In the course of the systematic search of Becquerel rays, Marie Curie also discovered, on 24 February, that thorium compounds were also active. Pierre Curie was fascinated by Marie’s findings: On 18 March he abandoned his own research projects and joined his wife in the venture. At this stage, the hunt for the supposed element became a matter of paramount importance and urgency. Marie Curie noted, “This fact is quite remarkable and suggests that these minerals may contain an element much more active than uranium.” Her hypothesis was immediately confirmed: “I have prepared chalcolite with pure products this artificial chalcolite is not more active than other uranium salts.” She then concluded that an unknown element exists only in the uraniferous minerals that are more active than uranium. She found that all compounds and minerals that contained uranium were active and that pitchblende, a massive variety of uraninite from the Joachimasthal mine in Austria, as well as chalcolite, a natural uranium phosphate, were more active than metallic uranium itself. Beginning on 11 February 1898, she tested all samples at hand or borrowed from various collections, including a large number of rocks and minerals, taking the activity of metallic uranium as a reference. Marie Curie’s strategy is clearly expressed in her first note published on 12 April 1898 in the Comptes Rendus de l’Académie des Sciences: “I have searched if substances other than uranium compounds render air conducting for electricity” (Curie, M. Marie Curie’s First Publication: 12 April 1898 The emission of uranic rays could now be quantified from the weight and the time required for compensation of the charges produced in the ionization chamber. The compensation was followed by a second invention, the quadrant electrometer. He invented a device by which the charges produced by uranium in an ionization chamber were compensated for by opposite charges in known amounts produced by applying a weight to a leaf of quartz. In 1880, together with his brother Jacques, he had discovered piezoelectricity (i.e., the production of electric charges when pressure is applied to hemihedral crystals such as quartz). At this point, little progress would have been made without the genius of Pierre Curie. Becquerel had used electroscopes, but the measurements were unreliable. This later property was much more amenable to quantitative measurement. In addition to blackening a photographic plate, uranic rays rendered air conductive for electricity. The “Curie Laboratory”: left, chemistry bench right, ionization chamber and electrometer.
This short history of the discoveries is retraced from three laboratory notebooks in which one can distinguish the writings of Pierre and Marie (Adloff 1998) and from three notes published in the Comptes Rendus de l’Académie des Sciences (C.R. However, within eight months in 1898 she discovered two elements, polonium and radium, founding a new scientific field-radioactivity. The topic was moribund when Marie Curie entered the scene. One reason was the proliferation of false or doubtful observations of radiation similar to uranic rays in a variety of substances. Marie Curie, in a biography of Pierre Curie, confirmed, “we felt the investigation of the phenomenon very attractive, so much the more so as the topic was quite new and required no bibliographical research.”Īfter initial excitement, interest in the new rays had faded rapidly.
What was the source of this inexhaustible energy that apparently violated the Carnot principle that energy can be transformed but never be created or destroyed? Pierre Curie, already a famous physicist for his work on magnetism and crystal symmetry, had a feeling that the phenomenon was quite extraordinary, and he helped his wife reach a decision in her choice of thesis topic. Uranium compounds and minerals appeared to maintain an undiminished ability to blacken a photographic plate over a period of several months. On the other hand, the uranic rays, discovered in 1896 by Henri Becquerel, raised a puzzling problem. X-rays, discovered by Wilhelm Conrad Röntgen in 1895, were still a topical question, but had lost the charm of novelty. In 1897 at the age of 30, Maria Skłodowska, who had married Pierre Curie in 1895, concluded her studies at the Sorbonne in Paris and was thinking of a subject for a thesis. An illustration from Vanity Fair magazine, 1904 (Library of Congress).
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