Summary:
This theory was based on the idea that fire was due to a fiery principle called "phlogiston," which was given up during combustion.[1] The Theory of Phlogiston was universally accepted in 1780.[2]
Phlogiston theory states that phlogisticated substances are substances that contain phlogiston and dephlogisticate when burned. Growing plants then absorb this phlogiston, which is why air does not spontaneously combust and also why plant matter burns as well as it does.[3] Substances which burn were said to be rich in phlogiston. Other, such as metals, which do not really catch fire, but produce much ash or "calx" were said to be poor in phlogiston.[4] Metals were composed of calx and phlogiston. The function of air was merely to carry away the liberated phlogiston.[5]
Carbon was thought to be almost pure phlogiston, since it burns away and leaves little ash. The ash or calx of lead was regarded as the metal from which all phlogiston had escaped. Carbon and lead calx together therefore should produce the metal [lead calx + phlogiston = metallic lead].[4]
The Problem:
Quantitative experiments revealed problems, including the fact that some metals gained weight after they burned, even though they were supposed to have lost phlogiston. Some phlogiston proponents, like Robert Boyle, explained this by concluding that phlogiston has negative weight; others, such as Louis-Bernard Guyton de Morveau, gave the more conventional argument that it is lighter than air.[3]