Marie Curie discovered radium in 1898 without a pure sample

Marie and Pierre Curie, with the chemist Gustave Bémont, announced the discovery of radium to the French Academy of Sciences on 26 December 1898. They had not isolated it: their evidence was a barium-rich substance about 900 times more radioactive than uranium, which they argued must contain a new element.

The 26 December date is when their paper was presented to the Academy, the formal announcement. It was the couple’s second new element that year, after polonium, which they had proposed in the Academy’s proceedings that July.

“The new radioactive substance certainly contains a very strong proportion of barium; in spite of that its radioactivity is considerable. The radioactivity of radium therefore must be enormous.” — Curie, Curie and Bémont, 1898

Radioactivity alone was not proof of a new element. The spectroscopist Eugène Demarçay vaporised their samples in a flame and found a weak but distinct purple line at 381.48 nanometres that matched no known element; their polonium sample, by contrast, was too weak to show its line at all. Isolation took until 1902, when Marie, after thousands of crystallisations, extracted one decigram of almost pure radium chloride from several tons of ore residue.

The scale never got easier. In 1921, an industrial plant near Pittsburgh spent six months turning 500 tons of ore, plus 500 tons of chemicals, 1,000 tons of coal and 10,000 tons of water, into a single gram worth over $100,000. American women crowdfunded that gram. At the White House on 20 May 1921, President Warren Harding handed Curie a lead-lined, 130-pound case for it, though for security the radium itself was locked away in a nearby government lab.

Marie’s 1911 Nobel Prize in Chemistry was awarded for polonium and radium; the 1903 Physics prize she shared with Pierre and Henri Becquerel was for radioactivity itself. Their laboratory notebooks remain so radioactive that readers at France’s Bibliothèque nationale must sign a form accepting the risk before consulting them.