Web page: Marie Curie Bio (Nobelprize.org)
Web page: Polonium (Jefferson Lab)
Web page: Radium (Jefferson Lab)
Web site: Ford's Theatre (Official Site)
Web site: Abraham Lincoln Presidential Library and Museum (Official Site)
Web page: John Wilkes Booth Bio (NotableBiographies.com)
Web page: A Brief History of the Abacus (Ryerson University)