The phlogiston theory is an obsolete scientific theory of combustion. It was developed by J. J. Becher late in the 17th century and was extended and popularized by Georg Ernst Stahl, who (correctly, but for the wrong reasons) 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.
"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, quantitative experiments revealed problems, including the fact that some metals, such as magnesium, gained weight when they burned, even though they were supposed to have lost phlogiston. Mikhail Lomonosov attempted to reiterate Robert Boyle's celebrated experiment in 1753 and concluded that the phlogiston theory was false. He wrote in his diary: "Today I made an experiment in hermetic glass vessels in order to determine whether the mass of metals increases from the action of pure heat. The experiment demonstrated that the famous Robert Boyle was deluded, for without access of air from outside, the mass of the burnt metal remains the same."
Some phlogiston proponents explained this by concluding that phlogiston had "negative weight"; others, such as Louis-Bernard Guyton de Morveau, gave the more conventional argument that it was lighter than air. However, a more detailed analysis based on the Archimedean principle and the densities of magnesium and its combustion product shows that just being lighter than air cannot account for the increase in mass.
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". The phlogiston theory states that all flammable materials contain phlogiston that is liberated in burning, leaving the "dephlogisticated" substance in its "true" calx form. In the modern theory, on the other hand, flammable materials (or unrusted metals) are "deoxygenated" when in their pure form and become oxygenated when burned.
http://en.wikipedia.org/wiki/Phlogiston_theory