Phlogiston theory

Phlogiston is also the name of a substance in the fictional setting Spelljammer.

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The phlogiston theory is a obsolete scientific theory of combustion.

Development

The theory was developed by J. J. Becher late in the 17th century and extended and popularized by Georg Ernst Stahl who declared the rusting of metal to be a combustion process.

Theory

The theory holds that all flammable materials contain phlogiston (derived noun form of the Greek phlogistos, meaning flammable), a substance without color, odor, taste, or weight that is liberated in burning. Once burned, the "dephlogisticated" substance was held to be in its "true" form, the calx.

The theory is related to alchemical notions of the classical elements: fire, water, air and earth. All substances were held to be a combination of these four elements.

"Phlogisticated" substances are those that contain phlogiston and are "dephlogisticated" when burned. Since any substance could be observed to burn for only a limited time with limited air (for instance in a sealed container), air was thought to have a specific capacity for phlogiston.

Joseph Black's student Daniel Rutherford discovered Nitrogen in 1772 and the pair used the theory to explain his results. The residue of air left after burning, in fact a mixture of nitrogen and carbon dioxide, was sometimes referred to as phlogisticated air, having taken up all of the phlogiston.

Conversely, when oxygen was first discovered it was thought to be "dephlogisticated air", capable of combining with more phlogiston and thus supporting combustion for longer than ordinary air.

Challenge and demise

Eventually, experiments revealed problems, including the fact that some metals gained weight when they burned, even though they were supposed to have lost phlogiston. Still, phlogiston remained the dominant theory until Antoine Laurent Lavoisier showed that combustion requires oxygen, solving the weight paradox and setting the stage for the new caloric theory of combustion.

In some respects, the phlogiston theory can be seen as the opposite of the modern "oxygen theory". Whereas the phlogiston theory states that all flammable materials contain phlogiston that is liberated in burning, and the "dephlogisticated" substance was now in its "true" calx form. In fact, flammable materials (or unrusted metals) are "deoxygenated" when in their pure form and become oxygenated when burned.

See also

• Science – Discoveries of the chemical elements, Combustion (Calx)
• People – Antoine Laurent Lavoisier Joseph Priestley, Georg Ernst Stahl, J. J. Becher, Daniel Rutherford, Mikhail Lomonosov, Joseph Black
• Theories / Beliefs – Caloric theory, Caloric, Eliminative materialism, Obsolete scientific theory, List of alternative, speculative and disputed theories