In 1895 Rontgen drew closer attention to the Crookes tube by discovering the rays which he called X-rays, but which now bear his name. They differ from ordinary light-waves in their length, their irregularity, and especially their power to pass through opaque bodies. A number of distinguished physicists now took up the study of the effect of sending an electric discharge through a vacuum, and the particles of "radiant matter" were soon identified. Sir J. J. Thomson, especially, was brilliantly successful in his interpretation. He proved that they were tiny corpuscles, more than a thousand times smaller than the atom of hydrogen, charged with negative electricity, and travelling at the rate of thousands of miles a second. They were the "electrons" in which modern physics sees the long-sought constituents of the atom. No sooner had interest been thoroughly aroused than it was announced that a fresh discovery had opened a new shaft into the underworld. Sir J. J. Thomson, pursuing his research, found in 1896 that compounds of uranium sent out rays that could penetrate black paper and affect the photographic plate; though in this case the French physicist, Becquerel, made the discovery simultaneously' and was the first to publish it. An army of investigators turned into the new field, and sought to penetrate the deep abyss that had almost suddenly disclosed itself. The quickening of astronomy by Galilei, or of zoology by Darwin, was slight in comparison with the stirring of our physical world by these increasing discoveries. And in 1898 M. and Mme. Curie made the further discovery which, in the popular mind, obliterated all the earlier achievements. They succeeded in isolating the new element, radium, which exhibits the actual process of an atom parting with its minute constituents. The story of radium is so recent that a few lines will suffice to recall as much as is needed for the purpose of this chapter. In their study of the emanations from uranium compounds the Curies were led to isolate the various elements of the compounds until they discovered that the discharge was predominantly due to one specific element, radium. Radium is itself probably a product of the disintegration of uranium, the heaviest of known metals, with an atomic weight some 240 times greater than that of hydrogen. But this massive atom of uranium has a life that is computed in thousands of millions of years. It is in radium and its offspring that we see most clearly the constitution of matter.