Phlogiston Theory: Up in Smoke

The theory of Phlogiston states that all combustible mater contains an invisible and undetectable substance known as phlogiston. Its existence is what supposedly explains fire and rust and other oxidation processes. 

The theory of Phlogiston was developed in the 17th century by JJ Becher, a doctor and alchemist. However, the theory was disproved in 1753 by Robert Boyle. 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 still clung to the theory, arguing that phlogiston had a negative weight and thus could not fall into Boyle's study, though the idea fell out of favor by the end of the 18th century.