Joseph Priestley. Cartoon of Joseph Priestley (1733-1804), British chemist. This cartoon, of 1791 portrays Priestley, who supported the French revolution, as a political agitator. 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. He discovered that animals consume oxygen and plants produce it. Priestley was persecuted for his religious and political beliefs. He left England in 1794.