1 Roy G. Neville'
Krebs Engineers
Steps Leading to the Distovery of Oxygen, 1774 A bicentennial tribute to Joseph Priestley
On August 1, 1774, the English scientist Joseph Priestley (1733-1804) isolated gaseous oxygen, and this momentous experiment was later to prove (in the hands of Lavoisier) of the utmost importance to the development of modern chemistry. Combined with other elements and in the free state oxygen comprises approximately 50% of the total matter of the Earth's crust, forming by weight more than one-fifth of the atmosphere, approximately twothirds of the human body, and eight-ninths of all the water on this planet. The preparation, isolation, and characterization of this important element represents, therefore, an achievement of the greatest significance in the history of chemistry. The recognition~ofthe role oxygen plays in the calcination (i.e., oxidation) of metals, in plant and animal respiration and life ~rocesses. and in combustion. marked a turning point in the history of chemical the& and signalled the beginning of the modern period of its development. It is the purpose of this bicentennial tribute to review briefly the sequence of events preceding the actual discovew of oxvwn: ." . and to show that. as in all sctentific discovery, momentous experiments are usually the culmination of a long series of earlier experiments, intuitions, and hypotheses. The circumstances surrounding the discovery and isolation of oxygen by Priestley in 1774 are shown to he no exception, and the experiments he carried out are described largely in his own words, taken from his books and papers. Oxygen Foreshadowed As long ago as 1630, Jean Rey (-1582-after 1645), a physician a t PQrigord in France, published a small hook on the increase in weight of tin and lead on calcination (1). He attempted to explain this phenomenon by postulating that the increased weight of the calx was due to "thickened air" which "mixed itself with the calx." Rey did not state that the formation of the calx was due to the uniting of the air, in whole or in part, with the metal, and that the increase in weight was due to this union. Rey's contribution was, therefore, merely a focussing of attention on the relationship that exists between the calcination of metals (e.g., tin, lead) and the necessity of air for that process. By the middle of the seventeenth century, in England, both Robert Boyle (1627-91) and his colleague Robert Hooke (1635-1702), possessed some idea of the part played by air in combustion. In his famous hook The Sceptical Chymist (London, 1661), Boyle said that when charcoal is strongly heated in a closed retort it does not disappear, hut becomes black again on cooling (2). If air is admitted, the hot charcoal burns and crumbles to a white ash. Boyle concluded, therefore, that air plays an important hut unknown role in combustion f3), and he later spoke vaguely of a "volatile nitre" in the air (4). In his great work Micrographin (London, 1665), which
'For reprints: Gas Kinetics Division, 1205 Chrysler Drive, Menla Park, California 94035. 428
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Figure 1.
A portrait of Joseph P r e s t e y at about age 60
does not provide details of his chemical experiments on calcination and combustion, Hooke put forward in 12 propositions a theory in which he stated that combustion occurs by the agency of "a substance inherent, and mixt with the air, that is like, if not the very same, with that which is fixt in saltpeter." He claimed, but without experimental confirmation, that "in this dissolution of bodies by the air, a certain part is united and mixt." In 1677-79, Hooke did carry out quantitatiue experiments on the combustion of charcoal, on the structure and burning of candle flames, and the fusion of nitre (5). Had he continued experiments of this kind it is possible that Hooke might have discovered oxygen, which he termed "nitre air" since he suspected its presence in nitre and other salts. However, he would undoubtedly have been hampered in his investigations as the techniques for the successful handling of gases were not discovered until the careful work of Stephen Hales (1677-1761) during the first quarter of the eighteenth century, long after the death of Hooke (61. Exactly 100 years before Priestley's preparation and isolation of oxygen, John Mayow (1643-79) published his Tractatus Quinque Medico-Physici (Oxford, 1674), in which he advanced a theory of combustion similar
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Cover.--------------
The cover symbol, prepared to commemorate the bicentennial of Joseph Priestley's discovery of oxygen on August 1, 1974, also is the medallion for the Third Biennial Conference on Chemical Education to be held at the University Park Campus of The Pennsylvania State University July 30-August 3 of this year. A highlight of this conference is the Second "Centennial of Chemistry" Celebration featuring "Oxygen Day," August 1 and including an excursion to Priestley House in North&nberland,Pennsylvania where appropriate ceremonies are planned. The celebration also marks the 100th anniversary of the "Centennial of Chemistry" at Priestley House on July 31-August 1, 1874. This was the first national chemical congress held in the US., and at this meeting the committee that later founded the American Chemical Society was formed.
E X P E R I M E N T S A N D
O B S E R V A T I O N S ON DIFFERENT KINDS OF
A
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By JOSEPH PRIESTLEY, LL. D. F;R. S.
L O N D O N :
Printed for J. Jom~aow. No. 72, in St. Paul's Church-Yd.
MDCCLXXIV. Figure 2. Photograph at title page of volume I of Priestley's great six volume work on gases 1774-86.
to Hooke's, hut supported by elegant and ingenious experiments (71. Mayow concluded that air consists of at least two components, one of which is identical with Hooke's "nitre air." Mayow termed this component "nitro-aerial spirit," and stated that i t is this spirit that supports comhustion, is responsible for the calcination of metals, and is essential for respiration. Despite these correct insights concerning the role of "nitre air" in comhustion, calcination, and respiration, neither Hooke nor Mayow actually isolated a specimen of "nitre air" (i.e., oxygen), which is freely evolved as bubbles on fusing nitre. Again, this was most probably due to their inability to conceive a way of collecting the evolved gas. The Phlogiston Period
Boyle, Hooke, Mayow, and other investigators of the period were all brilliant and inventive men, but it must he remembered that during the 17th century and throughout most of the 18th century chemical theory (with certain modifications) was dominated by three all-embracing concepts: (1) the Aristotelian theory of the four "elements" (uiz. air, earth, fire, water); (2) the Paracelsian tria prima or "three principles" (uiz. philosophical "salt," "sulfur," and "mercury"); and (3) the theory of phlogiston. As accurate analytical methods had not yet been developed the chemical composition of even the simplest salts was not known in many cases. The concept of a chemical element had not yet been conceived (81, and there was no chemical atomic theory (9). Indeed, throughout much of the 18th centurv, the existence of atoms was actively questioned, and no r&ahle means for establishing their quantitative relationship existed. Although a large numhei of salts, and a few organic compounds; had heen prepared and descrihed, chemistry was still an "art" consistine of a heteroeeneous assemhlaee of IareeIv - . unconnectei empirical facts and ohsemation~. Promess made in the 17th century by the English scientists ~ o y l e Hooke, , and Mayow toward the correct explanation of the calcination of metals, and the role of oxygen in comhustion and respiration was interrupted, unfortunately, by the advent of the theory of phlogiston. Introduced by two German chemists, Johann Joachim Becher (1635-82) and his pupil Georg Ernst Stahl (1660-17341, the theory of phlogiston was a totally false, though temporarily useful, postulate which held its ground for almost a
Figure 3 . Photograph of frontispiece of volume I of Priestiey's work on gases referred to in Figure 2. This photograph aepicts the simple apparatus w e d in Priestley'e studies.
century. In his Physicae Subterraneae (1669). Becher stated that the components of all matter are "air," "water," and three "earths." One of the "earths" is inflammable (terra pinguis), the second mercurial, and the third fusible (or vitreous). These "earths" corresponded with the "sulfur," "mercury" and "salt" of the earlier alchemists. Becher postulated that, on comhustion, the "fatty earth" hums away, leaving the "mercurial" and "vitreous" earths (i.e. calx, ashes, etc.). Stahl refined the terra ~ i n e u i sconcent. statine that substances burn because they-contain ''phlogistonG (the ~ r i n c i ~ lof e inflammability), which is released during comhustion. Thus, when a metal is calcined phlogiston esy capes; when the original metal is recovered by reduction, phlogiston is restored to it. The loss of phlogiston on calcination implied that metals are mixtures, in accord with 18th century theories of the composition of matter. When zinc, for example, was heated in air to produce its calx, the reaction was explained as: zinc metal + air + heat = calx of zinc + phlo&on. The correct explanation is: Zn + Ik02 = ZnO. The increase in weight of metals on calcination was largely ignored by Stahl and most 18th century chemists. When asked why a calx weighed more than the metal from which it was formed, chemists said that phlogiston had "negative weight," and "buoys up" the metal, thus com~oundinethe error! With few exceptions. 18th century chemists were not aware of the importanceof the balance to chemistw, until Lavoisier in the 1770's proued its importance. ~ e k p i t eits logical inconsistenci&, the theory of phlogiston was the first "theory" to he used in chemistry with reasonable success, for it did appear to account for a large number of previously unrelated chemical observations, reactions and processes. For these reasons i t sewed a useful, if temporary, purpose. Priestley and the Discovery of Oxygen
It was against this background of chemical "theory" and largely empirical observation that Priestley began his classic experiments on gases. Stephen Hales had shown earlier (61 that "airs" (gases) could he collected in vessels filled with water and inverted over water. In 1770 Priestley wrote to his friend T. Lindsay: "I am now taking up some of Dr. Hales's enquiries concerning air" (10); and, in 1772, he published an important paper Obseruations on D~fferentKinds of Air (11). In that paper he described a fairly modem pneumatic trough for collecting gases over water, as well as the preparation of several new "airs" (e.g., "nitrous air," NO; "phlogisticated air," Nz; "nitrous vapour," NOz; "nitrous air diminished," N20; "acid air," HCI (collected over Hg); and "mephytic Volume 5 1 . Number 7, July 1974 / 429
air," COz). Priestley and other chemists of this period referred to gases as "airs," and he appears to have been the first to collect gases over mercury. In this manner several gases which are soluble in (and react with) water were isolated for the first time (e.g. NH3, SOz, SiFd. The discovery, preparation, isolation, reactions, and experimental handling of these gaseous compounds are described in a series of six volumes t h a t Priestley puhlished between 1774 and 1786 (12). Today these hooks are recognized to be among the great classics of early chemical literature (13). As early as 1771 (14), Priestley knew that air which had been "spoiled" (by putrefaction, the burning of candles, the breathing of animals, etc.) could he restored quickly to its original freshness by growing green plants (e.g., mint). Later (in 1778) Priestley discovered that aquatic plants growing in water containing dissolved "fixed air" (COz) gave off gaseous oxygen, and he said The injury which is continually done to the atmosphere by the respiration of such a large number of animals . . . is.. . repaired by the vegetable creation.
Priestley did not a t first comprehend the importance of light in the generation of oxygen by plants, and was anticipated in this discovery by the brilliant work on photosynthesis of J a n Ingen-Housz (1730-991, published in 1779 (15). In 1775 (16), Priestley stated that although air was regarded by all natural philosophers (scientists) as heing elementary (i.e., one of the Aristotelian "elements"), he was
. . . soon satisfied that atmospherical air is not an unalterable thing . . . (and that) . . . atmospherical sir is alterable, and therefore that it is not an elementary substance, but a composition. The crucial experiment by which Priestley first prepared gaseous elementary oxygen was carried out on August 1, 1774. This date should he remembered by all modern chemists, for it marks the birth of~"modern" chemistry, although the significance of this date was not appreciated a t the time. Earlier in 1774 Priestley had been given a large convex lens ("burning glass"), and he had attempted to extract "air" from a miscellaneous assortment of chemicals donated to him by his friend John Warltire (17391810). One of these chemicals was "red precipitate" (mercurius calcinatcls per se, i.e., mercuric oxide, HgO). This was prepared by the prolonged exposure to air of mercury heated to a temperature a few degrees helow its boiling point. The bright, orange-red "scales" were skimmed off the hot mercury, and it was this suhstance that Priestley investigated. The exact nature of this material had remained a mystery since the time of Boyle (17). Priestley heated a small quantity of the "red precipitate" by focussing the rays of the Sun on it contained in small phials filled with, and inverted over, mercury. He described this important experiment a s follows (18) Having afterwards procured a lens of twelve inches diameter, and twenty inches f w d distance, I proceeded with great alacrity to examine, by the help of it, what kind of air a great variety of substances . . . would yield . . . With this apparatus . . . on the 1st August, 1774, I endeavoured to extract air from mercurius eolcinatus per se; and I presently found that, by means of this lens, air "Liver of sulfur," made by fusing potash (KzCOd with sulfur, contains mixed polysulfides; K&, where n = 2-5. Priest1ey's"nitrous acid" was nitric acid, HN03 (not HNOA. J. J. Magalhaens (1722-SO), usually called Magellan, emigrated from Portugal to England in 1764. He carried out a great deal of important work on gases, and published several books. 5L. C. Cadet de Gassieourt (1731-9% an eminent French chemist, and director of the S h e s porcelain factory. $A. L. Lavoisier (1743-94), a brilliant chemist who was primarily responsible for overthrowing. the phlogiston theory and estahlishing the foundations of modern chemistry. 430
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was explled from it very readily. Having got about three or four rimes as much as the bulk of my materials. I admrtted water to it, and found that it war not imbihed hy rt. But what surprized me more than I can well express, was, that a candle burned in this air with a remarkably vigorous flame, very mueh like that enlarged flame with which a candle burns in nitrous air, exposed to iron or liver of s u l p h ~ r hut ; ~ as I had got nothing like this remarkable appearance from any kind of air besides this particular modification of nitrous air, and I knew no nitrous acid was used in the preparation of mercurius calcimtus, I was utterly at a loss how to account for it. Priestley also observed that
.. . 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, and it consumed very fast . . . That Priestley was totally a t a loss to account for the chemical behavior of this new "air" is brought out in the following statement At the same time that I made the above-mentioned experiment, I extracted a quantity of air, with the very same property, from the common red precipitate, which heing produced hy a solution of mercury in spirit d nitre, made me conclude that this peculiar property, heing similar to that of the modification of nitrous air ahove mentioned, depended upon something being eommunicated to it by the nitrous and since the mercurius calcimtus is produced by exposing mercury to a certain degree of heat, where common air has access to it, I likewise concluded that this suhstance had collected something of nitre, in that state of heat, from the atmosphere. Priestley discussed these experiments with his friend Warltire, telling him that the "air" he had obtained might be due to some impurity in the "red precipitate" (which had heen purchased "at a common apothecary's"), and might still contain a residue derived from the "nitrous" (i.e., nitric) acid. Warltire gave Priestley a fresh specimen of red precipitate, warranted to he "genuine" (i.e., pure), and
.. . this heing treated in the same manner as the former, only hy a longer continuance of heat, I extracted mueh more air fmm it than from the other. Priestley remained unconvinced even by this later experiment, however; and
.. . being at Paris in the October following, and knowing that there were several very eminent chymists in that place, I did not omit the opportunity, by means of my friend Mr. Magellan,'to get an ounce of mercurius calcimtus prepared by Mr. Cadet,J of the genuineness of which there could not possihly be any suspicion; and at the same time, I frequently mentioned my surprize at the kind of air which I had got from this preparation to Mr. Lav~isier,~ .. . and several other philosophers, who honoured me with their notice in that city; and who, I dare say, cannot fail to recollect thecircumstance. Since, in 1774, Priestley had kept his new "air" (made from HgO) completely confined over mercury he did not yet realize that this "air" was respirable I had no sus~icionthat the air . . . was even wholesome. so far was 1 from knowing w a r it was that I had really found: takrng rt for granted, that it was nuthlng more than such kmd ul air as I had brought nitrous air to b~ by rhe prueesdes above ment~oned
He mentioned in passing that he had also obtained this "air" from red lead (Ph304). and from minium (PhO). After his return from Paris, Priestley carried out further experiments (using Cadet's specimen of pure red precipitate) a t intervals until March, 1775, when it occurred to him to mix "dephlogisticated air" (as he termed oxygen)
with "nitrous air" (nitric oxide, NO) over water. As some absorption took place, Priestley concluded that his new " . alr might he respirable. He placed a mouse in the gas, a n d found that it lived comfortably for half an hour before hecomine unconscious. On exposure to fresh air, the mouse revived. Similar experi.ments witb an equal vdume of atmosnheric air had shown that, after breathing it for a quarter of an hour, another mouse (identical in size and weight) died. Even after a mouse had breathed the "air" fmm red precipitate for half an hour, the residual air was still capable of supporting the combustion of a candle. Priestley then courageously breathed some of this new " . a n himself, and said that his ". . . breast felt peculiarly light and easy for some time afterwards." He therefore recommended that i t might find use in medicine
sphere is spirit of nitre. Nothing I ever did has surprized me more, or is mare satisfactory.
3,
>9
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 privilegeof breathing it.
He adds, like the true theologian he was, that ". . . the air which Nature has provided for us is as good as we desewe." Although he had prepared gaseous oxygen as early as 1771, by the action of heat on nitre (KNOd, Priestley a t that time confused it with atmospheric air (19). To his credit, however, was the observation that a candle burned very brightly in this "air" from nitre. He attempted to explain the vigorous burning of a candle in this "air" (contained in a bell-jar) by postulating tbat during combustion the candle gave off phlogiston, which saturated the "air" in the jar, eventually extinguishing the candle. Like other eminent scientists of the time, Priestley believed tbat atmospheric air supports combustion because i t is only partially saturated witb, and is capable of absorbing more, phlogiston. Substances were said to bum in air with only a moderate flame because phlogiston is present; whereas in the "air" (i.e. oxygen) from nitre, etc., which contains less phlogiston, the flame is much brighter. Priestley concluded that his new "air" (from HgO) contained little or no phlogiston, and therefore named it "depblogisticated air." By the same reasoning, the gas remaining after a combustible substance had burned was named "phlogisticated air" (e.g., nitrogen). Thus, by Priestley's reasoning
In his memoir on respiration (21), Priestley said that the function of the blood is to discharge phlogiston from the system; and he demonstrated that air can react with blood through a thin animal membrane. Conclusion
Although Priestley's numerous experiments with both old and new gases were unquestionably brilliant, and will forever remain a monument to his experimental genius, his theoretical inferences were frequently erroneous. He had been raised with an almost blind faith in the validitv of the~phlogistontheory, and sought to explain his epoc6makin, discoveries in terms of this old hvnothesis. Priestley liGd until 1804, but died with an &shaken faith in the ~hloeistontheow (22). It remained for the eenius of ~avoisier-and his associates to interpret ~ r i e s t l e ~discovg eries correctlv, and to show that his "de~hlogisticatedair" a key role in accounting for-the oxidation (oxygen) of metals and non-metals, combustion processes, and the respiration of plants and animals. Literature Cited (11 Roy. J.. "Esssvs d s lean Rai." Barax. tES(1; reminted in faeaimilc bv McKie. D.. " T h ; ~ u a ~ ; ~ fJean R ~ . ' E . Arnold & Ca., h n d o n , 1951. ~ r n t i & t i ~da& e on the inercssa in weight of vsrioua metal. on calcination arc given by: Sprat, J., "The Histaw of tho Rwal Soeietv." landon. 1667. on. 228-229. facsimile reo.tot. d i t d &J. J. copeend H. w:jonea,kndan. 1966: (21 Neville,R.G.. J. CHEM.EOUC.,38.106(19611. (31 Boyle believed that combustion and respiration -re related, and that life luas analogous to a slow~bvningfleme. See: Boyle. R.. "Relation Betwixt Plamr and Air."London. 1672: " E s a y ~ oEffluviums," f London. 1673. (1)The concept of a "volatile nitre,"-released on heating nitre, was deaeribd by: Clsrka, W.,"The Natural History of Nitre," London, 1670. See also: Royle, R., "Suspicion8 about Some Hidden Qualities ofthe Air."landon, 1674. (51 Birch, T., "Hiataryofthe Royal Soeirty." landon, 1756-7, voi.3. pp.4M-69. (61 H e k , S., " V w t e b k Stafieks." London, 17W: "Heemsntatidr%" London, 1735 All ~ a s o swere "aim" to Hales. and although he prepared several chemicslly diiand eollaeted them ovei water, accurately mesavring their voiumes, ferent g-,
phlogisticated air (nitrogen) = air + phlogiston dephlogisticated air (oxygen) = air - phlogiston Although Priestley carried out many experiments on his new "air" (oxygen) from August, 1774, onwards, be confused it with nitrous oxide (NzO), which also supports combustion though not as vigorously as oxygen. Priestley admitted (20) that he was not aware of the true nature of "dephlogisticated air" (02) until after his return from Paris late in 1774. On March 1, 1775, he mixed "nitrous air" (nitric oxide) with his new "air," which immediately produced reddisb-brown "nitrous vapour" (nitrogen dioxide): 2NO + 0 2 = 2N02. Priestley quickly established that "nitrous vapour" (NOz) is slightly soluble in water, giving an acid solution. When he mixed "nitrous air diminished" (nitrous oxide, NzO) witb "nitrous air" (NO) there was no reaction. These experiments convinced him that the "air" he had prepared by heating mercurius calcinatus per se (mercuric oxide, HgO) was indeed an entirely new gas. Writing in April, 1775, Priestley said I have now discovered an air five or six times as good as common air . . . (which was) . . . got first from m e r e u r h calcinotus per se, red lead, &c.; and now, from many substances, as quicklime (and others that contain little phlogiston), the spirit of nitre, and by a train of experiments demonstrate that the basis of our atmo-
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~orihumborland.Pa 1800.
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