Chemical Education Today
From Past Issues
The Discovery of Oxygen and Other Priestley Matters by Kathryn R. Williams
The silhouette of Joseph Priestley, shown with his portrait in Figure 1, appeared on the cover of the July 1974 Journal of Chemical Education. In the same issue, Roy G. Neville’s article “Steps Leading to the Discovery of Oxygen, 1774” heralded the bicentennial of Priestley’s best known contribution to chemistry, the discovery of oxygen (1). This month’s focus on the atmosphere presents a fitting occasion to revisit Neville’s article and several other papers on Priestley and his many and diverse accomplishments. For an overview of Priestley’s life, readers may consult this Journal’s Profiles in Chemistry for September 1987 (2). However, I highly recommend reading a complete biography, for example, the books by Thorpe (3) and Holt (4). Born in 1733 in Fieldhead, near Leeds, Priestley grew up in surroundings heavily influenced by the English Dissenting religious movement. 1 His formal education at Daventry Academy (1752–1755) prepared him for a career in the clergy, and he served as minister to congregations in Needham Market and Nantwich (1755–1761), Leeds (1767– 1773), and Birmingham (1780–1791). Throughout his life, the ministry remained Priestley’s avowed profession, but his theological views aroused the disapproval of religious leaders, both the Anglican establishment and other Dissenting clergy. His nonconformist views extended to the realm of politics and government as well (5, 6). He actively spoke and wrote against the British monarchy, and he supported both the American and the French revolutions. Priestley’s radical opinions found disfavor in public circles as well as the press. For example, in the 1791 caricature shown in Figure 2, the artist, pseudonym Norbal Scratch, attacked Priestley from many sides (7). The title, “Dr. Phlogiston, the Priestley Politician or the Political Priest”, refers sarcastically to the theory Priestley championed throughout his scientific activities. The book under Priestley’s foot is open to a page labeled “Bible Explained Away”, and the two smoldering manuscripts bear the titles “Essay on Government” and “Political Sermon”. The paper in Priestley’s left coat pocket, labeled “Revolutionary Toasts”, refers to a dinner in Birmingham (which, in truth, Priestley did not attend) that commemorated the second anniversary of Bastille Day. In July 1791, public prejudice in Birmingham, fueled by insinuations like those in the cartoon, incited a raging mob to demolish Priestley’s church and home, including his laboratory and extensive holdings of scientific apparatus. He and his family escaped without physical harm, but negative public sentiments and threats of violence continued, and three years later Priestley and his wife emigrated to the newly formed United States of America. Although not entirely free from personal rebuke (8), he spent his final ten years with his sons in the peaceful, but remote, settlement at Northumberland, PA.2 Priestley’s interest in science originated with his teaching activities to supplement his early ministerial income and
Figure 1. Portrait (left) of Joseph Priestley at about 60 and his silhouette (right) that was prepared to commemorate the bicentennial of his discovery of oxygen. J. Chem. Educ. 1927, 4, 140 (left) and J. Chem. Educ. 1974, 51, July cover (right).
became more evident while he taught language and history at Warrington Academy (1761–1767). During that period, Priestley made yearly visits to London, where he met Benjamin Franklin, who encouraged and supported Priestley’s endeavors in natural philosophy. Franklin convinced Priestley to write his History and Present State of Electricity, which resulted in Priestley’s election to the Royal Society in 1766 (just one year after he received the degree of LLD from the University of Edinburgh for his Chart of Biography). When Priestley received the call to minister at Mill Hill Chapel in Leeds in 1767, he quite fortuitously moved into a house next door to the local brewery. Curious about the “fixed air” (CO2) emanating from the vats, Priestley investigated its properties and even discovered the refreshing taste of water impregnated with the gas. Thus began Priestley’s extensive studies of “pneumatic chemistry,” the predominant avocation for the remainder of his life. In 1773, William Fitzmaurice, second Earl of Shelburne, persuaded Priestley to leave Leeds to become his secretary and literary companion. The open-ended nature of the position left Priestley with adequate time for his scientific pursuits, and it was during this period that he produced his most important experimental work, including the discovery of oxygen. Neville’s article (1) traces the oxygen story to a centuryand-a-half before Priestley, when French physician Jean Rey noted the necessity of air for the calcination (oxidation) of metals. Later in the 17th century, Robert Boyle realized that the atmosphere plays an important role in combustion, and his colleague Robert Hooke proposed that the essential substance is also present in saltpeter (KNO3). In 1674, exactly one century before Priestley’s discovery, John Mayow concluded that atmospheric air consists of at least two components, one being Hooke’s nitre air.3 However, neither scientist
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Chemical Education Today
From Past Issues isolated oxygen, because methods to collect evolved gases were not available until the early 18th century, when Stephen Hales invented the method of water displacement. It was Priestley, however, who developed Hales’s idea into the pneumatic trough (Figure 3), which he used in his preparation of several “airs,” including oxygen. (He replaced the water with mercury to collect gases, such as NH3 and SO2, that react with water.) Priestley recorded his first preparation of oxygen on August 1, 1774. He had recently acquired a large convex lens (known as a “burning glass”), which he used to focus solar rays to heat various substances. On that day, he tested mercurious calcinatus (HgO), and “I presently found that by means of this lens, air was expelled from it readily…what surprised me more than I can well express, was, that a candle burned in this air with a remarkable vigorous flame…” (1). Astounded by these observations, Priestley at first thought that the gas resulted from an impurity in the mercurious calcinatus. However, in the fall of that year, while touring France with Lord Shelburne, Priestley obtained a sample of “the genuineness of which there could not possibly be any suspicion” (1). After returning to England, he verified his previous experiment. By March 1775, Priestley realized that the “air” was indeed something new, and that it could keep a mouse alive much longer than atmospheric air. After breathing the new air himself, he wrote, “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 be a fashionable article of luxury. Hitherto only two mice and myself have had the privilege of breathing it.” (2). Unfortunately, in the 18th century the budding science of chemistry was dominated by the Phlogiston Theory of Georg Ernst Stahl. According to Stahl, a substance burned because it contained phlogiston (the fundamental property of inflammability), which escaped during combustion. Extended to metals, this resulted in the backwards notion that during calcination phlogis-ton escapes; when the calx is reduced to the metal, the phlogiston is restored. In accord with the Phlogiston Theory, Priestley believed that atmospheric air was only partially saturated with phlogiston. Thus it could absorb more phlogiston from a burning substance. The flame was much brighter in the new gas because it contained no phlogiston (and hence could absorb much more). He named the new gas (O 2 ) “dephlogisticated air”, and called the part remaining after combustion (N2, CO2, H2O, etc.) “phlogisticated air”. Priestley clung steadfastly to the Phlogiston Theory, even after Lavoisier and others demonstrated its contradictions. Although Priestley devoted much of his final years to theological matters, his Northumberland writings include several articles and letters refuting the new system of chemistry (13, 14). How unfortunate that a man so open and forward in matters of politics and religion could not set aside a contra-
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dictory scientific theory, even though many of his own experimental results pointed to its fallacy! But, even though Priestley may have faltered as a chemical theorist, he more than made up for the weakness by his record of experimentation in a wide range of scientific disciplines and with his detailed and honest (almost childlike by today’s standards) written accounts of his observations and thoughts. To end this Figure 2. Caricature of Joseph Priestley enbrief look at titled “Dr. Phlogiston, the Priestley Politician Priestley the scien- or the Political Priest”, July 1, 1791. J. tist, I call attention Chem. Educ. 1931, 8, 2141. to his most important contribution to our knowledge of the atmosphere. Several years before the oxygen discovery, while still living at Leeds, Priestley tested a claim by the Count de Salucé that air spoiled by candle burning could be restored by exposure to cold. Priestley described his observations as follows: “Though this experiment failed, I have been so happy as by accident to have hit upon a method of restoring air which has been injured by the burning of candles, and to have discovered at least one of the restoratives which Nature employs for this purpose. It is vegetation. “…on the 17th of August 1771, I put a sprig of mint into a quantity of air in which a wax candle had burned out, and found that on the 27th of the same month another candle burned perfectly well in it. This experiment I repeated, without the least variation in the event, not less than eight or ten times in the remainder of the summer.” (3, pp 177–178).
Priestley also realized the consequence of his observations. In a subsequent paper, he wrote: “… plants, instead of affecting the air in the same manner with animal respiration, reverse the effects of breathing and tend to keep the atmosphere sweet and wholesome when it is become noxious in consequence of animals either living and breathing, or dying and putrefying in it.” (3, p 180).
Not only did Joseph Priestley discover the most important component of the atmosphere, he also understood the essential interplay of plants and animals in the cycling of oxygen in the environment.
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Chemical Education Today
From Past Issues Notes 1. The term Dissenters referred to a number of Protestant denominations that refused to conform to the tenets of the Church of England. During Priestley’s lifetime, Dissenters were allowed to hold services in licensed “meeting houses”. 2. On July 31 and August 1, 1874, a group of 37 chemists commemorated the first centennial of the discovery of oxygen with a meeting at Northumberland. During the event, several attendees, most notably Charles F. Chandler, suggested forming a society of American chemists. Although the official founding of the American Chemical Society took place in New York in 1876, the Priestley house is considered by some writers (9) to be the place where the Society was born (or, more properly, conceived). Due largely to the efforts of George Gilbert Pond of then Pennsylvania State College, the Priestley home was purchased and restored in the 1920s (10– 12). 3. In the 17th and 18th centuries, the word “air” referred to any gas. Nitre air was the gas (O2) obtained from nitrates.
Literature Cited 1. 2. 3. 4.
5. 6. 7. 8. 9.
Neville, R. G. J. Chem. Educ. 1974, 51, 428–431. Miller, F. A. J. Chem. Educ. 1987, 64, 745–747. Thorpe, T. E. Joseph Priestley; E. P. Dutton: New York, 1906. Holt, A. A Life of Joseph Priestley; Oxford University Press: London, 1931; reprinted by Greenwood Press: Westport, CT, 1970. Davis, T. J. Chem. Educ. 1933, 10, 348–349. Gordon, N., J. Chem. Educ. 1927, 4, 141. Newell, L. C. J. Chem. Educ. 1931, 8, 2138–2155. Newell, L. C. J. Chem. Educ. 1933, 10, 151–159. Goldschmidt, S. A. J. Chem. Educ. 1927, 4, 145–147.
Figure 3. Reproduction of the frontispiece of Priestley’s Experiments and Observations on Different Kinds of Air, showing a pneumatic trough and several other pieces of apparatus. J. Chem. Educ. 1974, 51, 429.
10. 11. 12. 13. 14.
Walker, W. H. J. Chem. Educ. 1927, 4, 150–157. Browne, C. A. J. Chem. Educ. 1927, 4, 159–171. Browne, C. A. J. Chem. Educ. 1927, 4, 184–199. Davis, T. L. J. Chem. Educ. 1927, 4, 176–183. Soloveichik, S. J. Chem. Educ. 1962, 39, 644–646.
Kathryn R. Williams is in the Department of Chemistry, University of Florida, P. O. Box 117200, Gainesville, FL 326117200;
[email protected].
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