His theological training gave him an independent and disputatious spirit that, combined with his hot temper, made him a dangerous opponent in debate.
Priestley's avocation was science. He discovered several gases, the most important being oxygen (later named by Lavoisier). The work that earned him election to the Royal Society, however, was the History and Present State of Electricity (1767). His most productive years of scientific work took place while in the service (1773-1780) of William Petty, 2d earl of Shelburne, in whose household he served as librarian and tutorial advisor. He traveled with Shelburne in Europe and met a number of scientists there. Priestley published a 6-volume account of his work, Experiments and Observations of Different Kinds of Air (1774-86). His studies of plant respiration anticipated the concept of photosynthesis.
Because of Priestley's positions as a minister and teacher, his strongly expressed opinions led to controversy. Much of his influence is due to his books, which total more than 70. His History of the Corruptions of Christianity (1782)--in which he not only attacked Roman Catholicism as a chief repository of error but also maintained that any departure from the original faith of Christ and the apostles was corrupt--caused an uproar. Moreover, Priestley attempted to prove that Apostolic Christianity was Unitarian. This book was officially burned in Dort, the Netherlands, by the public hangman. After more conflict a mob destroyed (1791) Priestley's church, house, and laboratory. In 1794 he fled England to settle in the United States.
