The Rise of the Phlogiston Theory of Fire
The Rise of the Phlogiston Theory of Fire
The late seventeenth century saw the rise of the phlogiston (pronounced FLO-jis-ton) theory of fire, which sought to explain the burning of objects. First proposed by Johann Joachim Becher (1635-1682) and Georg Ernst Stahl (1660- 1734), the phlogiston theory evolved into an complete theory of the chemical sciences. It was modified by later followers, giving it coherence, but also exposing its weaknesses. Phlogiston determined the direction that chemistry was to take for the next one hundred years, suggesting not only what experiments to perform, but how to interpret the results. The theory was eventually overturned by the concept of the combustion of oxygen, but only after a protracted series of debates and experiments.
The nature of fire has long been a source of wonder and mystery. Aristotle (384-322 b.c.) combined fire with air, water, and earth to explain the composition of all things. For Aristotle, when wood burnt the flame was the element of fire escaping, any vapor was air, moisture was water, and the ash that remained was the element of earth. Aristotle's system dominated medieval European thought, partly due to the Church's adoption and modification of his philosophical ideas.
The European Renaissance helped revive the works of other Greek philosophers, such as Plato (427-347 b.c.), who had been Aristotle's teacher. Plato had proposed a "burnable principle" that existed within inflammable objects. This fitted well with the alchemical notions of the Renaissance, and the burnable principle became associated with sulfur, or "some vague spirit of sulfur." A new system of elements evolved, with substances explained by a combination of sulfur, mercury, and salt. So wood burned because it contained sulfur, gave off flame because it contained mercury, and left ash because it contained salt. However, Aristotle's four elements were still used, sometimes in conjunction with the new system, creating a confused mix of explanations for the nature of fire.
Johann Becher was an alchemist and adventurer. He sold a process for turning silver and sand into gold to the Dutch government, told tales of handling a stone that made him invisible, and claimed to have seen Scottish geese that lived in trees and hatched eggs with their feet. In his science Becher followed alchemical ideas, but these were a confused body of work and he attempted to order and simplify some of the common notions of his time. First, he proposed an "oily spirit" as an essential principle of matter, but he also used the alchemical principles of mercury, sulfur, and salt. Later he altered Aristotle's four elements, keeping only water, and dividing earth into three separate substances. One of these, "oily earth" (terra pinguis), would form the basis of the phlogiston theory. Becher stated that in order for a substance to burn it must contain oily earth.
Becher's ideas were not clearly explained in his writings, and it is thanks to his disciple, Georg Stahl, that the theory was popularized. Stahl's writings were still heavy reading, but he published widely. Stahl's phlogiston was a material principle that escaped from objects when they burned. In the act of escaping the phlogiston caused violent motion, the flames, sparks and explosions of various combustions. So burning wood was explained as the substance phlogiston escaping from within the wood, causing the motion of flames and sparks as it left, and leaving behind an ash with little or no phlogiston.
The phlogiston theory became popular in the German states and then spread quickly across Europe. Its acceptance was wide and lasting, dominating the chemical sciences for a hundred years. The theory was successful for a number of reasons. It was a development of known theories of combustion. It appealed to those familiar with the alchemical notions by retaining the importance of sulfur, but also explained the combustion of substances that appeared to contain no sulfur. It was based on Aristotle's four elements, and used Plato's idea of a burnable principle. In a sense phlogiston united all these competing ideas, allowing supporters of each to easily adopt the new theory. Even the name phlogiston was not new; it was derived from the Greek word phlogistos (to burn), and had been coined early in the seventeenth century.
The phlogiston theory was passed on to entire generations of academics by Stahl's teaching. Stahl believed that his theory was divinely inspired, and that the common herd (which included his students) would not understand it. His intentionally uninteresting lecturing style made the details of phlogiston unclear, but his students dutifully copied and repeated his words. Stahl was also a very successful physician—he became the King of Prussia's personal doctor. The popularity of his medical text, The True Theory of Medicine (1708), also helped to spread the phlogiston theory.
Other theories of the nature of fire, some of them far closer to modern theories, lost out to phlogiston's popularity. There had been a strong British trend of thought that had suggested air as the supporter of combustion and life. However, the ideas of Robert Hooke (1635-1703), John Mayow (1643-1679), Stephen Hales (1677-1761), and others were not popular on the European continent. Opponents of phlogiston were smothered by the sheer weight of numbers of phlogistonists that trumpeted the new theory.
Phlogiston, although originally introduced to explain combustion, became the center of a whole system of chemistry. The wide scope of phlogiston's supposed attributes led to the theory becoming the first unifying principle of the chemical sciences. Breathing was explained as part of the process by which food was burnt in the body. Phlogiston would escape from the burning food and was expulsed by the lungs.
The theory also explained the process of metal calcination (rusting). When a metal rusted phlogiston escaped, leaving behind a lighter, more fragile, substance. Experiments were made to add phlogiston back to rusts, and were often successful. By adding a substance rich in phlogiston (charcoal) to certain rusts and heating them the metal was restored. Even more convincing was the fact that this process gave off a gas that was not capable of supporting fire or breathing. This was seen as proof that all the phlogiston had been taken out of the surrounding air and returned to the metal. Indeed, it is a simple and coherent explanation.
Phlogiston was also said to be the foundation of color. Many substances when burnt or rusted changed color. The change in color was explained by the phlogiston leaving during the process. Phlogiston was also said to be completely indestructible, nonelastic, dry, and imperceptible to all the human senses. Even thunder and lightning could be explained by phlogiston. Lightning flashes were the combustion of concentrations of phlogiston in the air, and thunder was the result of the collapsing dispersed air as the phlogiston escaped.
While the phlogiston theory had come from Becher's mind as conjecture based on older philosophical ideas, it helped inspire practical experimentation. Stahl performed a number of key experiments that were published widely. The results were easily explained by the internal logic of the phlogiston theory. Across Europe chemists dutifully repeated Stahl's work, and guided by his writings they also came to the same "obvious" conclusions. This new trend to finding practical support for a theory, no matter how misguided, is one of phlogiston's most important legacies.
Although phlogiston was considered a real substance, it was not originally conceived as having any weight. To Becher and Stahl it was a substance as insubstantial as sunlight, but like sunlight it could still have dramatic effects even if it could not be contained or measured.
Later supporters of phlogiston began to alter the theory from the original concepts of Becher and Stahl. Often this was because the complex, vague, and sometime contradictory writings of the founders were misunderstood. By the 1730s most phlogistonists regarded their imagined substance as having an actual weight. This had dramatic consequences for the theory. If phlogiston had weight then when it left a substance during combustion or rusting the remaining material should weigh less. However, a number of experiments gave results that conflicted with this. For example, when a metal rusted (gave off phlogiston) the material left behind often weighed considerably more than the original metal.
Some scientists suggested that phlogiston had a negative weight, and so its absence made materials heavier. However, this did not seem to apply to all situations—for example, when animals breathed (expelled phlogiston) they did not appear to gain weight. The negative weight was not accepted by all supporters, and alternative versions of the phlogiston theory began to appear.
The discovery of new gases, later identified as hydrogen and oxygen, which burned brighter and more fiercely that normal air, also caused problems for the phlogiston theory. More and more alterations were made by supporters to answer the critics. Fierce debates raged between the two sides. This had the effect of making the anti-phlogistonists more rigorous in their research and experimentation. In order to gain support for his alternative theory of combustion, Antoine Lavoisier (1743-1794) produced many papers carefully describing his methods and analysis. Over time the weight of this evidence convinced more and more scientists to abandon the phlogiston theory and adopt Lavoisier's new explanation of oxygen combustion.
The phlogiston theory died a lingering death, with some supporters like Joseph Priestley (1733-1804) maintaining its truth against all opposition. The theory also went through occasional revivals as late as the nineteenth century, often for metaphysical reasons as opposed to chemical ones. While it was a mistaken path, phlogiston is often seen as a halfway stage between alchemy and modern chemistry. For while the theory limited the analysis of results it also encouraged experimentation, and was eventually overturned by the weight of printed evidence.
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White, John Henry. The History of the Phlogiston Theory. London: E. Arnold, 1932, reprinted by AMS Press, 1973.