Phlogiston theory: Encyclopedia II - Phlogiston theory - Challenge and demise

Phlogiston theory - 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. Some phlogiston proponents explained this by concluding that it had "negative weight"; others, such as Guyton de Morveau, gave the more conventional argument that phlogiston 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. 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, but introducing a new substance, caloric.

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.

Adapted from the Wikipedia article "Challenge and demise", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki