The English chemist heated mercury oxide to produce “deflogged" air. In those days, science was dominated by the theory of phlogiston, the volatile principle of substances that is released during combustion and oxidation in the form of flame and heat. Priestley's discovery and Lavoisier's experiments buried the erroneous phlogiston theory, although Priestley himself continued to believe in it until the end of his life. Besides oxygen, he separated many other substances and, moreover, he invented carbonated water as well as a device for its production.
On August 1, 1774, English chemist Joseph Priestly discovered oxygen