ENGLISH THEOLOGIAN AND CHEMIST
Joseph Priestley was a dissenting Unitarian minister in England at a time when adherence to the established Church of England was of great importance. Preaching was a difficult career for Priestley—because his Unitarian views were unpopular and because he spoke with a stammer. Priestley published widely in a variety of subjects, including theology, education, history, politics, and science. Most often, Priestley is remembered as one of the discoverers of oxygen, but his impact on other lives went much further than this.
His first major science publication was The History and Present State of Electricity (1767), which gained him admission to the Royal Society ; it was followed by The History and Present State of Discoveries Relating to Vision, Light and Colours (1772). Both light and electricity were regarded as aspects of the Newtonian "imponderable fluid" or "ether." They were called "imponderable fluids" (or subtle fluids) because they had no detectable weight (no "poundage"), but still had some fluidlike characteristics. Another form of this "ether" was "phlogiston." Phlogiston was the postulated substance of fire, the active principle of acids, and the driving force behind chemical reactions.
As Priestley expanded his studies in chemistry he became active in the field of pneumatic chemistry, the study of air and gases. Priestley was the first to isolate and characterize a number of gases, including oxygen, nitrogen, hydrogen chloride, ammonia, sulfur dioxide, carbon monoxide, nitric oxide , and nitrous oxide. Priestley's names for these compounds were different from the modern names, in part because he never adopted the oxygen theory of chemistry. The names he used were in terms of the older "phlogiston theory." Priestley did this work using very simple apparatuses, such as saucers, glasses, tubes, cylinders, and tubs of water or mercury.
Among the chemical phenomena he investigated was the behavior of a gas or other substance in contact with fire. If fire was the visible escape of phlogiston from a burning substance, then some gases had a greater affinity for phlogiston than ordinary air and encouraged the flame. Other gases had a lesser affinity for phlogiston than ordinary air (or no affinity at all) and would extinguish the flame.
One gas was found to be especially able to support a flame. Priestley called this gas "eminently respirable air." He later called this same substance "dephlogisticated air," reasoning that because it had a large affinity for phlogiston, it must be particularly devoid of it, or dephlogisticated. Priestley found that the heating of a sample of "red precipitate" (a calx of mercury) to produce pure mercury generated very pure dephlogisticated air. Priestley's discovery of the large amount of "air" generated during the heating of red precipitate was similar to Joseph Black's discovery of "fixed air." The production of dephlogisticated air also fit Priestley's belief that a metal is phlogiston compounded with a calc. The dephlogisticated air liberated from red precipitate also fit well with the observation that when a candle burned out in a closed vessel, the volume of the air was diminished. It was thought that the presence of phlogiston decreased the "springiness" of air. Thus, adding phlogiston to air would cause it to contract, and removing phlogiston from air would cause it to expand. Priestley also found that air saturated with phlogiston could be "revivified" (or dephlogisticated) by green plants in the presence of sunlight. Dephlogisticated air would be renamed "oxygen" by Antoine Lavoisier, who made it the cornerstone of his theory of chemistry.
Priestley resisted the oxygen theory of chemistry to the end of his life. For Priestley, phlogiston was more than just the active principle of fire—it was the active principle of life. Here Priestley's scientific theory merged with some of his religious beliefs. If phlogiston were the active principle of fire, heat, light, electricity, acids, chemical reactivity, and life, then it might also be the active principle of spirit. This accorded well with his Unitarian belief in one omnipresent active principle in the universe. In his book Disquisition on Matter and Spirit (1777), he asserted that there was only matter and void in the universe—there were no immaterial spiritual influences. Thus, the material existence of phlogiston corresponded well with his religious beliefs.
Priestley also had strong convictions in favor of broad-based democratic reforms and freedom of thought. He advocated wider religious toleration in England. He supported the American colonists in their revolution against the British Crown and supported the French Revolution, even in the face of atrocities such as the Reign of Terror. Priestley made enemies as a result of his political beliefs, and in 1791 his house and laboratory in Birmingham were attacked and burned by a mob. Priestley fled to London and was able to emigrate from there to the United States in 1794. In the United States he was a renowned international figure. When he landed in New York, both the mayor and the governor greeted him. When he arrived in Philadelphia, he was received by President Washington.
In 1794 Priestley declined an offer to be a professor of chemistry at the University of Pennsylvania. He retired from public life in Northumberland, Pennsylvania, and died there in 1804. His home in Northumberland is now preserved as a historical landmark. According to Peter Miller (1993), "A work entitled 'Joseph Priestley in Context' would . . . far surpass the competence of any single chronicler."
There never was a widespread coherent theory of phlogiston. German chemist Johann J. Becher (1635–1682) brought the term "phlogiston" into use among European chemists in the middle 1600s. The word is based upon a Greek word used by Aristotle in his writings on matter. German chemist Georg Stahl (1660–1734) further articulated the phlogiston theory in the early 1700s.
According to the phlogiston theory, a flame was thought to be the visible escape of matter called phlogiston from a burning substance. Another key feature of the theory was that a metal was thought to be composed of phlogiston and earth. Luster, high heat conductivity, malleability, and ductility are all unusual characteristics for metals, but according to the theory, metals share these features because of their postulated phlogiston content. If the phlogiston was removed from a metal, the result was an earth called calc (plural, calx), often the metal's naturally occurring ore. Under certain conditions, phlogiston might even exhibit a negative weight! This anomaly became problematic after Sir Isaac Newton's 1687 Law of Universal Gravitation.
—David A. Bassett
Lindsay, Jack, ed. (1970). Autobiography of Joseph Priestley. Cranbury, NJ: Associated University Presses.
Miller, Peter, ed. (1993). Joseph Priestley: Political Writings. New York: Cambridge University Press.
Partington, J. R. (1962; reprint 1996). A History of Chemistry , Vol. 3. New York: Martino Publishing.
Partington, J. R., and McKie, Douglas (1981). Historical Studies on the Phlogiston Theory. New York: Arno Press.
Priestley, Joseph (1767). The History and Present State of Electricity. London.
Priestley, Joseph (1772). The History and Present State of Discoveries Relating to Vision, Light and Colours. London.
Priestley, Joseph (1774–1777). Experiments and Observations on Different Kinds of Air , Vols. 1–3. London.
Priestley, Joseph (1777). Disquisition on Matter and Spirit. London.
Priestley, Joseph (1779–1786). Experiments and Observations Relating to Various Branches of Natural Philosophy , Vols. 1–3. London.
Priestley, Joseph (1796). Considerations on the Doctrine of Phlogiston and the Composition of Water. Philadelphia.
Priestley, Joseph (1800). The Doctrine of Phlogiston Established and that of the Composition of Water Refuted. Northumberland, PA.
White, J. H. (1932). The History of the Phlogiston Theory. London: Edward Arnold.