Phlogiston theory

Phlogiston (from the Greek φλογιστός phlogistós burned ') is a hypothetical substance of which was suspected in the late 17th and 18th centuries that it escapes all combustible bodies in burning and penetrates when heated in it. The phlogiston theory was in the interpretation of reduction and oxidation processes, such as the different potential of various compounds of importance. In the 18th century, first refutations were held and began to describe the role of oxygen in combustion processes in more detail and to quantify. The phlogiston theory is therefore counted among the scientific errors, it would be more accurate to describe as a paradigm shift in the sense of Thomas S. Kuhn. The phlogiston theory presented less quantitative processes than the redox potential of the substances involved in the foreground.

The phlogiston theory

The phlogiston theory is an explanatory concept that sought to explain the (chemical) process of combustion. Phlogiston is a substance or postulated a hypothetical substance that dodge all combustible bodies during combustion.

The phlogiston theory was designed to work the chemist Johann Joachim Becher based. In his book Physica Subterranea ( 1667), the mineral substances and soils are classified

• Terra fluida or merkuralische earth, giving liquid, fineness, volatility and metallic properties of the substances
• Terra pinguis or greasy earth, this corresponds to the oily liquid of the alchemists, the oily Substances, sulfurous and bring combustible property
• Terra lapidea or glassy earth, which is the principle of meltability.

Air have in the formation of minerals no part. These three principles are then available for various materials, the terra fluida for the mercury (principle of volatility), the terra pinguis for the sulfur (principle of combustibility ) and the terra lapidea for the salt (the principle of strength). Georg Ernst Stahl now replaced the sulfur principle terra pinguis - color, flammability, oiliness and moisture. It gets in steel considerations of central importance, it becomes the phlogiston. All combustible body would contain phlogiston and during combustion was to be a decomposition into phlogiston, which is volatile and dodge, and the remaining, phlogistonfreien and incombustible part, the ashes.

Steel refined the theory of goblet in his work Zymotechnica fundamentalis (1697 ). It should be noted here that the subdivisions of materials and events to differ substantially at this time of our present ideas.

According to this theory of phlogiston was a fiery element in substances, that supports combustion, the oxidation of pure metals, the fermentation of organic matter, the decomposition of plants and animals. Chemists had observed that sulfur coal or burnt residue. These substances contained by former idea very much phlogiston. Other materials such as the metals copper, tin, zinc converted themselves into earthy, salty substances. Thus, the conclusion was obvious that these substances contained less phlogiston. Depending on the speed and extent of conversion in salt-like substances, the metals were different noble. Only gold and silver remained partially with the use of all chemicals unchanged, therefore they contained little or no phlogiston, they were noble, or unchanged from the perspective of former chemist. By heating with coal could be the metal out the phlogiston again, the metals were thereby revived with phlogiston.

The phlogiston theory difference, in contrast to the Cartesian embossed chemistry Robert Boyle, between mechanically prepared mixtures and so-called Mixts ( chemical compounds ). The properties of Mixts were not simply the sum of its components: metals contained phlogiston, but were not flammable themselves, because they still contained the principle of metallic earth.

Strengths and limitations of the theory

The influence of this theory was very large in the 18th century, because according to this theory, oxidation-reduction reactions, acids, bases and salts could be systematically investigated. Certain substances such as sulfur and phosphorus burned to gases, the acid dissolved in water. Other salts of Nature ( burnt lime, metallic calces ie metal oxides ) reacted with alkaline water. With litmus acids and bases could be detected. Were combined such opposed substances such as acids and bases, as neutral salts formed. With the help of the phlogiston theory, the acids (phosphorus, sulfur) and bases could systematize ( metallic calces ) from certain groups better. At the same time, the phlogiston theory preserved ancient alchemical ideas about the four primal elements (earth, water, air, fire ) to Empedocles. According to the phlogiston theory, there were only substances containing much or little phlogiston. There were no elements in the modern sense, but all were composite materials as well as less phlogiston - only phlogiston was a fundamental element for steel. In addition, the alchemical principles mercury came (liquid, shiny metal ) and sulfur ( flammable).

Robert Boyle had already put it another way in his book The Sceptical Chemist of 1661 in the 6th section of his book, the element conception and in contrast to the phlogiston theory develops a clearer idea of ​​the chemical element.

In addition, it was possible in the 18th century to describe the phlogiston theory many phenomena then known of chemistry. So this theory explained the finding that candles go out in closed vessels after some time. Air should then only can absorb a certain amount from the candle escaping phlogiston. Also, the realization that some of the air ( on the later recognition of oxygen) may support combustion longer was initially explained by Joseph Priestley that this dephlogestierte air is, which could therefore absorb more phlogiston.

Carl Wilhelm Scheele describes in his book published in 1777 Chemical Observations from the air and fire a combustion-supporting portion of the air and calls this fire air (oxygen). He also indicates several ways in which these fire air could be made ​​, for example by heating pyrolusite ( manganese dioxide) with concentrated sulfuric acid ( H2SO4). The time according to he interpreted these events as part of the phlogiston theory.

The combustion of organic matter proceeds (excluding the then unknown gaseous reaction products ) usually with weight loss. According to the phlogiston theory, the pre-recorded by the plants phlogiston is intended to be released again. Same explanations are also some non-metals, such as phosphorous or sulfur, possible. In metals, there were problems, as these generally form solid oxides and thus when burning heavier. The efforts made by Boyle attempts were, however, affected by the fact that it depended on the specific weight and not on the absolute weight. In addition, the experimental possibilities were limited in the 18th century, so many chemists reported due to the unobserved evaporation of a portion of the oxide of weight losses. The reduction of metal oxides with carbon to metals has been explained without contradiction by the absorption of phlogiston from the charcoal.

In the wake of the discovery of gaseous compounds and the use more accurate methods of measurement problems and errors of this theory were clearly increasing. In particular, there was no conclusive explanation for the weight gain in burns of metals. In order to save the theory, proponents tried, the phlogiston properties such as a negative mass attributable. Also, the hydrogen then discovered was partly held for phlogiston.

Was definitively refuted the theory only in 1785 by Antoine Lavoisier, who was able to show that all combustion phenomena could be explained without the use of extraordinary assumptions with his theory and oxidation by the oxygen gas. The final strong hypothesis of phlogiston theory, the explanation of the hydrogen formation on reaction of metals with acids, could from him by the realization that water is a compound of oxygen and hydrogen, can be ruled out.

Last yet possible explanations of phlogiston as " caloric " 1798 could be refuted by Benjamin Thompson in favor of the theory of the motion of the particles. He let dull drill steel in gun barrels. The tubes were again hot again and removed the allegedly present phlogiston by water. The heat could therefore not have been caused by an existing in the pipes exhaustible material.

The reality of phlogiston seemed proven at this time, so there were few attempts to explain the processes differently, since the adoption of the phlogiston everything seemed to be explainable. He had not been limited in this period of scientific research the claim of the remaining non- educational work of all the detail and knowledge was through the partially subjective observation opportunities. With the aim of holistic explanations science again asserted that " morally beautiful " with the older religious views of nature connect. Even researchers such as Joseph Priestley, who recognized the inherent contradiction to the phlogiston theory, remained with their attempts at explanation on this theory.

The important implications of this theory

Although the theory reversed the situation, according to current knowledge, much could be better understood and systematized with this interpretation in that period of history of science, or even explained.

• The phlogiston theory was sufficient to make the oxidation and reduction processes understandable.
• It suggested to the " finely distributed " phlogiston collect and examine and understand the gas properties.
• The theory enabled a systematization of material groups forming acids and bases.

• Scientists sat down with the phlogiston apart

Henry Cavendish (1731-1810) Figure with his signature. Title page from The Life of the Hon Henry Cavendish.

Friedrich Albrecht Carl Gren (1760-1798)

Antoine Laurent Lavoisier de (1743-1794) replaced the phlogiston theory of oxidation.

Joseph Priestley (1733-1804)

Carl Wilhelm Scheele (1742-1786) described the 1771-1772 fire air or oxygen later

Alessandro Volta (1745-1827)

Replacement by the oxidation theory

The phlogiston theory was replaced late 18th century by the oxidation theory of the chemist Antoine Lavoisier. Georg Ernst Stahl and his subsequent representatives of the phlogiston theory, that is, all Phlogistoniker, saw in the phlogiston, a substance that would be released when burned. One example described the transformation of metals by heating in air as calcification, it said the metal would lose phlogiston. Conversely phlogiston upon heating of an ore with coal would be planar, to form a shiny metal, taken from the latter.

Lavoisier convinced himself as a skeptic in the course of his experiments, that the substances during combustion (oxidation) bind oxygen. By correcting the theory based on the results of his experiments he could become more accurate expand the now acclaimed interpretation of reality contrary to prevailing opinion. Finally, other laws could be derived from this modified understanding, such as formulated by Joseph Louis Proust law of constant proportions (1797 ) or recognized by John Dalton in 1805, Dalton's law.

For the development of the knowledge Stahlsche phlogiston theory was an innovation. He realized that the burning of a substance ( in modern terms: a chemical process ) from a reaction between two substances was materially different, and recognized by the experiments which he had introduced to the other at the same time their reversibility or reciprocity. A substance A is the phlogiston from a substance other B takes on (incineration, in modern terms: oxidation). On the other hand, the recorded material B phlogiston again to give substance A ( reduction). Steel discovered the mutual conditionality of combustion (oxidation ) and reduction processes.

To contradict this theory in particular, led the neglect of the gaseous reactant these reversible processes in the following representatives of the phlogiston theory.

The observed phenomenon was related to the increase in weight of metals during combustion, was ultimately confirmed by Lavoisier in a variety of experiments and explained. When Joseph Priestley discovered gas in 1774, which was capable of all others, to support combustion, received the criticism of the phlogiston theory indirectly further support. Lavoisier showed that Priestley was gas one of the elements of the air, and called it Oxygène ( Oxygen ). From now on, the importance of air was clear during combustion. During combustion, phlogiston would not discharged, but air or rather their constituents absorbed. The combustion was no decomposition, but a compound which is going on, by a certain element of air is fixed with the combustible body. This takes on weight by burning ( oxidizing ), and the weight gain is exactly the weight of the added came gaseous body is equal.

Lavoisier investigated the changes in weight of different substances on oxidation and reduction, and discovered that the just discovered element oxygen plays a crucial role. Vain did the defenders of phlogiston theory, as Henry Cavendish, Joseph Priestley and Carl Wilhelm Scheele himself to keep the theory of steel by the same modified and claimed that the role of the air would be combustible bodies to withdraw the phlogiston.

Lavoisier showed by ensuring that

• On burning metals or sulfur these unite with oxygen,
• Doing as much oxygen is consumed, as is contained in the resulting oxides,
• You to recover metals from the oxides, do not add phlogiston, but must remove the oxygen.

Considering the criticism of the phlogiston theory about the concepts of Joseph Priestley dephlogisticated air or Antoine Laurent de Lavoisier oxygène so can be both easily concepts combine with the modern concept of the redox reaction or the Lewis acid-base concept. Acids are substances which release cations or anions or electrons can absorb. Thus, the oxidizing agent, which are reduced in a chemical reaction thereby. Repatriation Sets in the theory of phlogiston correspond to a Phlogistonaufnahme. There are thus electrophiles and correspond electrochemically positive ( anode ) or the oxygène or dephlogisticated air. Then the reciprocal assumptions apply to bases. Here it meant in the retranslation a Phlogistonabgabe.