The phlogiston theory was proposed by the German Ernst Stahl in the 17th century to explain the reason why some substances can burn. This philosopher claimed that things burned on fire because they had "phlogiston" inside.
The word phlogiston derives from the Greek "phlos", which means "flame", so "phlo-giston" means "what goes in the flame". Based on this principle, Stahl was convinced that something was "lost" or "went" from the material when combustion occurred.
Carbon combustion, which served as the basis for the phlogiston theory (Image by Alexas_Fotos at www.pixabay.com)
This theory was perhaps one of the first metatheories with some chemistry to be proposed, having as its predecessors the Aristotelian ideas that tried to explain that matter was composed of four elements: fire, air, water and earth.
However, the theory was very simplistic and was based on some alchemical principles that had a close relationship with that: materials could not be separated into their components in a simple and simple way, but could only be transformed from one mixture to another successively.
Georg Ernst Stahl was an iatrochemist (scientists who link medical and chemical knowledge) and philosopher, recognized as the first physician to the King of Prussia.
Stahl was not a methodical scientist who quantitatively followed the phenomena he studied, rather he always tried to give simple answers to the questions that disturbed him.
Origin
Ernst Stahl was a defender of the ideas of Johan Becher, who proposed that all matter (except metals) was composed of three "earths", namely: the base substance, the sulfurous earth and the mercury earth.
Becher's composition was based on Aristotelian ideas that affirmed that the sulfurous earth was the fire "asleep" in the bodies and that, once it "awakened", it consumed the sulfur of "Paracelsus" that was inside the bodies.
Portrait of Georg Ernst Stahl (Source: See page for author Via Wikimedia Commons)
Becher considered that metals were composed of different materials and, therefore, could be "transmuted". That is, to transform from one metal to another only through its heating, thereby changing the chemical relationships between the materials that made up each metal.
Based on these principles, Stahl focused on unraveling the mysteries that accompanied the combustion of organic bodies during the time. All the experiments he carried out were based on the incineration of metals and materials such as sulfur, coal and others.
By incinerating these compounds, Stahl documented that just by watching as the compound was being consumed, he could tell that "something" dissipated, vanished or disappeared. This "something" that Stahl observed was what he called "phlogiston."
In Aristotelian ideas, sulfur was the fire contained within matter and the "philosophical sulfur of Paracelsus" was completely lost when combustion activated that fire contained within sulfur or sulfurous earth in organic substances such as wood.
Stahl integrated the methods used by alchemists like Becher, Aristotelian ideas and his observations of combustion to propose, then, the theory of phlogiston.
Beginning
Stahl's theory gained strength among scientists and chemists of the time, since for them, if the body had the ability to burn or burn, these were made up of sulfur. For these scientists, sulfur was a material very similar to metals.
Moreover, the scientists of the time defined phlogiston as a "being" or "indestructible entity" that could be reincorporated into materials by trapping it in some way while the material from which it was detached was made to burn.
Another intrinsic property of phlogiston was its ability to be transferred from one material to another. This explained the way in which some bodies were burned and others were calcined, since some had the ability to transfer phlogiston and others did not.
Much research by Stahl and other scientists of the time focused on trying to isolate phlogiston. A few scientists associated phlogiston with "flammable air", claiming that it was.
This theory was widely disseminated during the time and seemed to charitably explain why the combustion of bodies occurred, the similarities that were observed between metals and the “fusion” of aspects such as oxidation and reduction in a single phenomenon: phlogiston.
An example widely used by defenders of the phlogiston theory was that of carbon on vitriolic acid, which is currently hydrogen sulfide. In this example, carbon “loses” its ability to burn (to phlogiston) and is transferred to sulfur, giving rise to vitriolic acid.
Objections to the theory
During the seventeenth century, this theory was classified as the most important of all chemistry, since it gave an explanation to all the observations that had been made in that field. Kant described it with an importance similar to that of Galileo on the fall of bodies.
However, for a methodical scientist using measurement strategies more profound than just observation, it was easy to find flaws in the phlogiston theory. This scientist was the French Laurent de Lavoisier.
Portrait of Antoine Lavoisier (Source: H. Rousseau (graphic designer), E.Thomas (engraver) Augustin Challamel, Desire Lacroix Via Wikimedia Commons)
Lavoisier was a fanatic of the physical sciences and measuring instruments. He decided to accurately understand the mechanism of combustion and the theory of phlogiston, finding that fire does not produce an immediate increase or decrease in the weight of materials.
Lavoisier accurately measured the combustion of different materials and determined that the weight of the residue after burning was very similar to that of the material before burning in fire.
In 1774, Lavoisier heard about the experiments of Joseph Priestley who used mercury dust and "dephlogistized" air.
This led him to conduct a series of rigorous experiments that he carried out between 1773 and 1775, in which he discovered that the dephlogistized air released from mercury dust was nothing more than the healthiest and purest part of the air we breathe. He named this part "vital air."
Lavoisier determined that the combustion and calcination processes were limited in time when they occurred in closed containers. Furthermore, the increase in material after combustion was due to the "vital air" that the material absorbed after combustion.
In 1779, Lavoisier published a work entitled General considerations on acids and on the principles of which they are composed, in which he baptized as "oxygen" the substance that, under certain conditions, originated all acids.
References
- Kamlah, A. (1984). A logical investigation of the Phlogiston case. In Reduction in science (pp. 217-238). Springer, Dordrecht.
- Rodwell, GF (1868). I. On the theory of phlogiston. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 35 (234), 1-32.
- Siegfried, R. (1989). Lavoisier and the phlogistic connection. Ambix, 36 (1), 31-40.
- Soloveichik, S. (1962). The last fight for phlogiston and the death of Priestley. Journal of Chemical Education, 39 (12), 644.
- Vihalemm, R. (2000). The Kuhn-loss thesis and the case of phlogiston theory. Science & Technology Studies.
- Woodcock, LV (2005). Phlogiston theory and chemical revolutions. Bulletin for the History of Chemistry, 30 (2), 57-62.