In 1896 the French physicist Henri Becquerel (1852-1908) was exploring the properties of a compound of uranium that glowed in the dark. He placed the compound in a dark drawer on top of a photographic plate that he had wrapped with heavy black paper to make it light-tight. The next morning he was surprised to find that the film was already exposed, or fogged. Becquerel presumed that some unknown rays had been emitted by the uranium compound, gone through the paper, and created the same chemical reaction in the plate that light would. In further experiments he found that a thin piece of metal would block the rays. It was soon found that compounds of both uranium and thorium emitted the rays, but that they did not emit light. In 1903 he shared the Nobel Prize in recognition of his discovery of what was later called radioactivity.

Marie Sklodowska Curie, born and raised in Poland, but working in France, used an electrometer to measure the ionization of the air caused by radioactive minerals. Because the amount of radioactivity produced by a uranium compound depended only on the amount of uranium present, she concluded that the atom itself must be the source. She found that the uranium-containing mineral pitchblende was more radioactive than the uranium itself and concluded that the mineral must contain a small quantity of another element that was more radioactive than the uranium. Her husband, Pierre, stopped his own work and joined Marie in searching for the element.

They started by grinding up 100 grams of pitchblende, but by the time they had found the element they had processed tons of the mineral. In July 1898 they announced the discovery of an element they named “polonium” in honor of Poland, where she had been born. In December of the same year they announced they had found an even more radioactive element that they named radium. It took until 1902 for them to separate one-tenth of a gram of radium chloride from a ton of pitchblende. In 1910 Marie announced that she had obtained pure metallic radium.

Pierre and Marie shared the 1903 physics Nobel Prize with Becquerel for their work on radioactivity. Marie won the 1911 chemistry Nobel Prize for her discovery of polonium and radium.

The dangers of radioactivity were unknown when the Curies did their work, but both were affected by it. The radon that was produced when they were processing the pitchblende would have caused lung cancer. But, the leaky windows in their workplace and their frequent bicycle rides in the country spared them. Pierre died in 1906 when he slipped on a wet street and was run over by a horse-drawn wagon. Marie’s death in 1934 was due to anemia, known now to be frequently caused by radiation.