Joseph Priestley’s chemical flask at the Smithsonian, Washington, D.C. I took this photo while on the first of my Thomas Jefferson history of ideas tours, 2015. Priestly discovered oxygen on this day, August 1, in 1774 (not 1775 as the placard says) and he became most famous for his discovery since he was the first to publish a description of it. It was, however, first discovered two years earlier by a Swedish chemist, Carl Wilhelm Scheele, independently.