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.