As researchers developed new ways to identify elements over the course of the 19th century, cry after cry sounded in the scientific literature — Gallium! Scandium! Holmium! Thulium! Praseodymium! Scientists soon discovered several elements that Mendeleev predicted. Three decades after Mendeleev published his diagram, a cry came from Pierre and Marie Curie: Polonium! A mere five months later, in December 1898, another call came from the Paris lab: Radium! It was one of a half-dozen elements discovered that year alone.

In this parade of new elements, some gave off radiation, rays or particles released by atoms as they decayed into other elements, known as their daughters. To understand how radioactivity works, think about the structure of an atom. At its core, or nucleus, it has a cluster of particles called protons and neutrons, and around them swirl a cloud of electrons. An atom of uranium 235, for instance, has 92 protons, 92 electrons, and 143 neutrons. But this arrangement is unstable. Soon the uranium will start to shed particles. It might shed two protons and two neutrons, a process called alpha decay, converting it to thorium, element 90. If thorium then turns a neutron into a proton and sheds an electron, called beta-minus decay, it becomes protactinium, element 91. This series of transformations from one element to another is called a decay chain. As the atom progresses down the decay chain, some routes are favored above others. Polonium 215 decays into lead almost all of the time, taking the alpha-decay route. Only a minuscule fraction of Polonium 215 takes the beta-decay route, to become astatine. The time it takes for half a sample of an element to decay, its half-life, can range from fractions of seconds to billions of years.

How quickly an element decayed and how it did so — meaning which of its component parts it shed — became the focus of researchers in radioactivity. Apart from purely scientific insights, there was a hope that radiation could lead to something marvelous. X-rays, a kind of radiation discovered by Wilhelm Roentgen and produced by accelerated electrons, had already been hailed as a major medical breakthrough and, in addition to showing doctors their patients’ insides, were being investigated as a treatment for skin lesions from tuberculosis and lupus. In her 1904 book “Investigations on Radioactive Substances,” Marie Curie wrote that radium had promise, too — diseased skin exposed to it later regrew in a healthy state. Radium’s curious ability to destroy tissue was being turned against cancer, with doctors sewing capsules of radium into the surgical wounds of cancer patients (including Henrietta Lacks, whose cells are used today in research). This enthusiasm for radioactivity was not confined to the doctor’s office. The element was in face creams, tonics, even candy. According to the Encyclopaedia Britannica article that Curie and her daughter wrote on radium in 1926, preliminary experiments suggested that radium could even improve the quality of soil.

And yet from the beginning, there were signs that radiation had sinister powers. In 1901, Henri Becquerel, the first person to observe radioactivity, reported strange burns he received from the vial of radium he carried in his waistcoat pocket. The burns appeared on the Curies’ hands as well. People who worked with X-rays at the beginning of the 20th century had a known tendency to lose their hair and develop burns on their skin and even cancer. In 1904, Clarence Dally, who was Thomas Edison’s X-ray assistant, died of cancer after having both his arms amputated to try to keep it from spreading. For all its anticipated promise in battling cancer, radiation was also clearly carcinogenic.

Perhaps the most tragic demonstration of this involved workers at the United States Radium Corporation factory in Orange, N.J., which in 1917 began hiring young women to paint watch faces with glow-in-the-dark radium paint. The workers were told that the paint was harmless and were encouraged to lick the paintbrushes to make them pointy enough to inscribe small numbers. In the years that followed, the women began to suffer ghoulish physical deterioration. Their jaws melted and ballooned into masses of tumors larger than fists, and cancers riddled their bodies. They developed anemia and necrosis. The sensational court case started — and won — by the dying Radium Girls, as they were called, is a landmark in the history of occupational health. It was settled in June 1928, four months before Marguerite Perey arrived at the Radium Institute to begin a 30-year career of heavy exposure to radiation.

Through all that, little seems to have changed at the institute, where a long tradition of lax safety practices continued. Two former Radium Institute chemists died in quick succession from brief, violent illnesses (“A New Victim of Science,” read a 1925 newspaper announcement of the second death). In 1927, Sonia Cotelle, a Radium Institute chemist who worked with polonium, began losing her hair rapidly. Cotelle later died from radiation exposure. Although radiation’s connection to cancer was known and the lab’s own employees had clearly suffered, the Curies made few adjustments to protocol. Marie Curie’s principal adaptations were to ask scientists to submit to blood tests and to encourage workers to take short breaks in the garden, which provided no real protection. When a journalist asked about the watch painters in New Jersey, she suggested that they eat calf’s liver to combat anemia. The great work went on.