phlogiston

n.
A hypothetical substance formerly thought to be a volatile constituent of all combustible substances, released as flame in combustion.

[From Greek, neuter of phlogistos, inflammable, from phlogizein, to set on fire, from phlox, phlog-, flame.]

Phlogiston was a name used in the 18th century to identify a supposed substance given off during the process of combustion. The phlogiston theory was developed in the early 1700s by the German chemist and physicist Georg Ernst Stahl (1660-1734).

In essence, Stahl held that combustible material such as coal or wood was rich in a material substance called "phlogiston." What remained after combustion was without phlogiston and could no longer burn. The rusting of metals also involved a transfer of phlogiston. This accepted theory explained a great deal previously unknown to chemists. For instance, metal smelting was consistent with the phlogiston theory, as was the fact that charcoal lost weight when burned. Thus the loss of phlogiston either decreased or increased weight.

The French chemist Antoine Laurent Lavoisier (1743-1794) demonstrated that the gain of weight when a metal turned to a calx was just equal to the loss of weight of the air in the vessel. Lavoisier also showed that part of the air (oxygen) was indispensable to combustion, and that no material would burn in the absence of oxygen. The transition from Stahl's phlogiston theory to Lavoisier's oxygen theory marks the birth of modern chemistry at the end of the 18th century.

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the hypothetical principle of fire regarded formerly as a material substance

The question of what fire really is has intrigued people since ancient times, but not until the Renaissance did scientists offer explanations for the phenomenon. Classical Greeks thought that fire is an "element" no more explicable than earth, water, or air.

Robert Boyle found, however, that fire depends on air. If one removes air in a vessel, combustion cannot take place. He also found that air is required for the respiration of small animals and for the production of oxides from metals by heating them. In this process chemical changes caused by heating a substance to a temperature below the melting point (calcination) results in a compound called a calx.

Around this time, however, chemists in Germany formulated a theory of combustion that did not require air. Johann Joachim Becher believed that combustible matter contains "oily earth," a substance released during combustion. Georg Stahl renamed this substance phlogiston. With the phlogiston theory these chemists could explain not only combustion but also the reverse of calcination, reduction. According to this theory, if calx of lead is brought into contact with charcoal and heated, the phlogiston flows from the charcoal into the calx of lead, changing it to metallic lead. Because charcoal burns so well, it was believed to contain a lot of phlogiston.

One of the first indications that something was amiss with the theory was evidence that metals gain weight upon calcination. Clearly, this contradicted the idea that upon calcination, phlogiston leaves the metal. Chemists saved the phlogiston theory by assuming that the substance had "negative weight."

Around the same time Joseph Priestley experimented with heating calx of mercury and found that it produces "air" with special properties. (The word gas had entered English in 1658 but was not common yet; Priestley used it sometimes, but felt he had to explain it.) If one introduces a candle into this "air", the candle burns vigorously. A mouse shut in a jar with it lives longer than one shut in a jar filled with ordinary air. Priestley believed in phlogiston, so he tried to explain this result with the phlogiston theory. Priestley called this "air" dephlogistonated air. He believed that air has phlogiston added to it by fires and by breathing.

Antoine Lavoisier discovered in 1772 that phosphorus and sulfur absorb a portion of air when burning instead of expelling something. Lavoisier also discovered that the reduction of calx of lead releases large quantities of "fixed air," air that does not sustain life or combustion (now known as carbon dioxide). That is, reducing a calx to a metal expels something, it does not absorb something. These results, and also the results of other experiments, led Lavoisier to formulate a theory of combustion that superseded the phlogiston theory. Lavoisier had realized that "dephlogistonated air," a gas that he subsequently termed oxygen, was responsible for the combustion of materials.

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The noun has one meaning:

Meaning #1: a hypothetical substance once believed to be present in all combustible materials and to be released during burning

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