[Wilhelm Conrad Röntgen and the discovery of X-rays].

W.C. Röntgen reported the discovery of X-rays in December 1895 after seven weeks of assiduous work during which he had studied the properties of this new type of radiation able to go through screens of notable thickness. He named them X-rays to underline the fact that their nature was unknown. The news of this discovery immediately aroused an immense interest in the public and also initiated intense research in several directions. Physicians and physicists began as early as January 1896 to use X-rays on patients to investigate the skeleton and subsequently the lung and other organs. This was the birth or radiology. Rapidly they observed skin erythema, which led to the idea of using X-rays against a variety of lesions. In June 1896 the first patient was treated by radiotherapy. J.J. Thomson (Cambridge, U.K.) showed that X-rays were able to ionize gaz and the study of this phenomenon led to the discovery of electrons in 1897. In order to understand the emission of X-rays, H. Becquerel (Paris) investigated the role of the phosphorescence of the glass of the tube and while doing so discovered radioactivity in March 1896. X-rays and radioactivity were at the origin of the scientific revolution at the end of the 19th and the beginning of the 20th centuries. Research on radioactive materials demonstrated the existence of atoms which had been till then only a convenient hypothesis for explaining chemical reactions, but whose reality was considered as dubious by most physicists. Moreover, interaction of particles emitted by radionuclides and atoms enabled first the study of the structure of the atom and subsequently its nucleus. Matter, elements which were thought to be immutable were found to be transmutable, and eventually to disintegrate. The origin of the energy transferred to the radiation which was emitted appeared as a mystery and in order to explain it the physicist had to accept that matter could convert energy. In 1903 Einstein established the equivalence between matter and energy. Matter, energy, electricity, light which were formerly considered as continuous quantities were found to be discrete: there are particles of matter (elementary particles), energy (quanta, Planck 1905), electricity (electron), light (photons). Radioactive decay, particle interactions imposed a probabilistic physics which progressively replaced classic deterministic physics. Radioactivity can be used as a clock to measure time in the universe. Datations were made for fossils, art masterpieces and also for the earth, the solar system and universe. X-rays diffraction proved to be a powerful tool for studying crystals and molecules, in particular protein, and in 1953 enabled to demonstrate the DNA double helix. Hence X-rays and radioactivity originated a revolution in physics and science and in the vision of nature. The imperceptible and yet so powerful rays demonstrated the deficiencies of our senses. Mathematical entities and instrumentation must complement our sensations. The huge increment in our knowledge is accompanied by a divorce between the scientist and the layman who now often has great difficulties understanding new concepts not only in physics but also in biology.