Some of Joseph Priestley's laboratory equipment, 1775. Joseph Priestley (1733-1804) was an English theologian, natural philosopher, chemist, educator, and political theorist. His most famous scientific research was on the nature and properties of gases. By clever design of apparatus and careful manipulation, Priestley isolated and characterized eight gases, including oxygen, and out of this work emerged his most important scientific texts: the six volumes of Experiments and Observations on Different Kinds of Air (1774-86), from which this image is taken. He also contributed to the understanding of photosynthesis and respiration.