In 1896, Henri Becquerel (1852–1908) had discovered, by chance, the phenomenon of radioactivity, after he found that uranium salts left on top of covered photographic plates produced an image on the plates when they were later developed. Soon afterwards, thorium was also found to be radioactive. In 1898 Marie Curie (née Sklodovska) realized that certain minerals were more ‘radioactive’ (a term she first introduced) than could be rationalized by the amount of uranium or thorium that they contained. She guessed that they might contain trace amounts of an even more radioactive element, and during the long purification process, she eventually realized that two such elements were present. The naming of the first of these, discovered in July 1898, is described by her daughter Eve Curie in her biography of her mother: . . . ‘You will have to name it,’ Pierre said to his young wife, in the same tone as if it were a question of choosing a name for little Irène [their first daughter]. The one-time Mlle Sklodovska reflected in silence for a moment. Then, her heart turning toward her own country which had been erased from the map of the world, she wondered vaguely if the scientific event would be published in Russia, Germany and Austria—the oppressor countries—and answered timidly: ‘Could we call it “polonium”?’ . . . Marie Curie named the element after her homeland, Poland, but the country did not exist as a separate entity at that time, and her choice was something of a political statement. The second element discovered by Marie and Pierre Curie was found to be millions of times more radioactive than uranium. This element they called ‘radium’ because of its intense radioactivity. Over three and a half years later, when they finally isolated a tenth of a gram of purified radium salts from tonnes of pitchblende ore, the Curies were delighted to find that the substance was spontaneously luminous. After the discovery that uranium and thorium were radioactive, in September 1899, Ernest Rutherford (1871–1937) made a further discovery: ‘In addition to this ordinary radiation, I have found that thorium compounds continuously emit radio-active particles of some kind, which retain their radio-active powers for several minutes.
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