Lanthanum (LA) - atomic weight, 139 - is a rare metallic element which occurs in cerite (q.v.) and some other rare allied minerals. The metal, which was discovered by Mosander in 1839, has a steel-grey colour, and oxidises if exposed to moist air. It possesses moderate ductility and malleability. It forms salts corresponding to the oxide La203, which is a white powder somewhat resembling lime.