The existence of an colourless, odourless substance that was given off during combustion was first proposed during the 17th century. In 1700 Georg Ernst Stahl gave this substance the name phlogiston. He derived it from the greek phlogistos, meaning flammable.

It was believed that substances that could be burnt did not exist in their true form. Their true form was what was left after combustion, and the pre-combusted form was a combination of the substance and phlogiston. Burning the object liberated the phlogiston, and enabled the object to assume its true form (the calx). This process was known as dephlogistination.

A major blow to the theory of phlogiston emerged when it was discovered that a metal such as magnesium actually gains mass when burnt. This lead to phlogistians proposing that Phlogiston had negative mass! This meant that the release of phlogistion from a burning object would cause it to gain mass, thus fitting the observations.

Eventually Lavoisier provided a much more satisfactory explanation, that of oxidation, which is still in use today. The calx is actually the metal oxide, and the gain in weight is due to the addition of oxygen to the substance.

Phlo*gis"ton (?), n. [NL., fr. Gr. burnt, set on fire, fr. to set on fire, to burn, fr. , , a flame, blaze. See Phlox.] Old Chem.

The hypothetical principle of fire, or inflammability, regarded by Stahl as a chemical element.

This was supposed to be united with combustible (phlogisticated) bodies and to be separated from incombustible (dephlogisticated) bodies, the phenomena of flame and burning being the escape of phlogiston. Soot and sulphur were regarded as nearly pure phlogiston. The essential principle of this theory was, that combustion was a decomposition rather than the union and combination which it has since been shown to be.

<-- this theory is now discredited and superseded by the theory of chemical reaction between oxidizable substances and oxidants as an explanation of combustion -->

 

© Webster 1913.

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