Almost forgotten in the twenty-first century, the phlogiston theory proposed by Georg Stahl became the dominant model explaining combustion and fermentation in eighteenth century chemistry. Although the theory was later replaced after the discovery of oxygen, the phlogiston theory for a time was the basis for all serious chemical experimentation and research. It constituted the first systematic and comprehensive theory of chemistry.

Chemistry at the beginning of the eighteenth century was still based primarily on the principles of Aristotle Aristotle and the ancient idea that all matter was composed of four elements: earth, water, fire, and air. Alchemy, Alchemy using a mixture of Aristotelian ideas and magical principles in its relentless search for a way to produce gold from “base” metal, dominated chemistry. Two unsolved mysteries of chemistry at the time involved the mechanisms of fermentation and combustion. A strong practical interest in metallurgy drove scientists to seek answers to the latter problem in particular. [kw]Stahl Postulates the Phlogiston Theory (1723)
[kw]Theory, Stahl Postulates the Phlogiston (1723)
[kw]Phlogiston Theory, Stahl Postulates the (1723)
[kw]Postulates the Phlogiston Theory, Stahl (1723)
Phlogiston theory
[g]Germany;1723: Stahl Postulates the Phlogiston Theory[0630]
[g]Prussia;1723: Stahl Postulates the Phlogiston Theory[0630]
[c]Chemistry;1723: Stahl Postulates the Phlogiston Theory[0630]
[c]Science and technology;1723: Stahl Postulates the Phlogiston Theory[0630]
Stahl, Georg Ernst
Becher, Johann Joachim
Lavoisier, Antoine-Laurent
Priestley, Joseph

Two phenomena associated with both combustion and fermentation had been observed: Both processes gave off heat (albeit extremely slowly in the case of fermentation), and both released some substance into the air. In the case of fermentation, the production of a colorless and odorless material was inferred from the bubbles it produced. In combustion, the release was more intense, as a great deal of heat was also produced. It was believed that combustion, like fermentation, released a colorless and odorless material in addition to the tangible smoke that it also produced. Smoke was known to contain airborne particles that later settled out of the air as soot or other substances.

A German chemist, Georg Ernst Stahl, developed a theory to account for these observations, the phlogiston theory. Although he had discussed this theory in earlier works, the publication of Fundamenta chymiae dogmaticae et experimentalis
Fundamenta chymiae dogmaticae et experimentalis (Stahl) (1723; the fundamentals of dogmatic and experimental chemistry) represented a more complete description. As a theory of combustion, Combustion which could also be generalized to fermentation, Fermentation it revolutionized chemistry in the eighteenth century. Stahl’s basic proposal was that a principle he called “phlogiston” (from the Greek phlogistos, meaning “burnt”) existed, to a greater or lesser degree, in all substances. During combustion, phlogiston was freed from the burning substance and released into the air. He considered phlogiston to be an intangible “principle,” rather than an element or substance with mass.

Many of Stahl’s concepts about chemistry came from his teacher Johann Joachim Becher, whose ideas combined Aristotelian philosophy and alchemy. Both Becher and Stahl were obsessed with the usual alchemical pursuit of the philosopher’s stone Philosopher’s stone[Philosophers stone] (a mythical substance that could turn base metals into gold), and they had strong interests in metallurgy and mineralogy. They considered all substances to be mixtures of a few simple elements and a handful of “principles.” Principles were indeterminate entities without apparent mass and included salt, sulfur, and mercury, as well as phlogiston. (Although the names of some of these principles, originated by Paracelsus [1493-1541], resemble the names of modern compounds or elements, they were not the same substances.)

Throughout the middle of the eighteenth century, the phlogiston theory was almost universally accepted. When applied qualitatively, it provided a plausible theory for what occurred during combustion. Substances were considered to have varying amounts of phlogiston: Flammable substances contained the most phlogiston, which was why they burned. During combustion, phlogiston was “squeezed” out into the air. The resulting substance, such as wood ash resulting from burning wood, was said to be “dephlogisticated,” as all of its phlogiston had been released.

The theory also helped to explain how living organisms Respiration used air when breathing. Normal air was considered low in phlogiston. Stahl thought that a person, for example, breathed in normal air and later exhaled air with elevated levels of phlogiston. This release of phlogiston from the living system was required for survival. Once air had become saturated with phlogiston, it was no longer able to accept more, which helped to explain why air became “used up” by a mouse in a closed container and why fumes from a fire were unbreathable; in both cases, the air was fully phlogisticated. “Bad air,” which was unhealthy to breathe, became equated with elevated levels of phlogiston.

Even metals contained some phlogiston, according to Stahl. When a metal, such as lead, is burned, it becomes oxidized. According to the phlogiston theory, this process, called calcination, Calcination represented loss of phlogiston. The calcinated metal was referred to as a calx and was considered to be a simpler substance than the original metal. Phlogiston could be returned to a calx by heating it with a burning material high in phlogiston, such as charcoal; this was the same process used in smelting ore.

A major problem with the phlogiston theory was that the calx weighed more than the original metal: If a calx represented a simpler substance created by taking phlogiston out of metal, one would expect the calx to weigh less, just as the ash left behind after burning a log weighs less than the original log. Stahl himself never seemed to be concerned with this problem, probably because he did not consider phlogiston to have any weight of its own. Thus, the weight gain, which had been discovered long before the birth of the phlogiston theory, was assumed to occur by some other, unknown process. As the phlogiston theory became widely accepted, however, other chemists modified it, attributing mass to phlogiston. Once phlogiston was believed to have mass, the oxidation or calcination of metals came to represent a serious problem for the theory.

Two main theories were advanced to explain the weight gain of calx. The first was that phlogiston was so light that it had the property of “levity,” which made substances less dense. Thus, as phlogiston left a metal, the metal became denser and therefore weighed more. This explanation involved a common misunderstanding about the difference between weight and density, however: An increase in density does not change the weight of a given amount of a substance. Also, the oxidized form of a metal is actually less, not more, dense than the original pure metal.

The second theory involved ascribing a negative weight to phlogiston. If phlogiston’s weight were negative, then removing phlogiston from a metal would indeed increase the weight of the resulting calx. Even in the eighteenth century, however, this explanation was considered outlandish by most chemists, and it remains a source of ridicule in many modern discussions of the theory. Those who rejected both these explanations yet still chose to accept the theory typically considered the phenomenon a mystery to be solved at a later time.

Doubts about the phlogiston theory gradually became more widespread in the last few decades of the eighteenth century, dividing chemists into two opposing camps, phlogistonists and antiphlogistonists. Joseph Priestley was the most prominent of the phlogistonists, defending the theory until his death in 1804. In one of his most famous experiments, he heated some calx of lead (mercuric oxide) in a closed container and found that the air released by the calx caused a candle to burn more vigorously and that a mouse placed in the container could breathe longer than when placed in normal air. Priestley called the air inside the container “dephlogisticated air”: He assumed that, by heating lead calx, he had removed phlogiston from normal air and caused it to be reabsorbed by the calx. In fact, rather than removing something from the air, Priestley had added to it when the heated mercuric oxide gave off oxygen.

Antoine-Laurent Lavoisier, the most prominent antiphlogistonist, believed that calcination produced a weight gain in metals because something in the air combined with the metal. He repeated the experiments of Priestley, interpreting the change in the lead calx as the loss of a gas into the air. This explanation solved the problem of weight gain experienced by metals when calcinated: Lavoisier reasoned that the same gas that a calcinated metal released had bonded to it when it was originally calcinated, and the weight of the gas had been added to the weight of the pure metal at that time. Lavoisier named this new gas “oxygen.” Oxygen Although Lavoisier was never able to dissuade Priestley from believing in phlogiston, he did convince most other chemists of his day that the theory needed to be discarded.

By the end of the eighteenth century, most chemists had discarded the phlogiston theory in favor of Lavoisier’s oxygen theory. The existence of oxygen could explain the same phenomena as phlogiston, without the latter theory’s serious flaws. Weight loss during combustion was due to oxygen in the burned substance joining together with carbon to form carbon dioxide, a gas given off as a by-product. In the case of metals, for which combustion caused weight gain, oxygen joined together with the metal to form a new material, the calx form of the metal. This process of metal bonding with oxygen is now known as “oxidation.” Oxidation A metal calx could be converted back to its pure form by heating it with charcoal, which catalyzed the release of oxygen from the metal atoms. Even respiration was easily explained once oxygen was recognized as the component of air that gets used up in the respiration process and carbon dioxide was found to be the gas that is exhaled in that process.

Some philosophers of science consider the phlogiston theory a serious setback for chemistry in the eighteenth century. Because of its roots in alchemy and its proponents’ blindness to the theory’s deficiencies, progress in experimental chemistry was stalled. One reason for this stalemate was that the theory was developed at a time when chemistry was primarily qualitative, so few chemists were aware that metals gained weight when calcinated, or if they were aware did not recognize the significance of the weight gain.

Throughout the eighteenth century, chemistry became progressively more quantitative, which brought the weight gain problem to the forefront of the field, encouraging chemists to look for alternative theories. Consequently, some philosophers of science consider the phlogiston theory a useful step in the development of modern chemistry that helped highlight the importance of quantitative experimentation. Moreover, the theory was in many ways the first systematic theory of modern chemistry, and, despite ultimately being discredited, it represented a transitional paradigm between those of the Middle Ages and Renaissance and those of the nineteenth and twentieth centuries.

• Conant, James Bryant. The Overthrow of the Phlogiston Theory: The Chemical Revolution of 1775-1789. Cambridge, Mass.: Harvard University Press, 1967. An analysis of the fall of the phlogiston theory with special emphasis on it as an example of a paradigm shift.
• Djerassi, Carl, and Roald Hoffmann. Oxygen. Weinheim, Germany: Wiley-VCH Verlag, 2001. A play using the discovery of oxygen as its theme.
• Donovan, Arthur. Antoine Lavoisier: Science, Administration, and Revolution. New York: Cambridge University Press, 1996. A comprehensive biography of Lavoisier with ample discussion of his debunking of the phlogiston theory.
• Golinski, Jan. Science as Public Culture: Chemistry and Enlightenment in Britain, 1760-1820. New York: Cambridge University Press, 1992. Focuses on the history of chemistry in England during the period of the fall of the phlogiston theory.
• Kuhn, Thomas S. The Structure of Scientific Revolutions. 3d ed. Chicago: University of Chicago Press, 1996. Kuhn’s seminal text discusses the phlogiston and oxygen theories as examples of competing paradigms and explains why the latter replaced the former.
• McCann, H. Gilman. Chemistry Transformed: The Paradigmatic Shift from Phlogiston to Oxygen. Norwood, N.J.: Ablex, 1978. Follows up on the work of Kuhn to provide in-depth analysis of the eighteenth century paradigm shift in chemistry.
• Partington, James Riddick, and Douglas McKie. Historical Studies on the Phlogiston Theory. New York: Arno Press, 1981. A reprint of papers from the Annals of Science reviewing several aspects of the phlogiston theory.
• Yount, Lisa. Antoine Lavoisier: Founder of Modern Chemistry. Berkeley Heights, N.J.: Enslow, 1997. Book about Lavoisier written for high school students.

Fahrenheit Develops the Mercury Thermometer

Geoffroy Issues the Table of Reactivities

Réaumur Discovers Carbon’s Role in Hardening Steel

Bernoulli Proposes the Kinetic Theory of Gases

Nollet Discovers Osmosis

Black Identifies Carbon Dioxide

Priestley Discovers Oxygen

Ingenhousz Discovers Photosynthesis

Cavendish Discovers the Composition of Water

Lavoisier Devises the Modern System of Chemical Nomenclature

Proust Establishes the Law of Definite Proportions

Joseph Black; Henry Cavendish; Antoine-Laurent Lavoisier; Joseph Priestley; Georg Ernst Stahl. Phlogiston theory