The world of chemistry was shaken Monday by lanthanum's announcement that the popular 57th element will quit Transition Group IIIb of the periodic table at the end of the summer. 'I have nothing but good things to say about my time with the periodic table,' said the ductile, silvery-white metal, speaking from the site of its discovery by Carl Gustav Mosander in 1839. 'Nevertheless, I will be stepping down after Labor Day to focus on my own earth-metal solo projects.' Rumors of a longtime feud with molybdenum and the constant demands of lens manufacturing are believed to be behind the departure. 