CHAPTER III. The Discovery of “dephlogisticated Air” by Priestley and by

THE DISCOVERY OF “DEPHLOGISTICATED AIR” BY PRIESTLEY AND BY SCHEELE--THE OVERTHROW OF 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’s laboratory 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, “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:--“I perceived the necessity of a knowledge of fire, because without this it is impossible to make any experiment; and without fire or heat, it is 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.

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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 a History of Electricity, which first brought him into notice, and which procured for 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 his History 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 of Experiments 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 several papers to the Philosophical Transactions of 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.

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 was possible 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 it oxidised, 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:--“Being now satisfied with respect to the nature of 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 much dephlogisticated.” 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.”

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 the metallic earths, if free from phlogiston, are the most proper, and next to them the calcareous 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

Phlogisticated air, and weighed 7 dwts. 15 grs. Nitrous air " " 7 " 16 " Common air " " 7 " 17 " Dephlogisticated air " " 7 " 19 "

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.” 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.”

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.

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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 a catalogue 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 favourite pursuit. 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 on Fire 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 the Royal 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 on Fire and Air was 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 his death, 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 his Treatise on Air and Fire, 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.” 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 it properties 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.”

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 yellow sulphide 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.” The conclusion that such air, which had received phlogiston and had contracted in 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.”

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 that they 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.” 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, and found 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.”

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.” 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 call it 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.” 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) in aquafortis (nitric acid), or by distilling “mercurial nitre” (mercuric nitrate). The cheapest 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 potassium carbonate 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 is simply made red-hot.” 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.” 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 if a “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 the acid, which manifests itself as aerial acid, must be produced.” 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. And although 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, but it 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 those who 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.”

There is no account in Hales’ work of his reducing litharge in closed vessels. It is to be presumed that Lavoisier heated in a retort a mixture of litharge and charcoal, and that the air which he speaks of was a mixture of oxides of carbon. This account does not inform us of Lavoisier’s views on combustion, but merely shows the date at which he had first obtained what he supposed were results new to science. We recognise that Mayow had anticipated him in this.

It was not until Priestley, when dining with him in the autumn of 1774 (being in Paris with Lord Shelburne at the time), had informed Lavoisier of his discovery of “dephlogisticated” air, that the ideas of the latter upon the subject became precise. Priestley’s own words are:--“Having made the discovery some time before I was in Paris, in the year 1774, I mentioned it at the table of Mr. Lavoisier, when most of the philosophical people of the city were present, saying that it was a kind of air in which a candle burned much better than in common air, but I had not then given it any name. At this all the company, and Mr. and Mrs. Lavoisier as much as any, expressed great surprise. I told them I had gotten it from precipitate per se, and also from red-lead. Speaking French very imperfectly, and being little acquainted with the terms of chemistry, I said plombe rouge, which was not understood till Mr. Macquer said I must mean minium.”

Shortly after this, Lavoisier repeated Priestley’s experiments and confirmed their truth; and this led to the true explanation of experiments of which an account is given in the Memoirs of the French Academy for 1774, and which were fundamental in their character. They referred to the calcination of tin in hermetically-sealed retorts. The tin was placed in a retort which was heated on a sand-bath until the metal had melted. The beak of the retort, previously drawn out into a capillary, was then sealed, the air expelled having been collected and its weight noted. The retort was then cooled and weighed. It was again heated, and the temperature was maintained until the calcination of the tin stopped. With a large retort the calcination was more complete than when a smaller one was employed, this implying that the degree to which the calcination proceeded was dependent upon the amount of air present. After cooling the retort a second time, it was again weighed, when it was found to have undergone no change of weight. The beak was then broken, and air entered with a hissing noise. The gain in weight was now about 10 grains with a large retort. The tin and its calx were next weighed, and it was found that the gain in weight of the tin was always equal to the loss of weight of the air in the retort, measured by the quantity of air which entered on breaking the beak of the retort, less the air driven out of the retort before hermetically sealing it. From this Lavoisier concluded that calx of tin is a compound of tin and air.

Lavoisier’s next research, communicated to the Academy in 1775, and published in 1778, was entitled “On the Nature of the Principle which combines with Metals during their Calcination, and which increases their Weight.” In this he describes experiments showing that when metallic calces are converted into metals by heating with charcoal, a quantity of fixed air is expelled; and here for the first time he points out that fixed air is a compound of carbon with the elastic fluid contained in the calx. He then describes the preparation of oxygen by Priestley’s process of heating red oxide of mercury (mercurius precipitatus per se), and shows that the red oxide, when heated with charcoal, manifests the properties of a true calx, inasmuch as metallic mercury is formed, and a large quantity of fixed air is produced.

His next paper, which appeared in 1777 in the Mémoires of the Academy, deals with the combustion of phosphorus; and here he recapitulates Rutherford’s experiments, and shows that one-fifth of the air disappears, and that the residue, to which he gave the name “mouffette atmosphérique,” is incapable of supporting combustion. It will be remembered that Rutherford named this residue “phlogisticated air,” inasmuch as he imagined it to have absorbed phlogiston from the burning phosphorus; Scheele, too, had made a similar experiment with a similar result. From these observations, Lavoisier concluded that air consists of a mixture or compound of two gases, one capable of absorption by burning bodies, the other incapable of supporting combustion.

This paper was immediately followed by another, also published in 1777. Its title is, “On the Combustion of Candles in Atmospheric Air, and in Air eminently respirable.” In this memoir he distinguishes between four kinds of air:--1, Atmospheric air, in which we live and which we breathe. 2, Pure air, alone fit for breathing, constituting about one-fourth of atmospheric air, and termed by Priestley “dephlogisticated air.” 3, Azotic gas, identical with Rutherford’s “mephitic air,” and of which the properties were then unknown. 4, Fixed air, which he proposed to call “acide crayeux,” or acid of chalk, discovered twenty-five years previously by Black.

By this time his theory was well developed. He accounted for the phenomena of combustion without having recourse to the phlogistic hypothesis: the calx was produced by the union of the metal with the active constituent of air; and when carbonaceous material burned, the carbon united with this same constituent, producing fixed air. But there were still difficulties in his way: it was known that in dissolving metals in dilute vitriol or muriatic acid, a combustible and very light air was evolved; and that the metals were thereby converted into calces in combination with the respective acids. This fact was not explained even by the supporters of the phlogistic theory, but it had the effect of preventing them from accepting Lavoisier’s views. Some considered that hydrogen and phlogiston were identical, and that on dissolving a metal the calx was formed by the escape of the phlogiston; while others had a hazy idea that hydrogen was a compound of water and phlogiston; but of this more hereafter.

Lavoisier’s objection to such a theory was that the calx was heavier than the metal, and that hydrogen, though light, still possessed weight. Moreover, he had ascertained that the calces of mercury, tin, and lead are compounds of these metals with active air, and that as fixed air is produced by heating such calces with carbon, fixed air must be a compound of carbon and vital air, or, as he named it, the “oxygine principle,” inasmuch as its combination with phosphorus, sulphur, and carbon resulted in the formation of acids (=ὀξύς=, an acid).

In 1777 he read another memoir, “On the Solution of Mercury in Vitriolic Acid, and on the Resolution of that Acid into Aeriform Sulphurous Acid, and into Air eminently respirable.” Priestley had already shown that this process yielded sulphur dioxide; Lavoisier carried the temperature higher, and, decomposing the sulphate of mercury, produced metallic mercury, sulphur dioxide, and oxygen. It appeared therefore that sulphurous differed from sulphuric acid in containing a smaller proportion of oxygen.

He also experimented with iron pyrites, and his experiments recall those of Boyle. Boyle found that “marcasite,” a disulphide of iron, on exposure to air, gained in weight, while vitriol of iron was formed. Lavoisier performed the same experiment, not “in a very pure air,” as Boyle did when he left the pyrites exposed in a quiet dust-free room, but in a confined quantity of ordinary air; and he found that the air was rendered incapable of supporting combustion, or, in other words, its oxygen was removed.

In the same volume of the Memoirs of the Academy for 1778, another of Lavoisier’s papers--“On Combustion in General”--is to be found. In this he showed that oxygen gas is the only substance which supports combustion; that during the burning of combustible substances in air a portion of the oxygen disappears, and converts the burning substance into one of two kinds of compounds: either an acid, such as sulphuric acid from sulphur, phosphoric acid from phosphorus, or carbonic acid from carbon (for in those days the term “acid” was applied to what we now term an anhydride); or in the case of metals a calx, or compound of oxygen with the metal. The processes are analogous, but differ in the rate at which they take place; for the calcination of metals is a much slower operation than the combustion of sulphur or phosphorus. It is the rapidity of the action which leads to actual inflammation. He next examined and attacked the theory of phlogiston, and maintained that the existence of phlogiston is purely hypothetical, and quite unnecessary for the explanation of the phenomena. But his papers were received with doubt. The change demanded was too great; the trammels of custom were too firmly bound. He gained no converts.

Until the true nature of hydrogen had been explained, the attack on the phlogistic theory could not be said to be complete. This combination of hydrogen and oxygen to form water was first proved by Cavendish. And as soon as Sir Charles Blagden, in 1783, had communicated Cavendish’s results to Lavoisier, the latter at once saw their bearing on the new theory which he was endeavouring to uphold, and perceived how they would give a final blow to the adherents of the theory of phlogiston. For it had been frequently adduced as an objection to his new views, that they were incapable of explaining why hydrogen should be evolved during the solution of metals in acids, or why it should be absorbed during the reduction of calces to the metallic state. Lavoisier at once repeated Cavendish’s experiments on a large scale, and was assisted on that occasion by Laplace, Sir Charles Blagden also being present. A considerable quantity of water was produced, and the volumes of the combining gases were found to be 1 of oxygen to 1·91 of hydrogen. Shortly after, in conjunction with Meunier, he performed the converse operation, in decomposing steam by passing it over iron wire heated to redness in a porcelain tube. The iron withdrew the oxygen from the water, while the hydrogen passed on and was collected in the gasholder.

The explanation of the solution of metals in acids was now easy: it depended on the decomposition of water. While the oxygen united with the metal to form a calx, the hydrogen was evolved; the calx dissolved in the acid, forming a salt of the metal. And the operation of producing hydrogen by the action of steam on red-hot iron met with an equally simple explanation: the oxygen and iron united to form an oxide--the ancient ethiops martial--while the hydrogen escaped. The converse took place during the reduction of a calx to the metallic state by hydrogen. Here the hydrogen seized on the oxygen of the calx, removed it in the form of water, and the metal was left. These experiments were due to Cavendish; all that Lavoisier did was to show the true nature of the phenomena. The opponents of the new doctrines, Priestley chief among them, did their best to disprove the view that water was a compound of oxygen and hydrogen. But in vain. Many of Lavoisier’s opponents had to admit the justice of his views; and in 1787 De Morveau, Berthollet, and Fourcroy joined Lavoisier in reconstructing the nomenclature of chemistry on a new basis, which is substantially that in use at the present day. Black, too, was a convert, but Priestley and Cavendish remained true to their old faith, and one of Priestley’s last acts was to publish a defence of the phlogistic theory. We shall see later how Cavendish carefully considered the rival theories, and what reasons induced him to cast his vote for the older one.

Among the numerous memoirs which Lavoisier communicated to the Academy during the ten years between 1772 and 1782, one still remains to be mentioned. It was published as early as 1777, but it must be remembered that many of these memoirs were antedated. It referred to the respiration of animals; and Lavoisier concluded, on the ground that the phenomena of respiration are essentially similar to those of combustion and calcination, that the only portion of the air which supports animal life is the oxygen. The azote or nitrogen is inhaled along with the oxygen, but is exhaled unaltered. The oxygen, however, is gradually converted into carbonic acid; and when a certain amount, but by no means the whole, has been thus changed, the air becomes unfit for respiration. If the carbonic acid is withdrawn by means of lime-water or caustic alkali, the residue is air poor in oxygen, and the azote is the same as that left after the calcination of metals, or the burning of a candle, in air.

At the time of his impeachment, Lavoisier was engaged in experiments on perspiration, along with Séguin. He had nearly finished his experimental work, but had drawn up no account of it. His request that his life might be prolonged until he had compiled a statement of his results was refused; but Séguin, who was fortunately spared, undertook the task. The facts collected do not, however, bear directly on our subject, and shall not be further alluded to here.

This account of Lavoisier’s researches would be incomplete without a reference to his text-book of chemistry, Traité élémentaire de Chimie, in which his views are stated in order, and with great clearness. The nomenclature current at the time was so cumbrous, that it was almost, if not quite, impossible for the supporters of the new theory to express their meaning in an intelligible manner. De Morveau had suggested a nomenclature for salts; Black, too, had invented one; but neither of these systems was adapted to represent the new views. It was partly with the object of avoiding such embarrassment that Lavoisier wrote his Treatise.

He begins with a clear statement of what is generally termed “the states of matter”--solid, liquid, and gaseous--and points out that solids and liquids are almost all capable of change into the aeriform state by the addition of “caloric.” Proceeding next to the consideration of the nature of air, he shows that it must necessarily contain all those gases capable of existence at the ordinary temperature; and he explains how water-vapour must be one of them, seeing that even though water is a liquid at the ordinary temperature, it is capable, like many other liquids, of existing as vapour, when mixed with other gases. He next treats of the analysis of air, and describes his classical experiment of heating four ounces of mercury for twelve days in a retort communicating with a bell-shaped receiver, standing in a mercury trough. Having marked the initial height of the air in the jar by means of a piece of gummed paper, he found that, after twelve days’ heating close to the boiling-point, the air had diminished in volume by about one-sixth, and that the mercury had become covered with a red deposit of mercurius calcinatus per se, which, when collected, weighed 45 grains. The residual air in the retort and in the jar was incapable of supporting life or combustion; but the red precipitate, when heated, lost 3½ grains of its weight, yielding 41½ grains of metallic mercury, while it evolved 7 or 8 cubic inches of oxygen, capable of supporting the combustion of a candle vividly, and of causing charcoal to burn with a crackling noise, throwing out sparks. Oxygen was thus successfully separated from air, and obtained from it in a pure condition for the first time, in a single series of operations.

In Lavoisier we see a master mind, not only capable of devising and executing beautiful experiments, but of assimilating those of others, and deducing from them their true meaning. Although his additions to the known chemical compounds were few in number, and cannot be compared with those of Scheele or of Priestley, yet his reasoning in disproof of the phlogistic theory was so accurate and so exact that it rapidly secured conviction. With the exceptions already mentioned, almost all the eminent chemists of the day accepted his conclusions; and one, Kirwan, who had written a formal treatise in defence of the phlogistic theory, was so fair-minded, that after his work had been translated into French and published with comments, he acknowledged that the old theory was dead, and that truth had conquered.

It will be interesting now to trace Cavendish’s part in developing the history of the discovery of the constituents of air, and to note his arguments in favour of the phlogistic theory. Although Cavendish never publicly acknowledged its insufficiency, yet he had ceased to occupy himself with chemical problems at the time when its adoption was universal, and his true opinions have never been recorded.

Experiments and Observations on Different Kinds of Air, vol. ii. By Joseph Priestley, LL.D., F.R.S. Second edition (1776), p. 29.

Chemical Treatise on Air and Fire (1777), § 3.

Loc. cit. p. 46.

Loc. cit. p. 55.

Loc. cit. p. 94.

Loc. cit. p. 102.

The accurate translation of Scheele’s Treatise published by the Alembic Club (William F. Clay, 1894) has been made use of here.

§ 4.

§ 5.

§ 16.

§ 16.

§ 22.

§ 23.

§ 24.

§ 29.

§ 80.

§ 87.

§ 93.

This, as previously remarked, had already been noticed. In Maquer’s Élémens de Chymie-pratique, published in 1752, a work which ran through many editions, we read (p. 307): “There happens during all these calcinations, and especially in that of lead, a very strange phenomenon for which it is very difficult to assign a reason. It is that those bodies, which lose no small proportion of their substance, whether by the dissipation of phlogiston, or because part of the metal is exhaled as vapour, yield calces increased in weight after calcination; and this increase is by no means inconsiderable.... Physicists and chemists have devised many ingenious systems to account for this phenomenon, but no one of them is absolutely satisfactory. As no well-established theory has been devised, we shall not undertake to attempt an explanation of this singular fact.”