Priestley Discovers Oxygen
Joseph Priestley's discovery of oxygen, a significant milestone in the history of chemistry, emerged from his amateur exploration of gases in the 18th century. Initially studying gases like carbon dioxide and hydrogen, Priestley employed innovative methods, such as using a pneumatic trough over mercury, to isolate various gases that had previously eluded observation due to their solubility in water. His key discovery occurred on August 1, 1774, when he focused sunlight on mercuric oxide, producing a colorless gas, which he initially thought was a different gas he had studied. This gas exhibited remarkable properties, such as supporting combustion and respiration more effectively than ordinary air. Priestley’s work ultimately drew the attention of chemists like Antoine-Laurent Lavoisier, who would later expand on his findings and rename the gas oxygen. Although Priestley remained committed to the phlogiston theory throughout his life, his investigations laid foundational insights into gas chemistry and air purity measurement. His legacy is complex, as he has been celebrated as a pioneer in pneumatic chemistry despite the contentious nature of his claim to the discovery of oxygen, overshadowed by contemporaries who reached similar conclusions.
Priestley Discovers Oxygen
Date August 1, 1774
By heating a brick-red compound of mercury, Joseph Priestley produced a gas whose properties of enhanced support of combustion and animal respiration led him to believe that he had discovered an amazing new substance: dephlogisticated air, or oxygen.
Locale Calne, Wiltshire, England
Key Figures
Joseph Priestley (1733-1804), English natural philosopher and Unitarian ministerCarl Wilhelm Scheele (1742-1786), Swedish chemist who, independently of Priestley, discovered “fire air,” or oxygenHenry Cavendish (1731-1810), English physicist and chemist who discovered inflammable air, or hydrogen, and the composition of waterAntoine-Laurent Lavoisier (1743-1794), French chemist who made oxygen the central element of his new chemistrySecond Earl of Shelburne (William Petty-Fitzmaurice; 1737-1805), British opposition leader, 1768-1782, and later prime minister, 1782-1783
Summary of Event
English natural philosopher Joseph Priestley came late to the study of gases (which he called airs), approaching them as an amateur rather than a professional. After discovering that “fixed air” (carbon dioxide) formed an effervescent liquid when dissolved in water (now known as soda water), he studied “inflammable air” (hydrogen), which was discovered by English physicist and chemistHenry Cavendish. Following a suggestion by Cavendish, Priestley began collecting other airs in a pneumatic trough over mercury rather than water, and he was thus able to isolate several new gases whose solubility in water had prevented previous chemists from seeing them. One of the first such gases he found was nitrous air (nitric oxide), which he prepared by combining “spirit of nitre” (nitric acid) with various metals. This substance provided him with a method for testing the “goodness” of common air for combustion and respiration. The quantitative measure of this goodness was the volume of a brown gas (nitrogen dioxide) that formed when he reacted nitrous air with the air in question.
After discovering an “acid air” (hydrogen chloride), Priestley delivered a paper in 1772 about his observations on the different kinds of airs. These discoveries, along with his liberal social and religious views, brought him to the attention of the second earl of Shelburne (later the First Marquis of Lansdowne and England’s prime minister), who offered him a position as his companion and librarian. The years Priestley spent in Shelburne’s service proved to be the most productive of his life. He continued his studies of new gases at Shelburne’s summer estate at Calne, near Bowood in Wiltshire. There Priestley collected over mercury an alkaline air (ammonia) that he obtained by heating a mixture of sal ammoniac (ammonium chloride) and quicklime (calcium oxide).
During this early period in Shelburne’s employ Priestley made the greatest discovery of his life, although in retrospect he realized that he had actually prepared the new gas earlier, in 1771 and 1772, but had not recognized it because he then believed that nothing was purer than ordinary air. Around the same time, Carl Scheele in Sweden prepared a gas that he called “fire air” (oxygen), but his results were not published until 1777. In subsequent accounts, Priestley claimed that his discovery of “dephlogisticated air” (oxygen) was the result of chance, not planning. He had purchased a large magnifying lens and was using it to concentrate the Sun’s heat on a variety of substances to see what gases were produced. A friend had given him an interesting brick-red substance, mercurius calcinatus per se, or red calx of mercury (mercuric oxide), and on August 1, 1774, Priestley focused sunlight on this red powder and observed that globules of liquid mercury and a colorless gas were generated. He collected this gas in an inverted vessel in a pneumatic trough filled with mercury and then studied its fascinating properties. It was not very soluble in water, but a candle flame burned faster and brighter in it than in common air. Initially, he thought that the gas might be “dephlogisticated nitrous air” (nitrous oxide), a gas he had earlier studied, but additional research showed that it behaved far differently from this other gas.
In the fall of 1774, Priestley accompanied Lord Shelburne on a trip to continental Europe, including a stay in Paris, where he met chemist Antoine-Laurent Lavoisier and told him about his experiments with the gas generated from red calx of mercury. This meeting proved to be fortuitous for Lavoisier and the future of chemistry, since Lavoisier would eventually make this elemental gas, which he named oxygen, the centerpiece of his reform of chemistry.
Upon his return from the Continent, Priestley discovered additional wonderful properties of dephlogisticated air, and some scholars date his effective discovery of oxygen to March, 1775, because it was then that he recognized oxygen as much better than ordinary air. His test involving two mice was particularly significant. He found during the test that a mouse confined in two ounces of the “new” air lived twice as long as a mouse confined in two ounces of ordinary air. Priestley even experimented on himself, experiencing a feeling of exhilaration when he breathed in the gas. Because he was an ardent believer in the phlogiston theory, one of whose pivotal doctrines was that combustible substances contained a weightless material, called phlogiston, Priestley named his new gas dephlogisticated air, since he considered it to be common air that had been deprived of its phlogiston and thus was able to readily absorb phlogiston escaping from burning materials.
During his later career, in England and America, Priestley continued to believe in the superiority of the phlogiston theory until his death in Pennsylvania in 1804. On August 1, 1874, Priestley’s great-grandson and many American chemists gathered at his grave site to commemorate the centennial of his discovery of oxygen. This meeting proved to be the beginning of what came to be called the American Chemical Society.
Significance
It is a central irony of Joseph Priestley’s career that the discovery he hoped would buttress the phlogiston theory ended up, in the hands of Lavoisier, totally undermining it. Besides this ironic significance, Priestley’s work on oxygen and several other gases contributed to solving the chief chemical problems of the time: calcination, the role of gases in combustion, the respiration of plants and animals, and the composition of common air. Because of his discoveries some scholars have called him the father of pneumatic (gas-related) chemistry. The test that he devised for the “goodness” of air has led to his designation as the father of eudiometry, the science of measuring air’s purity. Medical doctors have honored him because he was prophetic in his suggestion that oxygen be investigated as a potential healer of diseased lungs.
Priestley’s being credited for the discovery of oxygen has been met with controversy: Scheele had prepared oxygen prior to Priestley (though he failed to publish his findings before Priestley), and Lavoisier, who prepared oxygen after Priestley, nevertheless understood oxygen better than anyone. Furthermore, both Priestley and Scheele, as phlogistonists, interpreted their results in terms of a theory whose deficiencies had become obvious to Lavoisier and many others. Still, Priestley did bring reason to a new intellectual territory, that is, to the realm of different kinds of gaseous substances, and, in effect, he became the Christopher Columbus of this “new world” of chemistry.
Bibliography
Gibbs, F. W. Joseph Priestley: Revolutions of the Eighteenth Century. Garden City, N.Y.: Doubleday, 1967. In this biography for general readers, Gibbs portrays Priestley as the quintessential Enlightenment figure, whose rational approach to theology, politics, and science sheds light on various revolutionary changes during the Age of Reason. Illustrated, with a bibliography and an index.
Jaffe, Bernard. Crucibles: The Story of Chemistry. New York: Dover, 1998. This reprint of a classic work originally published in 1930 recounts chemistry’s history through the lives and achievements of the great chemists, including Priestley. Includes a sources section and an index.
Partington, J. R. A History of Chemistry. Vol. 3. London: Macmillan, 1962. The seventh chapter of this comprehensive history of chemistry is devoted to the life and contributions of Priestley, with many references to primary and secondary sources in the footnotes. Includes indexes of names and subjects.
Priestley, Joseph. A Scientific Autobiography of Joseph Priestley (1733-1804). Edited by Robert E. Schofield. Cambridge, Mass.: MIT Press, 1966. This collection of Priestley’s scientific correspondence, with helpful commentary by the editor, makes available in a narrative format many primary materials on Priestley’s career. Includes three appendices, a general bibliography, and an index.
Schofield, Robert E. The Enlightened Joseph Priestley: A Study of His Life and Work from 1773 to 1804. University Park: Pennsylvania State University Press, 2004. A comprehensive two-volume biography written by a leading Priestley authority. The first volume recounts Priestley’s early years in England; the second volume includes discussion of the final forty years of his life, examining his discovery of oxygen and his days in the United States.
Strathern, Paul. Mendeleyev’s Dream: The Quest for the Elements. New York: Berkeley Books, 2000. This popular account of the history of chemistry centers on the discoveries of the elements, and Priestley’s discovery of dephlogisticated air is an important part of this story. Includes a section on further reading and an index.