The son of a Nonconformist minister, Priestley was born in Yorkshire, England. As a young man he had many interests. He taught himself several languages, including Arabic and Hebrew, as well as philosophy and science. He had radical religious beliefs and eventually became a Unitarian Minister.
In 1766, he met Benjamin Franklin who was on a visit to London to attempt to settle the dispute over taxation between the American colonists and the British government. Franklin had been studying electricity and this meeting inspired Priestley to begin his own research in the field. He was the first to show that graphite is a good conductor of electricity.
Later, he turned to chemistry. As a result of his experiment on the decomposition of mercury oxide by heat, he discovered oxygen. He was the first to collect gases over mercury, and he thus discovered several water-soluble gases, including ammonia, hydrogen chloride, and sulfur dioxide.
An outspoken man with very liberal religious and political views, he was sympathetic to the American and French revolutions and was viewed with suspicion by the conservative British majority. In 1791 his house and laboratory were burned down by an angry mob. Priestley managed to escape, went into hiding for a time, and eventually emigrated to the United States. He spent the last ten years of his life in relative seclusion in Pennsylvania.