makes a noise resembling the crack of a pistol. Now, this discharge can be used in a variety of ways to excite various transformers in order to produce the best conditions for exciting the X rays. The method of using this powerful discharge to excite a transformer seems at present the most promising one in seeking the best conditions for obtaining rays of high penetrating power.
There is still another method of obtaining the rays yet in its infancy—the simplest method of all, for no apparatus is required.
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 arc 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.
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