Marie Curie
In 1897 Marie Curie began studying the same uranium salts that Becquerel reported on the year before and she asked herself “by what process can uranium furnish the same rays [Röntgen’s x-rays] without expenditure of energy and without undergoing apparent modification?” (i.e. without a Crookes’ tube?), and most importantly “Is uranium the only body whose compounds emit similar rays?”. Curie was more thorough and methodic than Becquerel; instead of relying on photographic plates, she measured the conductivity of the air around the salts—showing that their emanations were independent of temperature or exposure to light (the latter being a red herring that Becquerel was not able to let go). Furthermore, she identified that it was the metal in the compounds that was doing the work, not the compound itself. Steadfast, she proceeded to explore other metals, identifying thorium as another substance that could emit these rays. Later, she noticed that “pitch-blende (oxide of uranium ore) was four times more radioactive than oxide of uranium itself”. Hence, “The ore must contain a substance more radioactive than uranium and thorium, and this substance must necessarily be a chemical element as yet unknown”. She and her husband set out to isolate this substance, but did not expect the task to be as momentous as it was. It took them years of work to process fifty kilograms of raw material and obtain a few minute crystals. They named it Radium, and through this work, Curie surmised that as small as they were, each gram of radium released one hundred calories per hour. She benevolently tried to explain what this meant, writing that it “can melt in an hour its weight in ice”; more practically, however, 100 grams of Radon have more energy than 100 tons of coal.
Source: Curie, M Radium and Radioactivity Century Magazine (January 1904), pp. 461-466
Image by Jhriscones under CC BY-SA 4.0
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!