J. R. Parlington University
of London England
The Discovery of Oxygen
M a n y careful chemical authors1 say that oxygen was "discovered by Priestley on 1st August, 1774." At least as early as 1846* the opinion was expressed that, although he had obtained oxygen on August 1, 1774, Priestley did not then kuow that he had prepared a new gas and that his "effective discovery" of oxygen should be dated in March, 1775. I t is the object of the first part of this paper to give reasons for accepting the earlier date. The meaning of "discovery" is well known in chemistry. When a substance is prepared for the first time by a clearly described method, and sufficient details are given which distinguish it from other substances, it is s a i d t o be 'Ldiscovered." Glycerol was discovered by Scheele and not by Chevreul or Berthelot, ethylene by the Dutch chemists and not by Dalton. In a modern laboratory a student would be credited with an identification of oxygen if he had shown that: (I) a taper burnt in the gas with a very vigorous flame, (2) a glowing chip inflamed in the gas, and (3) the gas was practically insoluble in wat,er. Such a gas could not be nitrous oxide; the taper flames are quite different (as anyone who has seen them knows) and nitrous oxide is appreciably soluble in water. If the student were sent hack to try the action of nitric oxide he might think this was a little unfair, although useful. Priestley's Discovery of Oxygen
Priestley wrote up his discovery of oxygen early in 1775 and it was published in that year.s As usual, he told the story fully, clearly, and accurately, but it is important to appreciate that he had then formed a theory of the "true nature" of oxygen; it was a compound of nitric acid and an earth. We know from his
' DAW, SIR H., "Elements of Chemical Philosophy," 1812, p. 227; BRANDE,W. T.,"A Manual of Chemistry," Vol. 1, C., "A 1848, p. hi"; ROSCOE,SIR H. E., AND SCHORLEMMER, Treatise on Chemistry," Vol. 1, 1920, p. 240. Phil. Mag., 28, 498 (1846); HARCOURT, REV. W. VERNON, RODWELL,G. F., Nature, 27, 8 ( 1 8 8 2 t f u l l of errors; HARTOG, SIRP. G., Annak of Science, 5 , 31 (1941); and others. 3 PRIEST~Y J., , "Experiments and Observations on Different Kinds of Air," London, Vol. 2, 1775, pp. 29f; Alembic ClubReprint, No. 7 (1901). The illustration of the apparatus given in CONANT, J. B., "On Understanding Science," Yale University Press, New Haven, Conn., 1947, p. 79, Fig. 10; "Hasvsrd Case Histories in Experimental Science," Cambridge, Mass., 1, 1957, p. 95, Fig. 2, does not agree with Priestley's description. J., "Experiments and Observations on Different PRIESTLEY, Kinds of Air," London, 1774, p. 216 (this volume, the first in the series, is not numbered and the size is smaller than that of the remaining volumes).
own statement that Priestley's discovery was made in Calne, in Tiltshire, where he was staying (in his own house) in attendance on his patron, Lord Shelburne, and (as he repeatedly said) that it was accidental. He had been given a large burning glass by Parker, in London, and was trying the effect of heating wit,h it a number of substances given him by Warltire, among which was red oxide of mercury made by heating the metal for a long time in air, and hence called red precipitate, or mercury calcined, per se. Priestley put some of the substance into a small bulb filled with mercury, inverted the bulb in mercury, and heated the substance a t the top with the burning glass.&
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With this apparatus, after s variety of other experiments, . on the 1st of August, 1774, I endeavoured to extract air from mereurius calcinatus ner se lmercuric oxidel : and I vresentlv found that, by means oi this leis, air was expefied from-it very iesdily. Having got about three or four times as much as the bulk of my materials, I admitted water to it, and found that it was not imbibed by it. But what surprized me more than I can well express, was, that a candle burned in this air with a remarkably vigorous Hame, very much like that enlarged Hame with which a candle burns in nitrous air, exposed to iron or liver of sulphur as I know that no nitrous acid [i.e. in nitrous oxide]; but was used in the preparation of mweurius ealeinatus, I was utterly a t a loss how to account for it. In this case, also, though I did not give sufficient rtt,tention to the circumstance at that time, the flame of the candle, besides being larger, burned with more splendor and heat than in that species of nitrous air; and a piece of red-hot wood sparkled in it, exactly like paper dipped in a solution of nitre. . . ; an experiment which I had never thought of trying with nitrous air.
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Priestley had previously5 described the flame in the gas made by the action of iron on nitrous air (his name for nitric oxide); this
. . . makes it not onlv to admit a candle to hurn in it, but en-
The coveings on inorganic chem"istry as the work of a true scholar and teacher. He is now professor emeritus of Queen Mary College, University of London, having held the chair of chemistry there since 1919. His early work with Nernst sot his pattern of investigating reactions with the utmost precisian by physical methods. The same sense of integrity and precision is the ou& standing quality in his language far kommunication. The literature of chemistry is richer because he continues to make his encyclopedic knowledge available in presentations of meticulous clarity.
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I n November, 1774, Priestley found the same results with a specimen of red precipitate per se obtained from Cadet in Paris, which he was sure had not been made by heating mercuric nitrate, and at the same time he found that the gas after repeated agitation with water still supported the combustion of a candle with a strong flame, and hence was not nitrous oxide. He also obtained oxygen by heating red lead, which is not made with nitric acid. When he wrote up the results in 1775 Priestley was convinced that oxygen was a compound of nitric acid and earth, and he thought that red precipitate per se, made by heating mercury in air, and red lead. made bv heatinz litharze in air. had both imbibed nithc acid irom the atmosphere; ' in April, 1775, he wrote that he had obtained oxygen:
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...first from mermrius ealcinatus per se, red lead, &c.; and now, from many other substances, as quick-lime (and others that contain little phlogiston,) and spirit of nitre [nitric acid], and by a train of experiments demonstrated that the basis of our atmosphere is spirit of nitre. Nothing I ever did has surprized me more, or is more satisfactory.
When we are told that Priestley had not rerognized the "true nature" of oxygen until early in 1775 we should remember that by this he meant its composition from nitric acid and earth. not that it was a gas d~fferent from nitrous oxide, which he knew in the autumn of 1774. What he did in 1775 was to show by his nitric oxide test that the new gas was "between four and five times as good as common air" (we should say that air contains a quarter or a fifth of its volume of oxygen), and that it supported the respiration of a mouse longer thail common air. These were observations with a gas already known. For Priestley, the new gas was atmospheric air deprived of phlogiston, dephlogistieated air, its degree of phlogistication being one-fourth or one-fifth that of common air. When Cavendish d ~ s covered hydrogen in 1766, describing its preparation and its characteristic properties, he thought it was pure phlogiston, bnt this misconception of its "true nature" does not deprive him of the discoveiy of hydrogen. My argument is, therefore, that Priestley on August 1, 1774, had found properties which distinguished oxygen from the only other gas it could be (nitrous oxide), and that he had described these properties Even if he misunderstood the results, this does not deprive him of the discovery of oxygen on that date. When he discovered oxygen in August, 1774, Priestley had not formed his theoly of its "true Nature," that it was a compound of nitric acid and earth, for he says: I will frankly acknowledge, that, a t the commencement of the experiments recited in this section [the one with mercuric oxide beine ihe first he describesl. I was so far from havinc formed anv lnyputIst~~i+ t l u t led t o tie Jiwowriw I mtde i u p u r m i n ~tht~w, tlmr t l ~ \vould ? 1lilr.t.appc:srtd w r y i n p n h l r l e I,, n w I d I I r w u told of them.
Although not directly connected with the discovery of oxygen, I would like to mention another experiment by Priestley which does not seem to have attracted much attention. Boyle had made experiments on the calcination of tin in sealed retorts, but owing to his manner of working he thought the increase in weight when he weighed the retort after it had been opened, when he heard air rush into it, was due to the fixation of ponderable particles of fire. Lomonosov pointed 124
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out that the sealed retort should have been weighed after the calcination, and some work which he did not publish showed that there was no change in total weight. In describing some experiments "made in the Year 1773, and the Beginning of 1774," Priestley says:s That the cakes of metals contain air, of some kind or other, and that this air contributes t o the additional weight of the cnlces, above that of the metals from which they are made, had been observed by Dr. Hales; . . I had likewise found, that no weight is either gained or lost by the calcination of tin in a close glass vessel; but I purposely deferred making any more experiments on the subject, till we should have some weather in which I could make use of a large burning lens, which I had provided for that and other purposes; hut in the meantime I was led t o the discovery in a different manner. . I immediately filled a small phial with the red-lead, and heating i t with a candle, I presently expelled from it a quantity of air ahout four or five times the bulk of the lend, the air being received in a vessel of quicksilver.
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Priestley had obtained oxygen here, but he had not discovered it, since he gives none of its properties. Scheele's Discovery of Oxygen
The only independent discoverer of oxygen, apart from Priestley, was Scheele. His discovery was made before Priestley's, probably in 1771-72,' as is known from his laboratory notes, i.e. a t least two years before Priestley's. Scheele's book, "On Air and Fire," describing his discovery, was not published until 1777, but the manuscript was completed a t the end of 1775 and Bergman, in an introduction to the book, says that the work described in it was finished in 1775. Early in 1776 the printer sent the manuscript to Bergrnan, and the latter in his introduction says that he had repeated, with some modifications, the experiments described in it, and had confirmed them. One reason for t.he delay in publication was thus the time taken by Bergman in repeating the experiments. Bergman says: Chemistry teaches that, the elastic fluid which surrounds the earth a t all times and in all places has a unique composition, comprising three different materials, vis. goad air [oxygen], vitiated air [nitrogen] and aerial acid [carbon dioxide]. The first Priestley called, not incorrectly hut in a very forced way, dcphlogisticated air, Scheele on many grounds fire air, since it alone supports fire, whilst the other two extinguish it . . I h a w also repeated, with different modifications, the principal experiments on which he [Scheele] based his conclusions, and found them completely correct. If in consequence, in small subsidiary considerations, some slight rectification might be necessary, this is no way invalidates the main conclusions, based on many confirmatory researches. Heat, fire, and light have fundamentally the same components, good sir and phlogiston . . Of the kinds of matter now known, good air is the mast effective in removing phlogiston, which appears to be a true elementary matter which enters into most materials. For this reason I have placed good air a t the top of the phlogiston column in my new table In conelmsion I must mention that this of attractions . masterly work has been complete for two years [i.e. in 17751, although for various reasons, which it is superfluous to name here, it is only now published. In consequence, it has happened that Priestley, without knowledge of Scheele's work, has previously described various new properties belonging to the air. They are seen, however, to be of a different kind and presented in a different connexion.
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We know that Bergman corresponded with Priestley in the period when Scheele was doing his work, and it Zbid., p. 192.
' I have discussed the matter of date in the forthcoming volume of my "History of Chemistry," Vol. 3, pp. 219f.
was suggested that he may have informed Priestley of some of the latter. Priestley, in his account of his discovery of oxygen says: I am not conscious to myself of having concealed the least hint that was suggested to me by any person whatever, any kind of assistance that has been given me, or any views or hypotheses by which the experiments were directed, whether they were verified by the results or not.
No one who appreciates Priestley's absolute truthfulness can doubt his word, and I think it is certain that neither Priestley nor Scheele knew anything of the work of the other. The way in which Scheele was led to his discovery, the manner in which he made it, and the hypothesis which he proposed to explain his results, are well known8 and need not be repeated here. It will be sufficient to say that he obtained oxygen in a variety of ways: the decomposition of nitric acid vapor by heat, heating mercuric oxide and silver carbonate, heating manganese dioxide with sulphnric or arsenic acid, etc., and that he explained his results by the assumption that heat is a compound of fire air [oxygen] and phlogiston. He also, like Cavendish, identified phlogiston with hydrogen, so that in burning hydrogen in air he thought the product of the union of hydrogen and fire air was heat; he missed the formation of water since he worked over hot water.
passing into a jar of water.IO Then the bottom of the bulb was modertttely heated and more than half the jar was filled with air, which did not render lime water a t all turbid, but powerfully supported combustion and respiration. The calx at the same time was reduced to running mercury. Whence came this air? I reply, from the decomposition of the heat passing through the pores of the vessel, giving up phlogiston to the metal calx, which done, the air loses its faculty of penetrating the glass. The air was not contained in the ealx, since the calces of both nohle and common metals require phlogiston for reduction, not that this occurs differently but the former can attract it with such s. greater force that they can decompose heat in an experiment like this. The calces of silver and gold can similarly recover the metallic form and provide pure sir. The ealees made, not by heating but by precipitation with alkali [slkdi ccahonate], do not give pure air but this contaminated with aerial wid [carbon dioxide], which can eaaily he separated by lime water from the mixed air liberated by heat. With well washed gold calx this does not happen, sinceit rejects aerial acid.
We have previously noted the great force with which air removes phlogiston from iron and copper. Nitric acid has also a great affinity for this principle Nitrous air [nitric oxide] over water in an inverted bottle is an elastic, pellucid, colarlesa gas. When mixed with pure sir [oxygen] the mass instantly reddens and, as if with effervescence, heat is produced and the whole volume disappears. With common air more than a quarter disappears, corresponding with an equal goodness. These phenomena. are ascribed to the migration of phlogiston from the acid to the air, and are easily explained by what has been so well demonstrated hy the experiments of Mr. Seheele, that the matter of heat is nothing else than phlogiston intimately united with pure air, in which combination originate the heat prw duced and the diminution of the volume previously occupied. Nitric acid destroys platinum in the dry way [by fused nitre]. Then follow the cakes of nohle metals, which form the metals solely by the action of heat and without addition of phlogiston. Here the matter of heat is resolved into its principles, which Mr. Scheele has so shly demonstrated are pure air and phlogiston intimately united. Following him, I carried out an experiment in a somewhat modihed form. Half an ounce of mercuric nitrate was previously calcined by heat to red precipitate. This was introduced into a. small retort to which was adapted a long tube with the curved end
Scheele's theory was that a calx could be reduced to a metal in two ways. With calces of common metals, phlogiston had t o be supplied by a substance rich in it, such as charcoal. With calces of noble metals, which can be reduced by heating alone, the phlogiston was supplied by the decomposition of heat, which was a compound of fire air [oxygen] and phlogiston. The phlogiston reduced the calx to metal and the fire air was set free. Bergman's memoir, giving a summary of Scheele's work, was almost certainly published in 1775. Professor Gunnar Malmquist, Secretary of the Royal Society of Sciences of Uppsala, very kindly informs me that the date on which the memoir was communicated, and the date when this volume of the "Nova Acta" was published, are not available in the Archives of the Society. At a meeting on February loth, 1774, the vignette for the titlepage of Vol. 2 was discussed and a t a meeting on February 23rd the titlepage was approved. At a meeting on June 16th the bill for the printing of Vol. 2 was approved. I t seems t o follow that the volume was actually in print on that date, and hence Bergman's communication of Scheele's discovery of oxygen was a t least three months prior to Priestley's discovery, and the publication was in all probability before that of Priestley. Although it is commonly said that Scheele had lost priority of publication by some two years as compared with Priestley, Bergman's publication restores to him a date of announcement of his discovery a t least not later than Priestley's. The subject of the discovery of oxygen is one which has, perhaps, occupied historians of chemistry more than any other. There are very many publications on it and it might have seemed that nothing more of any significance could possibly remain. It is hoped, however, that the present short communication will not be without interest in the matter.
Ibid., pp. 219-229. "Nova Acta Regiae Societatis Scientiarum Upsaliensis," Val. 2, Uppsala, 1775, pp. 232-4.
'0 Bergman refers to a figure in a paper on the Aerial Acid in the same volume, reproduced in my "History of Chemistry," Vol. 3, p. 126.
Bergman's Memoir, 1775
Less well known is the fact that Bergman had published in 1775 a summary of Scheele's experiments, of his discovery of oxygen, and of his theory.= This occurs in a section of Bergman's memoir on affinity. In the part on phlogiston he says:
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* BERGMAN, T.,
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