Timeline of Earth Elements By anaboisvert Jan 1, 1774 Johann Gahn discovers Magnesium Jan 1, 1774 Joseph Priestly discovers Oxygen Period: Jan 1, 1774 to Jan 1, 1825 Earth Element Discoveries Jan 1, 1807 Sir Humphrey Davy discovers Potassium Jan 1, 1825 Hans Christian Oersted discovers aluminum