The phlogiston theory was put forward in 1667 by Johann Joachim Becher as an attempt to explain the combustion process and the rusting of metals both of which are now known as oxidation.
The Phlogiston theory said that all flammable substances had phlogiston within them. It was something that did not have any colour, smell, taste or mass that was released upon burning. After everything was burned, only then would the dephlogisticated, substance would take its true form. For instance substances that could be burned in air were determined to be rich in phlogiston. When air was taken away, burning would cease leading to a conclusion that air had a capacity to absorb a finite amount of phlogiston.
Joseph Priestly was an 18th-century English theologian, Dissenting clergyman, natural philosopher, chemist, educator, and political theorist. Priestley’s science was integral to his theology, and he consistently tried to fuse Enlightenment rationalism with Christian theism.
During his lifetime, Priestley’s considerable scientific reputation rested on his invention of soda water and his discovery of several “airs” (gases), the most famous being what Priestley dubbed “dephlogisticated air” (oxygen). However, Priestley’s determination to defend the phlogiston theory and to reject what would become the chemical revolution eventually left him isolated within the scientific community.