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Philogiston Theory

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Phlogiston Theory

According to the phlogiston theory, propounded in the 17th century, every combustible substance consisted of a hypothetical principle of fire known as phlogiston,

which was liberated through burning, and a residue. The word phlogiston was first used early in the 18th century by the German chemist Georg Ernst Stahl. Stahl

declared that the rusting of iron was also a form of burning in which phlogiston was freed and the metal reduced to an ash or calx. The theory was superseded

between 1770 and 1790 when the French chemist Antoine Lavoisier showed that burning and rusting both involved oxygen and concluded that both ash and rust

were compounds of oxygen. Lavoisier's oxidization theory has been accepted by scientists from about 1800 to the present day.

The theory of phlogiston was predominantly German in origin, with much early work done in Mainz, though it was widely believed through much of the eighteenth

century -- two of the most prominent followers of the theory, Johann Joachim Becher and Georg Ernst Stahl (who first used the name phlogiston in 1700), were

Swedish. Phlogiston was not only widespread but deep-seated, and gave way to the atomic theory only slowly.

Phlogiston theorists identified three essences which comprise all matter: sulfur or terra pinguis, the essence of inflammability; mercury or terra mercurialis, the

essence of fluidity; and salt or terra lapida, the essence of fixity and inertness. In this respect phlogiston theory is similar to the ancient alchemical notions of earth,

air, fire, and water. The terra pinguis was renamed phlogiston. In this view, metals were made of a "calx" (or residue) combined with phlogiston, the fiery principle,

which was liberated during combustion, leaving only the calx. Air, according to the theory, was merely the receptacle for phlogiston; all combustible or calcinable

substances, in fact, were not elements but compounds containing phlogiston. Rusting iron, for instance, was believed to be losing its phlogiston and thereby returning

to its elemental state.

Phlogiston theory was widely supported throughout the eighteenth century, although it came under increasing attack as empirical research pointed up its difficulties.

When it was determined that some metals actually gained mass when burnt, partisans explained it by giving phlogiston a negative mass. Even Priestley believed in the

theory until his death, convinced that his discovery of oxygen was "dephlogisticated air." It was up to Lavoisier to realize the significance of his discovery.

Lavoisier made a symbolic break with phlogiston theory by burning all textbooks that supported the theory, just as Paracelsus had destroyed his copies of the works

of the medieval medical authorities. His theory of oxidation soon replaced phlogiston theory, and remains a part of modern chemistry.

Although he exaggerated its importance, Lavoisier was the first to understand the significance of

Priestley's work on oxygen, and is considered by some to have discovered the element. He

disproved phlogiston theory by demonstrating that oxygen is required for combustion, rusting, and

respiration. He combined his chemical abilities with an interest in zoology to produce pioneering

work on anatomy and physiology.

phlogiston theory , hypothesis regarding combustion. The theory,

advanced by J. J. Becher late in the 17th cent. and extended and

popularized by G. E. Stahl, postulates that in all flammable materials there is

present phlogiston, a substance without color, odor, taste, or weight that is

given off in burning. APhlogisticated@ substances are those that contain

phlogiston and, on being burned, are Adephlogisticated.@ The ash of the

burned material is held to be the true material. The theory received strong

and wide support throughout a large part of the 18th cent. until it was

refuted by the work of A. L. Lavoisier, who revealed the true nature of

combustion. Joseph Priestley, however, defended the theory throughout his

lifetime. Henry Cavendish remained doubtful, but most other chemists of the

period, including C. L. Berthollet, rejected it.

Phlogiston Theory

The failure to understand combustion was an insurmountable obstacle to real progress in

chemistry. Any theory of chemical change must be able to explain combustion and phlogiston

was the first real attempt to do so.

The fact that wood turns to ashes and metals become soft powders when heated and can be

changed back into metals in the presence of charcoal is hard to reconcile without imagining the

addition or subtraction of some substance.

Phlogiston was that substance.

The theory was simple, and although having serious contradictions, was better than no theory at

all. Besides, despite the quantitative work of Galileo and Newton, the importance of quantitative

measurements had not yet been impressed upon the chemists.

The phlogiston theory was really quite simple.

Metals and combustible substances contained an imponderable substance known as phlogiston



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