred. He reintroduced the same unfortunate mouse into the remainder of the air—a portion to which nitric oxide had not been added—when it lived for another half-hour, and was quite vigorous when withdrawn.
Subsequent experiments with nitric oxide showed that air from red precipitate or from “mercurius calcinatus” (red oxide of mercury in each case, although prepared in different ways) was “between four and five times as good as common air.” He proceeds:[6]— “Being now satisfied with respect to thenatureof this new species of air, viz. that being capable of taking more phlogiston from nitrous air, it therefore originally contains less of this principle, my next inquiry was, by what means it comes to be so pure, or, philosophically speaking, to be so muchdephlogisticated.” He therefore went on to heat the various oxides of lead, but without any special results worth chronicling. On moistening red-lead with nitric acid, however, and distilling the mixture, he obtained, in successive operations, air which was “five times as good” as common air. This process formed lead nitrate, which on distillation yielded nitric peroxide and oxygen; the gas was, of course, collected over water, which absorbed the peroxide, allowing pure oxygen to pass. He found that red-lead was not the only “earth” which produced this effect; but that “flowers of zinc” (zinc oxide),chalk, slaked lime, and other substances also gave a gas, when distilled with nitric acid, which was “better” than common air. In some cases he broke up nitric acid by heat into water, nitric peroxide, and oxygen; in others he heated nitrates. His conclusion is: “Atmospherical air, or the thing 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.”[7]
When such experiments were made by heating nitrates in a gun-barrel, “phlogisticated air” was obtained. This was nitrogen, for the iron had reduced the oxides of the latter, and combining with their oxygen, had formed nitrogen; moreover, it had absorbed to a greater or less extent the oxygen simultaneously produced.
Having concluded that respirable air was a compound of nitrous acid, phlogiston, and earth, Priestley endeavoured to ascertain what was the nature of this earth. He concludes “that themetallic earths, if free from phlogiston, are the most proper, and next to them thecalcareous earths.”
“Dephlogisticated air may be procured from any kind of earth with which the spirit of nitre will unite.” A few quantitative experiments would surely have refuted this erroneous conclusion. Those which he attempted to make were very crude. A bladder (of which he does not give the capacity) was filled with
He concludes (taking into consideration that inflammable air is very light) “that the less phlogiston any kind of air contains, the heavier it is; and the more phlogiston it contains, the lighter it is.”[8]Strange that this should not have led to the rejection of the phlogistic hypothesis!
Priestley had the curiosity to breathe his “good” air. He says: “My reader will not wonder that, after having ascertained the superior goodness of dephlogisticated air by mice living in it, and the other tests above mentioned, I should have the curiosity to taste it myself.I have gratified that curiosity by breathing it, drawing it through a glass syphon, and by this means I reduced a large jar full of it to the standard of common air. The feeling of it to my lungs was not sensibly different from that of common air, but I fancied that my breast felt peculiarly light and easy for some time afterwards. 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.”[9]
It will be seen from this account that Priestley’s work was to some extent that of an amateur. He performed experiments, often without any definite object; and he was not always successful in devising theories. As before remarked, his chemical pursuits were to him a recreation, and were undertaken during the intervals of his necessary work. His mind was therefore not given over to them alone; and this is to be seen from the character of his writings. His style is a delightfully familiar one: he exposes his inmost thoughts with perfect frankness, and his writings are therefore very readable.—We have now to compare his work with that of his contemporary, Scheele, whose mission in life was that of a chemist; and the reader will be interested in noting the different points of view which these two eminent discoverers adopted.
Carl Wilhelm Scheele was born on the 9th of December 1742 in Stralsund, the capital of Swedish Pomerania, where his father was a merchant and a burgess. He was the seventh of eleven children. After receiving his education, partly in a private school, partly in the public school (gymnasium) at Stralsund, he was apprenticed at the age of fourteen to the apothecary Bauch in Gothenburg. In those days an apothecary was in large measure a manufacturer as well as a retailer of drugs. He had to prepare his medicines in a pure state from very impure materials, as well as to mix them in order to carry out prescriptions; and, indeed, he himself often, as sometimes happens still, ventured to prescribe in mild cases. Scheele’s master taught him such methods, and in addition instructed him in the use of the chemical symbols in vogue at that date; these he afterwards freely employed in his manuscripts, and this renders them exceedingly difficult to decipher. There still exists acatalogue of the drugs his master kept; many of them are of a fantastic nature, such as “ointment of vipers”, “human brain prepared without heat”, etc.; but among them were many of the well-known salts of metals, and the commoner acids, besides phosphorus, sulphur, rock-crystal, some ores, and some carbon compounds; for example, benzoic acid and camphor. There was a fair chemical library, which included the works of Boerhaave and Lemery, and his master devoted much pains to his instruction. In a letter to Scheele’s father, however, he expressed a fear that too great devotion to study and experimental work would undermine the health of a growing lad.
In 1765 the business was sold, and Scheele obtained a situation in Malmö with an apothecary named Kjellström. His master testified that he had extraordinary application and ability, and related that he was in the habit of criticising all that he read, saying of one statement, “This may be the case”; of another, “This is wrong”; of a third, “I shall look into this.” His memory was prodigious: he is said never to have forgotten anything which he had read relating to his favourite subject. He took little interest in anything else, and both his employers appear to have encouraged him to the utmost in his favouritepursuit. In 1768 he left Malmö for Stockholm; but here the exigencies of his duties interfered with his leisure for experimentation. While there, in conjunction with his friend Retzius, he discovered tartaric acid, which up till then had never been separated from tartar, its potassium salt. Here too he made investigations on the acid of fluor-spar (hydrofluoric acid); but finding his time too greatly occupied with routine work, he took a situation at Upsala, the seat of the largest university of Sweden, in 1770. At that time Bergman was Professor of Chemistry there, and Linnaeus occupied the Chair of Botany; both had then achieved a wide reputation. With Bergman he soon established close relations, and Retzius wrote that it was difficult to say which was pupil and which teacher. While at Upsala he wrote his great work onFire and Air, which we shall shortly have to consider. From his laboratory notes it appears that before 1773 he had obtained oxygen by the ignition of silver carbonate, red mercuric oxide, nitre, magnesium nitrate, and from a mixture of arsenic acid and manganese dioxide. Here too he discovered chlorine, and made researches on manganese, arsenic, and baryta. In 1775 he was elected a member of theRoyal Swedish Academy of Sciences, an honour which much improved his social status. In the same year he became manager of a business at Köping, where he passed the rest of his days, in spite of urgent appeals to engage in more remunerative work; indeed, he was strongly pressed to go to Berlin, and also, it is said, to London, for his publications had led to his recognition as one of the greatest chemists of the age. His book onFire and Airwas not published for some years after the manuscript had been in the printer’s hands. We learn from his letters that he was much afraid of being anticipated in his discoveries, as indeed events showed that he had reason to be.
From his letters and from the verdict of his contemporaries, Scheele is depicted as an amiable and honourable man, singularly free from vanity and selfishness. Unfortunately no portrait of him has survived. His last memoir on the action of sunlight on nitric acid was published in 1786; he died suddenly at the early age of forty-three in May of that year, two days after his marriage to Sara Margaretha Pohl. His devotion to science had told on his health, and his death was caused by a complication of diseases. Yet he was during his life, as after hisdeath, regarded as one of the greatest of chemists: his great knowledge, extraordinary aptitude in experimenting, and high intellectual powers placed him among the foremost men of science of his day.
Near the beginning of hisTreatise on Air and Fire,[10]Scheele defines air. It is that fluid invisible substance which we continually breathe; which surrounds the whole surface of the earth, is very elastic, and possesses weight. “It is always filled with an astonishing quantity of all kinds of exhalations, which are so finely divided in it that they are scarcely visible, even in the sun’s rays.”[11]It also contains another elastic substance resembling air, termed aerial acid by Bergman (identical with Black’s fixed air). Since atmospheric air has not been completely converted into fixed air by admixture of foreign materials, “I hope I do not err if I assume as many kinds of air as experiment reveals to me. For when I have collected an elastic fluid, and observe concerning it that its expansive power is increased by heat and diminished by cold, while it still uniformly retains its elastic fluidity, but also discover in itproperties and behaviour different from those of common air, then I consider myself justified in believing that this is a peculiar kind of air. I say that air thus collected must retain its elasticity even in the greatest cold, because otherwise an innumerable multitude of varieties of air would have to be assumed, since it is very probable that all substances can be converted by excessive heat into a vapour resembling air.”[12]
After defining the properties characteristic of air, namely, its power of supporting combustion, its diminution by one third or one quarter during the combustion of any substance which does not produce any fluid resembling air, its insolubility in water, its power of supporting life, and the fact of its being favourable to the growth of plants, Scheele demonstrates that air must consist of at least two elastic fluids. This he proves by exposing it to “liver of sulphur” (polysulphide of potassium), when six parts out of twenty were absorbed. He obtained the same result by employing a solution of sulphur in caustic potash, and also by polysulphide of calcium, prepared by boiling lime-water with sulphur, and by means of yellowsulphide of ammonium. Nitric oxide, “the nitrous air which arises on the dissolution of metals in nitrous acid,” produces a similar contraction, and so also do oil of turpentine and “drying oils” in general. Dippel’s animal oil, obtained by distilling bones, and ferrous hydroxide, produced from “vitriol of iron” and “caustic ley,” or ferrous sulphate and caustic potash, may also be used as absorbents; as may also iron filings moistened with water, a solution of iron in vinegar, and a solution of cuprous chloride. “In none of the foregoing kinds of air can a candle burn or the smallest spark glow.”
He accounts for these results by the theory that all such absorbents contain phlogiston, which is attracted by the air, and, combining with it, diminishes its bulk. The alkalies and lime attract the vitriolic acid of the sulphides used, and the air attracts the phlogiston. “But whether the phlogiston which was lost by the substances was still present in the air left behind in the bottle, or whether the air which was lost had united and fixed itself with the materials, such as liver of sulphur, oils, etc., are questions of importance.”[13]The conclusion that such air, which had received phlogiston and had contractedin volume, ought to be specifically heavier than common air was, however, rudely dissipated by experiment. The air must therefore contain two fluids, one of which does not manifest the least attraction for phlogiston, while the other is peculiarly disposed to such attraction. “But where this latter kind of air has gone to, after it has united with the inflammable substance, is a question which must be decided by further experiments, and not by conjectures.”[14]
To decide this question, Scheele burned in air substances such as phosphorus, which do not produce by their combustion any kind of “air.” The result was that the air lost 9 volumes out of an original 30, or about one-third of its bulk. A flame of hydrogen burning in air caused it to lose one-fifth of its volume. On burning a candle, some spirits of wine, or some charcoal, in a confined quantity of air, very little, if any, diminution of volume was noticed; but on shaking the air with milk of lime, contraction ensued, but not to the same extent as when phosphorus was burnt in it. This greatly puzzled Scheele; we now know that such combustibles are not able to remove all the oxygen, but thatthey are extinguished when only a portion of each has entered into combination. Here, again, however, his memory comes to his help, for he says, “It is known that one part of aerial acid mixed with ten parts of ordinary air extinguishes fire; and there are here in addition, expanded by the heat of the flame and surrounding the latter, the watery vapours produced by the destruction of those oily substances. It is these two elastic fluids, separating themselves from such a flame, which present no small hindrance to the fire which would otherwise burn much longer, especially since there is here no current of air by means of which they can be driven away from the flame. When the aerial acid is separated from this air by milk of lime, then a candle can burn in it again, though only for a very short time.”[15]Thus the question was correctly solved. Scheele’s acumen led him at once to make experiments admirably adapted to discover the true reason; he was not turned aside by any imaginary difficulties, but went straight to the point. He next burned sulphur in confined air, andfound little alteration of volume, but on shaking with clear lime-water, absorption took place, and one-sixth of the air was removed. “The lime-water was not in the least precipitated in this case, an indication that sulphur gives out no aerial acid during its combustion, but another substance resembling air; this is the volatile acid of sulphur, which occupies again the empty space produced by the union of the inflammable substance with air.”[16]
The next set of experiments were devised “to prove that ordinary air, consisting of two kinds of elastic fluids, can be compounded again, after these have been separated from one another by means of phlogiston.”
“I have already stated that I was not able to find again the lost air. One might indeed object that the lost air remains in the residual air which can no more unite with phlogiston; for, since I have found that it is lighter than ordinary air, it might be believed that the phlogiston, united with this air, makes it lighter, as appears to be known already from other experiments. But since phlogiston is a substance, which always presupposes some weight, I much doubt whether such hypothesis has any foundation.”[17]He had formerly conjectured that hydrogen, the “air” obtained by the action of vitriol on zinc, might be phlogiston; “still, other experiments are contrary to this.”
Scheele next directs attention to acid of nitre, and points out that when prepared in absence of organic material, it is nearly colourless; but that if phlogiston be given to it, it becomes red. At the end of a distillation of pure nitre with pure sulphuric acid, however, red fumes are produced: “Where does the acid now obtain its phlogiston? There is the difficulty.”
He collected some of this “red air” in a bladder containing milk of lime, to prevent its corrosive action; and having tried whether the resulting gas, which was now no longer red, would support combustion, “the candle began to burn with a large flame, whereby it gave out such a bright light that it was sufficient to dazzle the eyes. I mixed one part of this air with three parts of that air in which fire would not burn; I had here an air which was like the ordinary air in every respect. Since this air is necessarily required for the origination of fire, and makes up about the third part of our common air, I shall callit after this, for the sake of shortness, Fire-air; but the other air, which is not in the least serviceable for the fiery phenomena, I shall designate after this with the name already known, Vitiated air.”[18]How history repeats itself! Here is Scheele, in 1772, reproducing Mayow’s name “fire-air particles” for the same substance of which Mayow had inferred the existence a century before, and which he had pointed out as being present in the acid of nitre, as well as in common air.
This air is not a “dry acid of nitre converted into elastic vapours,” for it does not produce nitre with alkalies; moreover, it can be prepared from substances which have nothing in common with nitre, no compound of nitre having been used during their preparation. Scheele next describes experiments proving that “fire-air” is produced by the distillation of black oxide of manganese with concentrated oil of vitriol, or with the “phosphorus acid of urine” (phosphoric acid), by distilling nitrate of magnesium, made by dissolving the “white magnesia employed in medicine” (magnesium carbonate) inaquafortis(nitric acid), or by distilling “mercurial nitre” (mercuric nitrate). Thecheapest and the best method of producing “fire-air” is to distil purified nitre in a glass retort. But Scheele also obtained it from “calx of silver” (silver carbonate) prepared from silver nitrate and “alkali of tartar” (potassium carbonate); during this process he got aerial acid, which had been present originally in the alkali of tartar; but it was easily removed by means of milk of lime. Similarly, “calx of gold,” obtained from a solution of gold with “alkali of tartar,” gave “fire-air” when heated; but no aerial acid, for that air escapes during the precipitation of the “calx.” The brown-red precipitate obtained by adding “alkali of tartar” to “corrosive sublimate” (potassium carbonate to mercuric chloride, giving a basic carbonate of mercury and potassium chloride) yielded a mixture of fire-air and aerial acid when heated. But if the “calx of mercury” had been prepared by means of the “acid of nitre,” or in modern language by heating mercuric nitrate, a pure “fire-air,” unmixed with “aerial acid,” was the product. And lastly, arsenic acid, when heated, gave ordinary white arsenic together with “fire-air.”
This fire-air was completely absorbed by “liver of sulphur” (a polysulphide of potassium, formed by heating together potassiumcarbonate and sulphur); and a mixture of four parts of “fire-air” with fourteen parts of “vitiated air” lost the whole of its fire-air on standing for fourteen days in contact with liver of sulphur. Dippel’s animal oil, and burning phosphorus, charcoal, and sulphur, all absorbed “fire-air”—completely if it was pure, incompletely if it was mixed with “vitiated air”; in short, the identity of “fire-air” prepared from calces, etc., with that in ordinary air was completely established.
As “vitiated air” is lighter than ordinary air, it follows that “fire-air” must be heavier; and experiment proved this to be the case.
To completely disprove the possible contention that nitre was necessary for the production of “fire-air,” some “calx of mercury” (or red oxide), which had been prepared by boiling mercury for a long time in contact with air, was heated. The products were metallic mercury and “fire-air”. “This is a remarkable circumstance, that the fire-air which had previously removed from the mercury its phlogiston in a slow calcination, gives the same phlogiston up to it again, when the calx issimply made red-hot.”[19]Is it not remarkable that the true explanation should not have forced itself upon Scheele’s mind, which was so acute, and so capable of forming true deductions?
The next set of experiments dealt with the phenomena of respiration. A rat, confined in air until it died, polluted the air with one-thirtieth of aerial acid. Respiration from Scheele’s own lungs had the same effect. A few flies, bees, and caterpillars also polluted the air in the same way. Peas, roots, herbs, and flowers all converted about one-fourth part of ordinary air into “aerial acid”. “These are accordingly strange circumstances, that the air is not noticeably absorbed by animals endowed with lungs, contains in it very little aerial acid, and yet extinguishes fire. On the other hand, insects and plants alter the air in exactly the same way, but still they convert the fourth part of it into aerial acid.”[20]And so he makes experiments which prove that it is the fire-air which is converted into “aerial acid” by peas; and that “fire-air” is absorbed by fresh blood, and acquires no aerial acid from it. And, further, he was able to breathe fire-air for a long time, especially ifa “handful of potashes” was put into the bladder. A couple of large bees, confined in “fire-air,” along with milk of lime, consumed practically the whole of the air in eight days. But plants, confined in “fire-air,” along with milk of lime, would not grow; however, they yielded a little aerial acid. Scheele is again puzzled here by the circumstance that the blood and the lungs have not the same action on air as insects and plants, inasmuch as the former convert it into vitiated air, and the latter into aerial acid. We now know that air will not support life of warm-blooded animals when the oxygen falls below a certain not very small amount, while insects appear to be capable of exhausting the oxygen to a great extent; and it is probable that the plants, under the unnatural circumstances in which they were placed, gave off a considerable amount of carbon dioxide. Scheele’s explanation in terms of phlogiston is not successful. He wrote:—“I am inclined to believe that fire-air consists of a subtle acid substance united with phlogiston, and it is probable that all acids derive their origin from fire-air. Now if this air penetrates into plants, these must attract the phlogiston, and consequently theacid, which manifests itself as aerial acid, must be produced.”[21]This is reversing what may be termed the true explanation on the basis of the phlogistic theory. For Scheele supposes that oxygen contains phlogiston, and by losing it, yields carbon dioxide. On the other hand, the consistent explanation would be that carbon is carbonic acid plus phlogiston, and that when it burns it loses phlogiston and becomes carbonic acid again. We see how confused the phlogistic ideas became after the discovery of oxygen, and how ripe the time was for Lavoisier to formulate the views which are now universally accepted.
In the concluding sections of his treatise Scheele describes experiments which prove the solubility of “fire-air” in water; he mentions a convenient test for free oxygen in solution, viz. a mixture of ferrous sulphate and lime, which turns dark green, and finally rust-coloured, when added to water containing oxygen; and he shows that water is deprived of oxygen by the presence of a leech, kept in it for two days.
It is impossible not to recognise in Scheele one of the most acute intellects and able experimenters whom the world has ever seen. Andalthough we cannot but feel surprise that his discoveries did not lead him to take the step of renouncing the hypothesis of phlogiston, it must be borne in mind that the doctrine was surrounded with the halo of old age, and sanctioned by many names of great repute in their time. We shall see later that Cavendish, one of the greatest of English chemists, on weighing the rival theories, decided in favour of the phlogistic hypothesis. The actual escape of flame, a visible entity, from a burning substance, may have had much to do with this decision; and the uncertainty concerning the nature of heat, and the doubt whether it was not a form of imponderable matter, may have led both Scheele and Cavendish to retain the older views. It was Lavoisier who first dared to throw off the shackles of tradition; and this he did before oxygen had been discovered, as early as 1772.
Antoine Auguste Lavoisier was born in Paris on the 26th of August 1743. His father was wealthy, and spared no expense on his education. In his twenty-first year he obtained a gold medal from the Academy of Sciences for an essay on the best method of lighting the streets of Paris, butit was some years before he made definite choice of his subject. He published memoirs relating to geology and to mathematics, before the fame of Black’s and Priestley’s discoveries reached him and induced him to turn his attention to scientific chemistry. Lavoisier’s life was divided between his researches and the performance of public duties. In his twenty-fifth year he was elected a Member of the French Academy of Sciences, and, somewhat later, became its treasurer. He drew up numerous reports for the Government on questions on the borderland of Science and Technology; for example, on the preparation of paper for bills, which would not admit of forgery; on experimental agriculture; and on the manufacture of gunpowder. In 1771 he married Marie Anna Pierette Paulze, the daughter of a “fermier-général” or collector of Government revenue; and after his death, she became the wife of Count Rumford, another distinguished scientific man. Made a “fermier-général” himself, it was during his tenure of this office that Lavoisier was accused—along with others holding similar positions—of misappropriating revenue moneys, with the result that, under the dictatorship of the infamous Robespierre, he and twenty-eight of thosewho held like office were guillotined publicly, on the 8th of May 1794. It is stated that Lavoisier’s last plea, presented by Hallé—for permission to finish a research—was refused by Coffinhal, with the brutal phrase, “La Republique n’a pas besoin de savants; il faut que la justice suive son cours.” Within twenty-four hours the execution took place.
Lavoisier was a tall, handsome man, with a remarkably pleasing face. He possessed great influence, and used it all for good.
The first account which we possess of Lavoisier’s revolutionary ideas, for revolutionary they were then deemed, was in a sealed note, placed in the hands of the Secretary of the Academy on the 1st of November 1772. It is to the following effect:—
“About eight days ago, I discovered that sulphur, when burned, instead of losing weight, gains weight; that is to say, from one pound of sulphur much more than one pound of vitriolic acid is produced, not counting the moisture gained from the air. Phosphorus presents the same phenomenon. This increase of weight is due to a great quantity of air which becomes fixed during the combustion, and which combines with the vapours. This discovery, which I confirmed by experiments which I regard as decisive, led me to think that what is observed in the combustion of sulphur and phosphorus might likewise take place with respect to all the bodies which augment in weight by combustion and calcination; and I was persuaded that the gain of weight in calces of metals proceeded from the same cause. Experiment fully confirmed my conjectures. I effected the reduction of litharge in closed vessels with Hales’ apparatus, and I observed that at the moment of the passage of the calx into the metallic state, there was a disengagement of air in considerable quantity, and that this air formed a volume at least a thousand times greater than that of the litharge employed. As this discovery appears to me to be one of the most interesting which has been made since the time of Stahl, I thought it expedient to secure to myself the property, by depositing the present note in the hands of the Secretary of the Academy, to remain secret till the period when I shall publish my experiments.”
Lavoisier.
“Paris,11th November 1772.”