Joseph Priestley, born 1733 is best knowned for his discovery of Oxygen gas or as he called it ‘dephlogisticated air’. Before Priestley’s discovery of oxygen, the science community believed in the phlogiston theory. The phlogiston theory stated that ‘phlogiston’ (Greek word for burned) was removed from a substance was removed when burned. This theory was first proposed by Johann Becher, who called it “combustible earth”. In the early 18th century chemist George Stahl elaborated on the theory of phlogiston by stating metal were made up of two things calx and phlogiston. Stahl suggested that when metal were heated phlogiston was removed, calx was the powdery substance remaining. Priestley was not the first chemist to discover oxygen, Carl Scheele Swedish chemist was. Although Priestley was credited for the discovery because he published it first. Priestley’s apparatus for pneumatic experiments.
Priestley discovered oxygen by using a 12-inch diameter lens on a substance (mercurius calcinatus per se) his friend John Warltire gave him. The lens allowed him to heat up the substance, and the substance separated into two parts Mercury liquid and what he called “dephlogisticated air”. The flaw of the phlogiston theory that lead Priestley’s discovery was that when Mercury was heated at low temperatures mercurius per se formed (HgO). The phlogiston theory stated when mercury was heated it was supposed to lose the mass of phlogiston, not gain the mass of something else. 2 HgO(s) → 2 Hg(l) + O2(g)
Although Priestley discovered more than dephlogonisticated air, he is never did completely abandon the phlogiston theory. Chemist Antoine Lavoisier was the one credit for denouncing the phloginist theory, and given oxygen’s its name. Despite this, Priestly is still credited for the discovery of chemicals that compose air such as Oxygen, Carbon Monoxide, Nitrogen, Nitrix Oxide, and Nitrogen Dixode. Priestley laid down the foundation for modern chemistry.