Joseph Priestley. Coloured historical engraving of the British chemist Joseph Priestley (1733- 1804). Priestley isolated and examined many new gases including hydrogen chloride, nitrous oxide, ammonia, nitrogen and carbon monoxide. He was also the first to isolate oxygen but did not correctly recognize its role in combustion, believing instead that it played a part in the 'phlogiston' (combustible element) theory. However, he did discover oxygen's role in respiration, showing that animals consume oxygen and plants produce it. Priestley was persecuted for his religious and political beliefs. He left England in 1794. Engraving by C. Cook.