Röntgen’s discovery of the rays that pass through metals and solids opaque to light was made in 1895, and in the following year, Becquerel, a distinguished French academician, discovered that salts of the metal uranium (substances that had long been used in coloring china and glass) emit invisible radiations capable of discharging electrified bodies and of producing skiagraphic images on sensitive plates; he found that potassio-uranic sulfate emits rays that pass through black paper and give photographic impressions in the same way as Röntgen rays. This property is not limited to the brilliantly fluorescent uranic salts, but is shared by the non-fluorescent uranous salts, and is exhibited by compounds whether phosphorescent or not, whether crystalline, melted or in solution, as well as by the metal itself. The permanence of this activity is amazing, substances kept in a double leaden box more than three years continuing to exert the power. Shortly after the announcement by Becquerel, experimenters found that other substances have the power of emitting ‘Becquerel Rays,’ such as calcium and zinc sulfids and compounds of thorium. In 1898 Mme. Sklodowska Curie, working in the laboratory of the Municipal School of Industrial Physics and Chemistry in Paris, devised a special apparatus for measuring the electrical conductivity of the air when under the influence of ‘radio-active bodies,’ and by its means studied the behavior of pitchblende (uraninite), and of other uranium minerals; finding that some specimens of pitchblende had three times as much energy as uranium itself, she came to the conclusion that the peculiar property is due to some unknown body contained in the minerals and not to uranium. Examining the mineral with the aid of her husband, the two found a substance analogous to bismuth, four thousand times stronger than uranium, which was named ‘Polonium,’ in honor of the native land of Mme. Curie. In December of the same year, the lady received the Gegner prize of 4,000 francs awarded by the Academy of Sciences, as a substantial appreciation of her discovery, and later in the same month Mme. and M. Curie announced that they had found a second body in pitchblende, which they named ‘Radium.’ More recently, M. Debierne, working under the auspices of Mme. Curie, has discovered a third body, which he calls ‘Actinium,’ an unfortunate appellation because ‘actinium’ has already been used for an element announced by Dr. Phipson and since discarded.