In 1794, Joseph Priestley - amateur scientist, ordained minister and radical thinker - set sail for America to escape persecution. Stephen Johnson tells his incredible story: the discovery of oxygen, the invention of a science, the founding of a church, and, with the great minds of his time, the development of the United States itself. But Priestley's revolutionary ideas put him in terrible danger. Johnson uses the progress of Priestley and his colleagues not merely to describe the wonder of discovery, but to show us how we have come to understand the world, how far we have travelled with the power of human enquiry - and how one man's curiosity can help build an entire country.