opaque substances, while casting shadows of others. Most wonderful seemed the statement that by these rays the bones of the hand could be seen. Seventeen years later these Röntgen rays are used in every hospital, and reveal the inmost secrets of the body. But this is not their interest to the physicist, but rather the fact that they have opened up a whole field of facts previously unsuspected, so that an investigator ignoring them would to-day be held the greatest of old fogies. How did Röntgen come to discover the X-rays? No doubt there was a certain element of chance. We are told that he had covered the discharge tube, the so-called Crookes tube, with black paper, so that no light should get out from it, and that Röntgen's attention was attracted by the fluorescence, or faint shining with light, of a piece of paper lying on the table, the paper being covered with the salt of barium platinocyanide. But why did this piece of paper coated with this uncommon chemical happen to be lying on the table, and why had Röntgen covered the Crookes tube with black paper? We find that barium platinocyanide was one of the substances that had been investigated by previous investigators as to its fluorescence, and that such paper was a commercial article in Germany. Röntgen must then have suspected that there was some property of the Crookes' tube that would cause fluorescence, so that the presence of this fluorescent paper was not accidental at all. This is then a striking example of what I have before stated. A further one is given by a discovery made the next year in Paris. Röntgen's discovery had set the world on fire, and had given rise to a renewed interest in the subject of fluorescence. Noteworthy among fluorescent substances are the salts of uranium, and these were examined by Henri Becquerel, the third generation of physicists of that name. Becquerel placed uranium salts against a photographic plate wrapped up in black paper, and soon found that the plate was affected, even through the opaque paper. At first Becquerel thought that the uranium had this property only after being exposed to the sun's light, but he soon found that the same properties were possessed by uranium salts that had been formed in the dark, and had never seen the sun. In short these salts are constantly emitting a new sort of radiation, now known as Becquerel rays. Physicists now began to look for other substances than uranium which had these properties, with the result that it was found that uranium-bearing ores were found to contain other substances having the properties in a far higher degree, and at last the Curies were able to separate a new element, which was named radium. The field of radioactivity thus opened up has become an enormous one, and many substances have been discovered having radioactive properties. Here is again an illustration of the impossibility of distinguishing between physics and chemistry, for although Mme. Curie is a chemist, the Nobel prize in chemistry was awarded a few years ago to Professor Rutherford, professor of physics
