Several physicists, including Arthur Schuster, Goldhammer, George Fitzgerald, tried rehabilitating the hypothesis of transverse waves refuted by Röntgen. Waves or particles? While some physicists agreed with Röntgen, a number of other hypotheses came to life, of which I'll briefly review only transverse waves and particles, as the most relevant for this paper. Röntgen decided that the only reasonable choice open was that the X-rays are produced by longitudinal waves in the ether. Nor could they be cathode rays exiting the tube, because X-rays penetrated air much easier than cathode rays, and they did not deviate even in a strong magnetic field. Röntgen concluded that, despite their ability to produce fluorescence and chemical reactions and discharge electrified plates, the X-rays could not be transverse waves similar to ultraviolet light. On the other hand, these rays considerably differed from light, in particular, they could not be regularly reflected or refracted and they showed no sign of either polarization or interference. On the one hand, the X-rays behaved like light, producing sharp images on a photographic plate. While it was the penetrative power of X-rays, revealed by Röntgen's photographs, that struck the imagination of everyone, scientists found another amazing feature of the new phenomena. Röntgen's discovery The discovery of invisible rays penetrating opaque bodies made by Wilhelm Konrad Röntgen became known to public early in January 1896: some scientists received personal communications from Röntgen, while the majority learned about it from newspapers. Because of those results, they still kept their faith in the secondary radiation hypothesis at the time when Rutherford and Soddy began to develop the disintegration theory of radioactivity.ġ. However, in 1900 Pierre Curie and Georges Sagnac investigated secondary X-rays and concluded that they contained both "soft" X-rays and a negatively charged radiation (similar to betarays). They carefully checked that discovery, and attempted to produce a magnetic deflection of X-rays, with negative results. When the magnetic deviation of the beta-rays of radioactive bodies was discovered, in 1899, this presented a challenge to their hypothesis. For the Curies, this hypothesis explained one of the anomalous characteristics of radioactivity-the continuous emission of energy without any noticeable change of the emitting bodies. This paper describes Sagnac's studies and how the acceptance of the secondary radiation hypothesis guided the study of radioactivity by the Curies. This hypothesis, together with other relevant assumptions, was suggested by Georges Sagnac's investigation on X-rays. They thought that the radiation emitted by uranium compounds (and, later, by other similar substances) was similar to the secondary radiation emitted by heavy metals when they are hit by X-rays. Their papers avoid indeed theoretical discussion, but it is possible to identify the main hypothesis that directed their work. Pierre and Marie Curie's main discoveries on radioactivity are usually regarded as empirical investigations that were developed without any theoretical guidance. The aim of this article is to discuss one particular episode of experimentation-Becquerel's study of the phenomenon we call radioactivity-and the methodological problems aroused by his mistakes. As the study of radioactivity developed, Becquerel reinterpreted his own early work, hiding his mistakes or ascribing to himself their correction. Becquerel's mistakes were gradually corrected by other researchers. Those and several other aspects of Becquerel's experimental work must nowadays be interpreted as experimental mistakes. He also described an increase in the emission of radiation when uranium compounds were stimulated by sunlight. Guided by his preconceptions, Becquerel described experiments that seemed to support the view that uranium radiation had the usual properties of known electromagnetic waves: reflection, refraction and polarization. What Becquerel expected to find was the emission of a penetrating electromagnetic radiation (something similar to ultraviolet rays) emitted by a special phenomenon of fluorescence or phosphorescence that violated Stokes' law. It was guided by his acceptance of Poincaré's conjecture concerning a possible relation between X rays and luminescence. Becquerel's study of uranium radiation was not casual or blind. In 1896, Henri Becquerel detected a penetrating radiation emitted by some uranium salts and met a phenomenon that nowadays we call "radioactivity".
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