Pierre Curie, b. May 15, 1859, d. Apr. 19, 1906, obtained his doctorate in the year of his marriage, but he had already distinguished himself (along with his brother Jacques) in the study of the properties of crystals. He discovered the phenomenon of piezoelectricity, whereby changes in the volume of certain crystals excite small electric potentials. Along with work on crystal symmetry, Pierre Curie studied the magnetic properties of materials and constructed a torsion balance with a tolerance of 0.01 mg. He discovered that the magnetic susceptibility of paramagnetic materials is inversely proportional to the absolute temperature (Weiss-Curie's law) and that there exists a critical temperature above which the magnetic properties disappear (curie temperature).
Since 1882, Pierre had headed the laboratory at the Ecole de Physique et de Chimie Industrielle in Paris, and it was here that both Marie and Pierre continued to work after their marriage. For her doctoral thesis, Madame Curie decided to study the mysterious radiation that had been discovered in 1896 by Henri Becquerel. With the aid of an electrometer built by Pierre and Jacques, Marie measured the strength of the radiation emitted from uranium compounds and found it proportional to the uranium content, constant over a long period of time, and uninfluenced by external conditions. She detected a similar immutable radiation in the compounds of thorium. While checking these results, she made the unexpected discovery that uranium pitchblende and the mineral chalcolite emitted about four times as much radiation as could be expected from their uranium content. In 1898 she therefore drew the revolutionary conclusion that pitchblende contains a small amount of an unknown radiating element.
Pierre Curie immediately understood the importance of this supposition and joined his wife's work. In the course of their research over the next year, they discovered two new spontaneously radiating elements, which they named polonium (after Marie's native country) and radium. A third element, actinium, was discovered by their colleague Andre Debierne. They now began the tedious and monumental task of isolating these elements so that their chemical properties could be determined.
In 1903, Marie Curie obtained her doctorate for a thesis on radioactive substances, and with her husband and Henri Becquerel she won the Nobel Prize for physics for the joint discovery of radioactivity. The financial aspect of this prize finally relieved the Curies of material hardship. The following year Pierre was appointed professor at the Sorbonne, and Marie became his assistant. She was deeply affected when Pierre died after being struck by a truck on a Paris street. She overcame this blow only by putting all her energy into the scientific work that they had begun together. The Sorbonne provided the opportunity by offering her the post that Pierre had held of lecturer and head of the laboratory. She thus became the first female lecturer at the Sorbonne, and in 1908 she was appointed professor. For the isolation of pure radium, Marie Curie received a second Nobel Prize in 1911, this time for chemistry.
During World War I, Madame Curie dedicated herself entirely to the development of the use of X rays in medicine. In 1918 she took upon herself the direction of the scientific department of the Radium Institute, which she had planned with her husband, and where her daughter Irene Joliot-Curie worked with her husband Frederic Joliot. Marie's research for the rest of her life was dedicated to the chemistry of radioactive materials and their medical applications. She frequently lectured abroad, and she labored to establish international scholarships for scientists. Her death, on July 4, 1934, of leukemia was undoubtedly caused by prolonged exposure to radiation.
The work of Marie and Pierre Curie, which by its nature dealt with changes in the atomic nucleus, led the way toward the modern understanding of the atom as an entity that can be split to release enormous energy.