How did Pierre Curie discover radium?

Answer 1

The Curies noted that pitchblende, one of many compounds of uranium, had significantly higher amounts of radioactivity than other compounds. From that, they concluded that pithcblende contained an element more radioactive than uranium. They chemically removed those parts of pitchblende with less radioactivity, and ended up with a microscopic amount of radium -- after processing literally tons of pitchblende.