In 1895, Röntgen modestly announced his discovery of the _X_ rays. This attracted immediate and intense interest. Among those who undertook to follow up these phenomena was Becquerel, who, because of the apparent connection with phosphorescence, tried the action of a number of phosphorescent substances upon the photographic plate, the most striking characteristic of the _X_ rays being their effect upon such sensitive plates. In these experiments he obtained no results until he tried salts of uranium, recalling previous observations of his as to their phosphorescence. Distinct action was noted. Furthermore, he proved that this had no connection with the phenomenon of phosphorescence, as both uranic and uranous salts were active and the latter show no phosphorescence. Becquerel announced his discoveries in 1896 and this was the beginning of the new science of radio-activity. Radio-active bodies The rays given off by uranium and its salts were found to differ from the _X_ rays. They showed no appreciable variation in intensity, no previous exposure of the substance to light was necessary, and neither changes of temperature nor any other physical or chemical agency affected them. At first uranium and its compounds were the only known source of these new radiations, but many other substances were examined and two years later thorium and its compounds were added to the list. In general the discharging action seemed about the same. Other elements and ordinary substances show a minute activity. Only potassium and rubidium have a greater activity than this, and theirs is only about one-thousandth that of uranium. An Atomic Property In the examination of uranium and thorium compounds it was found that the activity was determined by the uranium and thorium present; it was proportto the amount ofional these elements present and independent of the nature of the other elements composing the compound. The conclusion was, therefore, that the activity was an inherent property of the atoms of uranium and thorium, that is, an atomic property. This was a long step forward and introduced into science the conception of a new property of matter, or at least of certain forms of matter. Discovery of New Radio-active Bodies In examining a large number of minerals containing uranium and thorium, Mme. Curie made the important observation that many of these were more active than the elements themselves. In measuring the activity she made use of the electrical method which will be described later. In the following table giving her results for uranium minerals the numbers under _i_ give the maximum current in amperes. They serve simply for comparison.