Marie Curie (1867-1934) shows that thorium is radioactive and hypothesizes that radioactive emanations come from single atoms
Editors. "Theory of radioactivity starts being developed by Marie Curie". The Literary Encyclopedia. First published 01 May 2009; last revised 30 November -1.
[http://www.litencyc.com/php/stopics.php?rec=true&UID=6382, accessed 28 April 2015.]