Joseph Priestley (1733-1804) was an English theologian, natural philosopher, chemist, educator, and political theorist who published over 150 works. Priestley's first scientific work, The History of Electricity (1767), was encouraged by Benjamin Franklin. 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). He also contributed to the understanding of photosynthesis and respiration.