Phlogiston

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Phlogiston (from Greek φλογιστός phlogistós , burned ') is a hypothetical substance introduced by Georg Ernst Stahl , which was assumed in the late 17th and 18th centuries to escape from all combustible bodies when they are burned and penetrate them when heated . The phlogiston theory was important for the interpretation of reduction and oxidation processes and the different potential of different compounds (in a modern view their redox potential ) and was successful from the point of view of contemporaries. The first refutations appeared in the 1770s and the role of oxygen in combustion processes began to be described and quantified in more detail. The phlogiston theory, which was the predominant chemical theory from around 1700 to 1775, has therefore long been counted among the scientific errors and seen as an outdated scientific paradigm of its time.

The phlogiston theory

Johann Joachim Becher (1635–1682). Copper engraving, probably made around 1675
Georg Ernst Stahl (1659–1734), copper engraving

In his phlogiston theory, Stahl started out from the work of the chemist Johann Joachim Becher (1635–1682), who in turn was based on the teachings of the doctor and scientist Daniel Sennert (1572–1637). In Becher's work Physica Subterranea (1669) there are two actual elements, water and earth, the latter being the actual principle at work. He divides them into

  • terra fluida or mercural earth, which gives the substances fluidity, delicacy, volatility and metallic properties,
  • terra pinguis or greasy earth; this corresponds to the oily liquid of the alchemists , which gives the substances oily, sulphurous and combustible properties, and
  • terra lapidea or glass-like earth, which stands for the principle of meltability and which in practice occurs, for example, with the slag residues in the glass and metal works.

After that, air played no part in the formation of the minerals. These three earths took on a role similar to the three Paracelsian alchemical principles: the terra fluida for mercury (Mercurius, principle of volatility), the terra pinguis for sulfur (principle of flammability, sulfur ) and the terra lapidea for salt (principle the strength). Stahl ( Zymotechnica fundamentalis 1697) now replaced the sulfur principle with the phlogiston: all combustible bodies would contain phlogiston and when burned, it is broken down into phlogiston, which is volatile and escapes, and the remaining, phlogiston-free and incombustible part, the ashes. The oxidation of pure metals (formation of so-called metal limescale), the fermentation of organic substances, the putrefaction of plants and animals were explained by the escape of phlogiston, according to Stahl. Chemists had observed that coal or sulfur burned without residue. According to what was thought at the time, these substances contained a great deal of phlogiston. Other substances such as the metals copper, tin and zinc were transformed into earthy, salty substances. This led to the conclusion that these substances contained less phlogiston. Depending on the speed and strength of the transformation into salty substances, the metals were of different noble quality. Only gold and partly silver remained unchanged when all chemicals were used, so they contained little or no phlogiston, they were noble and unchangeable from the point of view of chemists at the time . The phlogiston could be returned to the metal by heating it with charcoal , the metals were revived with phlogiston (reduced from today's perspective).

Robert Boyle (1626-1692)

Basically, Stahl was a fan of atomic science like Robert Boyle . In contrast to Boyle, however, he saw the atoms assigned to different elements or principles. He did not determine this exactly yet, but he added the phlogiston and sometimes some earths (metal oxides). On the one hand, these formed chemical compounds, which he called mixtum (for example sulfur or metals after steel), substances composed of these mixta , which he called compositum (for example mercury sulfide), and decomposita made up of composites , including minerals. The properties of the parts were not carried over to those of the connections; so metals contained phlogiston but were not themselves flammable.

Strengths and Limits of Theory

The influence of this theory was very great in the 18th century, because with its help oxidation-reduction reactions , acids, bases and salts could be systematically investigated. Certain substances such as sulfur and phosphorus burned to form gases that dissolved acidic in water. Other salts of nature ( quick lime , metal lime , i.e. metal oxides ) reacted with water as a base. Acids and bases could be detected with litmus . If you combined such opposite substances as acids and bases , neutral salts were formed. With the help of the phlogiston theory, the acids (phosphorus, sulfur) and bases (metal limes) from certain groups of substances could be better systematized. At the same time, the phlogiston theory preserved old alchemical ideas about the four primordial elements (earth, water, air, fire) according to Empedocles or the three principles of Paracelsus . According to the pure phlogiston theory - there have been a number of modifications over time - there were only substances that contained a lot or little phlogiston. There were no elements as we know it today, everything was a composite material with a lot or little phlogiston - only phlogiston was a fundamental element after steel. There were also the alchemical principles of mercury (liquid, shiny metal) and sulfur (flammable).

Robert Boyle had already formulated the concept of elements differently in his book The Skeptical Chemist from 1661 in the 6th section of his book and, in contrast to the phlogiston theory, developed a clearer concept of the chemical element.

In addition, in the 18th century it was possible to use the phlogiston theory to describe many of the phenomena of chemistry known at the time. This theory explains the finding that candles in closed vessels go out after a while. Air should then only be able to absorb a certain amount of phlogiston escaping from the candle. The realization that part of the air (according to later knowledge, the oxygen) can sustain the combustion for a longer time was initially explained by Joseph Priestley by saying that it was dephlogestated air , which could thus absorb more phlogiston.

In his book Chemische Abhandlung von der Luft und dem Feuer , published in 1777, Carl Wilhelm Scheele describes a proportion of the air that promotes combustion and calls it fire air (oxygen). He also gives several ways in which this fire air could be produced, for example by heating brownstone ( manganese dioxide ) with concentrated sulfuric acid (H 2 SO 4 ). In keeping with the times, he interpreted these processes within the framework of the phlogiston theory.

The combustion of organic substances takes place (without the gaseous reaction products, which were still unknown at the time), mostly with weight loss. According to the phlogiston theory, the phlogiston previously absorbed by the plants should be released again. The same explanations are also possible for some non-metals - such as phosphorus or sulfur. In metals there were problems, as these generally solid oxides form and thus more difficult when burned. However, the attempts made by Boyle were invalidated by the fact that it was the specific gravity and not the absolute weight that mattered. In addition, the experimental possibilities were limited in the 18th century, so that many chemists reported weight loss due to the unobserved evaporation of part of the oxide. The reduction of metal oxides with coal to metals has been explained without contradiction by the absorption of phlogiston from the coal.

In the course of the discovery of gaseous compounds and with the use of more precise measurement methods, problems and errors of this theory became increasingly clear. In particular, there was no conclusive explanation for the weight gain when burning metals. In order to save the theory, proponents tried to ascribe properties to the phlogiston such as a negative mass (according to one of the last defenders of the doctrine of Friedrich Albrecht Carl Gren ) (compare also the law of mass action ). Another big problem was that it was not possible to detect the phlogiston directly. In the further development of chemistry in the 18th century, the hydrogen discovered at that time was partly mistaken for phlogiston, among others by its discoverer Henry Cavendish , or electricity ( Giambatista Beccaria ), based on his observation of the reduction of metal limes through electrical discharges.

The theory was finally refuted only in 1785 by Antoine Lavoisier , who was able to show that all combustion phenomena could be explained by his oxidation theory and by the gas oxygen without the use of extraordinary assumptions . The last strong hypothesis of the phlogiston theory, the explanation of the formation of hydrogen when metals react with acids, was invalidated by him through the knowledge that water is a compound of oxygen and hydrogen. Lavoisier organized a systematic campaign against the doctrine of phlogiston, which he programmatically criticized in his Reflexions sur la phlogistique of 1786 as well as in his textbook of chemistry. Lavoisier's teaching also came to be known as anti-inflammatory chemistry , a word coined by Richard Kirwan in 1787, one of the last great phlogistonists (he eventually surrendered along with other well-known phlogistonists such as Joseph Black ).

The last possible attempts to explain the phlogiston as “heat substance” (which Lavoisier also considered to be real regardless of the phlogiston theory) could be refuted in 1798 by Benjamin Thompson in favor of the theory of the movement of particles. He ran blunt steel drills in cannon barrels. The pipes kept getting hot and the supposedly existing phlogiston was drained away by water. The heat could therefore not have been produced by an exhaustible substance present in the pipes.

The reality of Phlogiston seemed proven at this time; so there were few attempts to explain the processes differently, for with the assumption of the phlogiston everything seemed to be explainable. In this period of scientific research one did not have the claim to complete clarification work of all detailed knowledge and was limited by the partly subjective observation possibilities. In order to attempt holistic explanations, the natural sciences continued to combine “morally beautiful” with the older ideological views of nature. Even researchers like Joseph Priestley, who recognized the internal contradiction to the phlogiston theory, stuck to this theory in their attempts to explain.

The important implications of this theory

Although the theory reversed the situation according to today's knowledge, with this interpretation much could be better explained and systematized in that period of the history of science.

  • The phlogiston theory was able to make the oxidation and reduction processes sufficiently understandable.
  • She encouraged people to collect and examine the "finely distributed" phlogiston and to understand the gas properties.
  • The theory made it possible to systematize groups of substances that form acids and bases.
First attempt at a table of the more recent opinions of various naturalists on the composition of simple substances.
Pocket book for vagina artists and pharmacists from 1791 . Twelfth year, Weimar
Fire Phlogiston water Hot air Phlogistic air Inflammable air Air acid metal Metal lime
today - [Warmth] Hydrogen + oxygen oxygen nitrogen hydrogen Carbon dioxide elements Metal oxide
Achard Freye fire matter A special substance - Water associated with fire matter - Elemental air, combustibles and acid Elemental air and a special acid Metallic earth and phlogiston Metallic earth
Cavendish Freye fire matter A special substance Clean and inflammable air burned water Nitric acid with phlogiston - - Metallic earth and phlogiston Metallic earth with water
Gren Freye heat and light matter Bound heat and light matter - Heat matter and water Pure air with phlogiston Heat, water, fuel, acid (vitriolic, hydrochloric or vegetable acid) Its own phlogistic acid Metallic earth and phlogiston Metallic earth
Lavoisier Freye fire matter - Dephlogistic and inflammable air Fire matter and oxygen Nitric acid modified by fire matter - Oxygen and carbon simple fabric Metal and oxygen
Priestley Freye fire matter Elementary substance Elementary substance - Pure air and combustibles Phlogiston bound to a fine earth Modified Vitriolic and Nitric Acids Metal earth and inflammable air Metallic earth
Scheele Phlogiston and fire air Elementary substance Elementary substance Pure air and combustibles Modification of nitric acid - - Metal earth and phlogiston Metallic earth and water
Volta Freye heat matter Heat bound with air acid Elementary substance Air acid oversaturated with combustibles - Flammable and living air - Metallic earth -

Replacement by the oxidation theory

The phlogiston theory was replaced at the end of the 18th century by the oxidation theory of the chemist Antoine Lavoisier . First one develops, u. a. also Lavoisier, the caloric theory ; it too is an outdated theory of heat . She postulated a caloric substance that was invisible, had no weight and stayed between the molecules and penetrated body boundaries. The "caloric substance" developed a repulsive force in itself, which explained that it was looking for the balance between high and low concentration, i. H. flow from the warmer to the colder body.

It was introduced by Lavoisier in 1783 , building on the work of Joseph Black . Georg Ernst Stahl and his subsequent representatives of the phlogiston theory, d. H. all phlogistonists saw in the phlogiston a substance that would be released when burned. The transformation of metals through heating in air was also known as calcification ; it was said that the metal was losing phlogiston. Conversely, when an ore is heated with coal , the phlogiston would be absorbed by the latter, with the formation of a shiny metal .

As a skeptic, Lavoisier convinced himself in the course of his experiments that the substances bind oxygen when they burn (oxidation). By correcting the theory on the basis of the results of his experiments, which had become more precise, he was able to expand the now recognized interpretation of reality, contrary to prevailing opinion. Finally, further laws could be derived from this changed understanding, for example the law of constant proportions formulated by Joseph Louis Proust (1797) or Dalton's law recognized by John Dalton in 1805 .

Joseph Louis Proust (1754–1826) drawn by Ambroise Tardieu (1788–1841) around 1795
John Dalton (1766-1844)

Stahl's phlogiston theory was an innovation for the development of knowledge. He understood that the burning of a substance consisted of a reaction between two materially different substances (in modern terms: in a chemical process) and through the experiments he introduced, he also recognized their reversibility or reciprocity at the same time. One substance A releases the phlogiston, another substance B absorbs it (combustion, in modern terms: oxidation). On the other hand, substance B can release the absorbed phlogiston back into substance A (reduction). Stahl discovered the interdependence of these combustion (oxidation) and reduction processes.

The following proponents of the phlogiston theory contradicted this theory by neglecting the gaseous reactants of these reversible processes.

The phenomenon observed, which related to the increase in weight of the metals during combustion, was ultimately confirmed and explained by Lavoisier in a number of different experiments. When Joseph Priestley discovered a gas in 1774 which was capable of sustaining combustion above all others, criticism of the phlogiston theory received indirect support. Lavoisier showed that Priestley's gas was one of the elements in air and called it Oxygène (oxygen). From now on the importance of air during combustion was clear. When burned, phlogiston is not given off, but air or rather its components are absorbed. The combustion was not a decomposition, but a combination that takes place in that a certain element of the air becomes fixed with the combustible body . This increases in weight as it burns (oxidizes), and the increase in weight is exactly equal to the weight of the gaseous body that has entered.

Lavoisier examined the changes in the weight of various substances during oxidation and reduction and discovered that the element oxygen just discovered plays a decisive role in this. The defenders of the phlogiston theory, such as Henry Cavendish, Joseph Priestley and Carl Wilhelm Scheele themselves, tried in vain to preserve Stahl's theory by modifying it and claiming that the role of air was to deprive flammable bodies of the phlogiston.

Lavoisier demonstrated that

  • When metals or sulfur are burned, they combine with oxygen,
  • as much oxygen is consumed as is contained in the oxides formed,
  • in order to recover metals from the oxides one does not have to add phlogiston, but rather remove the oxygen.

If one looks at the criticism of the phlogiston theory using the terms of Joseph Priestley's dephlogisticated air or Antoine Laurent de Lavoisier's oxygène , both concepts can easily be combined with the modern concept of the redox reaction or the Lewis acid-base concept . Acids are substances that can split off cations or accept anions or electrons . In other words , oxidizing agents that are reduced by a chemical reaction. Translated back into the theory of the phlogiston, this would correspond to a phlogiston recording. They are therefore electrophiles and electrochemically correspond to the positive pole ( anode ) or the oxygène or the dephlogisticated air . The reciprocal assumptions then apply to bases. Here in the back translation this meant a phlogiston delivery.

literature

  • Gilman McCann: Chemistry Transformed: The Paradigmatic Shift from Phlogiston to Oxygen. Ablex Pub, 1978, ISBN 0-89391-004-X .
  • Peter Laupheimer: Phlogiston or Oxygen. Scientific VG, 1992, ISBN 3-8047-1212-6 .
  • William H. Brock : Vieweg's History of Chemistry. Vieweg, Braunschweig 1997, ISBN 3-540-67033-5 .
  • Irene Strube , Rüdiger Stolz, Horst Remane : History of Chemistry. Deutscher Verlag der Wissenschaften, Berlin 1986, ISBN 3-326-00037-5 , p. 54 ff.
  • Günther Bugge (ed.): The book of the great chemists. Volume 1. 6th edition (1st edition 1929). Verlag Chemie, Weinheim 1984, ISBN 3-527-25021-2 , p. 198.
  • Ursula Klein, Wolfgang Lefevre: Materials in eighteenth-century science. MIT-Press, Cambridge 2007, ISBN 978-0-262-11306-9 .
  • James Riddick Partington: Historical studies on the phlogiston theory (The Development of Science). Arno Press, New York 1981, ISBN 978-0-405-13895-9 .
  • James L. Marshall, Virginia R. Marshall: Rediscovery of the Elements. Phlogiston and Lavoisier. In: Hexagon. Journal of the Department of Chemistry, University of North Texas, Spring 2005 Issue, pp. 4–7 ( online - free full text)
  • Jaime Wisnak: Phlogiston: The rise and fall of a theory. In: Indian Journal of Chemical Technology. Volume 11, September 2004, pp. 732–743 (PDF; 87 kB)
  • Arthur F. Scott: The Invention of the Balloon and the Creation of Chemistry. In: Spectrum of Science. 3 (1984), pp. 106–115 (PDF; 3.8 MB)
  • Strube: Georg Ernst Stahl , Teubner 1984

Web links

Wiktionary: Phlogiston  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ Károly Simonyi : cultural history of physics . Harri Deutsch, Thun, Frankfurt a. M. 1995, ISBN 3-8171-1379-X , pp. 239 . ; Reiner Ruffing: Small encyclopedia of scientific errors. Gütersloher Verlagshaus 2011, ISBN 978-3-579-06566-3 , pp. 123-125.
  2. ↑ Thomas S. Kuhn : The structure of scientific revolutions .
  3. ↑ Wolf-Dieter Müller-Jahncke : Phlogiston theory. In: Werner E. Gerabek , Bernhard D. Haage, Gundolf Keil , Wolfgang Wegner (eds.): Enzyklopädie Medizingeschichte . De Gruyter, Berlin 2005, ISBN 3-11-015714-4 , p. 1155.
  4. ^ William Hodson Brock: Vieweg's history of chemistry. Berlin 2000, p. 50 ff.
  5. ^ Antonio Clericuzio, Elements, Principles and Corpuscles, Springer 2000, p. 195
  6. ↑ Wolf-Dieter Müller-Jahncke: Phlogiston theory. In: Encyclopedia of Medical History . De Gruyter, Berlin 2005, p. 1155.
  7. ^ Strube, Georg Ernst Stahl, Teubner, p. 46
  8. ^ Günther Bugge: The book of the great chemists. Verlag Chemie, Weinheim 1955, Volume I, Robert Boyle p. 184; Martin Carrier: On the corpuscular structure of matter in steel and Newton. ( Memento of the original from February 22, 2014 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Franz Steiner, Sudhoffs Archive Volume 70 (Issue 1), Wiesbaden 1986. @1@ 2Template: Webachiv / IABot / pub.uni-bielefeld.de
  9. ^ Günther Bugge: The book of the great chemists. Verlag Chemie, Weinheim 1955, Volume I, p. 198.
  10. ↑ Strube, Stolz, Remane: History of Chemistry. Deutscher Verlag der Wissenschaften, Berlin 1986, p. 54 ff.
  11. ↑ From P. Köthner: From the chemistry of the intangible . Published by AW Zickfeldt, Osterwieck Harz 1906.
  12. ↑ Florian Wodlei, Regina Kleinhappel: Lecture series on the history and development of thermodynamics as part of the lectures Theoretical Hydrodynamics, Transport Theory . Held at the University of Graz in the field of theoretical physics in the winter semester 2007/2008, summer semester 2008. 2nd improved and revised edition.
  13. ^ Walter J. Moore, Dieter O. Hummel: Physikalische Chemie. Walter de Gruyter, Berlin 1986, ISBN 3-11-010979-4 , p. 135 f.
  14. ↑ Christian Blöss: Entropy: Universal aspects of a physical quantity. Books on Demand (2010), p. 34.
  15. ^ Charles Adolphe Wurtz: History of chemical theories from Lavoisier up to our time , German ed. by Alphons Oppenheim (1817–1884), Robert Oppenheim, Berlin 1870 (pdf; 8.6 MB).
  16. ↑ Dorothea Golze: Phlogiston vs. Oxygen , 2008 (pdf; 72 kB).
  17. ↑ the special cations, the protons or hydrons. See also the term acid according to Brønsted and Lowry .
  18. ^ Karl-Heinz Näser: Physical chemistry for technicians and engineers. 16th edition, Leipzig 1986, p. 158.