Joseph Priestley FRS (/ˈpriːstli/; 24 March 1733 – 6 February 1804) was an English chemist, natural philosopher, separatist theologian, grammarian, multi-subject educator, and liberal political theorist who published over 150 works. He has historically been credited with the independent discovery of oxygen in 1774 by the thermal decomposition of mercuric oxide, having isolated it. Although Swedish chemist Carl Wilhelm Scheele also has strong claims to the discovery, Priestley published his findings first. Scheele discovered it by heating potassium nitrate, mercuric oxide, and many other substances about 1772.