Uranium was discovered by Martin Heinrich Klaproth, a German chemist, in the mineral pitchblende (primarily a mix of uranium oxides) in 1789. Although Klaproth believed he discovered pure uranium, he actually discovered uranium dioxide (UO2). Eugène-Melchoir Péligot noticed pure uranium reacted oddly with uranium tetrachloride (UCl4) and decided to isolated uranium by heating uranium dioxide with potassium in a platinum crucible. Radioactivity was discovered by Antoine Henri Becquerel from a sample of uranium in 1896. Nowadays, uranium is usually sold in the form of sodium diuranate (Na2U2O7·6H2O), also known as yellow cake, or triuranium octoxide (U3O8) because there is little demand for uranium metal.