By the time her second daughter, Eve, was born in 1904, Marie had grown accustomed to the disdain of colleagues who thought she spent too much time in the lab and not enough in the nursery. Georges Sagnac, a friend and collaborator, eventually confronted her. “Don’t you love Irène?” he asked. “It seems to me that I wouldn’t prefer the idea of reading a paper by [Ernest] Rutherford, to getting what my body needs and looking after such an agreeable little girl.”
But read scientific publications she did. In labs across Europe, scientists were studying new and surprising phenomena. In 1895 Wilhelm Röntgen had discovered X-rays, and the mathematician Henri Poincaré sought to understand the luminescent rays that could pass through a hand and impress a ghostly image on photographic paper. Henri Becquerel was noting the emission of a different kind of mysterious rays, those from uranium salts. J. J. Thomson discovered negatively charged particles, which we now know as electrons (and which we now know are the source of X-rays).
Curie built on Becquerel’s observations of the element uranium. At first, she and other scientists were baffled about the source of the high-energy emissions. “The uranium shows no appreciable change of state, no visible chemical transformation, it remains, in appearance at least, the same as ever, the source of the energy it discharges remains undetectable,” she wrote in 1900. She wondered whether the emitted rays were violating a basic law of thermodynamics: the conservation of energy.
Finally, she posited a daring hypothesis: The rays emitted might be a basic property of uranium atoms, which we now know to be subatomic particles released as the atoms decay. Her theory had radical implications. Trish Baisden, a senior chemist at the Lawrence Livermore National Laboratory, describes it as a shocking proposal: “It was truly amazing and a bold statement at the time because the atom was thought to be the most elementary particle, one that could not be divided. It further meant that atoms are not necessarily stable.” Curie’s hypothesis would revise the scientific understanding of matter at its most elemental level.
Curie set out to measure the intensity of uranium’s rays by adapting the electrometer Pierre had invented with his brother. The device allowed her to measure extremely low electrical currents in air near mineral samples that contained uranium. She soon repeated the experiment with thorium, which behaved in similar ways.
But she was puzzled by data that showed that the intensity of the radiation emitted by uranium and thorium was greater than expected based on the amounts of the elements she knew to be in her samples. “There must be, I thought, some unknown substance, very active, in these minerals,” she concluded. “My husband agreed with me and I urged that we search at once for this hypothetical substance, thinking that, with joined efforts, a result would be quickly obtained.”
In 1898 she indeed identified one of the substances and named it polonium, after her homeland. Five months later, she identified a second element, which the world came to know as radium. Curie described the elements she studied as “radio-active.”
Pierre put his crystals aside to help his wife isolate these radioactive elements and study their properties. Marie extracted pure radium salts from pitchblende, a highly radioactive ore obtained from mines in Bohemia. The extraction required tons of the substance, which she dissolved in cauldrons of acid before obtaining barium sulphate and other alkalines, which she then purified and converted into chlorides. The separation of radium from the alkalines required thousands of tedious crystallizations. But as she wrote to her brother in 1894, “one never notices what has been done; one can only see what remains to be done.” After four years, Curie had accumulated barely enough pure radium to fill a thimble.
Working in a dilapidated shed with broken windows and poor ventilation, she nonetheless was able to make sensitive measurements. It is remarkable, says Baisden, that Curie calculated the atomic weight of radium so accurately given such deplorable conditions. “Large swings in temperature and humidity undoubtedly affected the electrometer...but Marie’s patience and tenacity prevailed.”
Both Curies were plagued by ailments—burns and fatigue—that, in retrospect, were clearly caused by repeated exposures to high doses of radiation. Both, too, were resistant to the suggestion that their research materials caused their ailments.