It has been discovered that certain substances, like the salts of uranium, have the power of emitting rays which have all the properties of the X rays. The list of such substances is constantly increasing, and they are called radio-active substances. It is possible to take a shadow picture of the hand through a board by placing the hand on a covered sensitive plate, resting the board on the back of the hand, and strewing the board with one of these radio-active substances in the form of a powder. Can it be that all the skill and industry which has been employed to perfect X-ray apparatus is to be supplanted by a powder? The peculiar property shown by the radio-active substances leads investigators to surmise that we have evidence of new substances, and we have the waves radium and polonium. [Illustration: FIG. 3.--The burning of an iron wire by the most powerful electric discharge yet produced.] The methods by which the X rays are detected in practical employment in surgery have not been essentially changed. The ordinary photographic plate, shielded in a plate holder, is still used to receive the shadow cast by the bones, and salts of barium or of calcium strewn on pasteboard serve as fluorescent screens to receive on their luminous surfaces these shadows and to make them evident to the eye. An interesting use of flexible sensitive films instead of glass plates has been made in dentistry. The films are put in the mouth, and the Crooke’s tube placed outside in such a position that the rays can pass through the jaw. In this way the accompanying photographs were taken (Fig. 4).[D] [D] Kindness of Dr. Dwight M. Clapp, Boston. [Illustration: FIG. 4.--1. Male patient, aged ten years. Temporary incisors in position with the permanent incisors nearly ready to erupt. The roots of the temporary teeth nearly absorbed. Right temporary central cut incisor destroyed by a blow five years previous, showing gutta-percha filling put in at the time which, with the dead root, has been absorbed (same as the live root of the other central). It is taught in the text-books that teeth with dead nerves are not absorbed. 2. Temporary molars in position with no signs of the permanent bicuspid which should take its place. 3. Patient, aged ten years. Shows the open ends of the incisor roots. 4. Shows one bicuspid and two molar teeth. The roots of the teeth pass through the floor of and into the cavity of the antrum. The spongy character of the bone is shown. 5 and 6. Superior jaw, right and left sides, showing temporary cuspid teeth in place, with the permanent cuspids imbedded in the jaw entirely covered.]