>>16729502 (OP)Radioactivity was discovered in the 1850s by Nicephore Niepce, while doing photographic research. He invented the first photographic plates using silver chloride, then developed a process using bitumen, he tested thousands of compounds for light sensitivity and eventually tried his luck with some uranium compounds. But to his surprise, his photopgraphic plates kept getting exposed while being enclosed in a box, in his lab. Somehow the uranium compounds gave off light that could cross the wooden boxes.
This was written about in "La lumieres ses causes et ses effets", by Edmond Becquerel, father of Henri Becquerel, alleged discoverer of radioactivity.