INTRODUCTION
Highly important as are the phenomena of Radioactivity from the physical, chemical, medical, and philosophic points of view, they are hardly comparable in their relations to the affairs of our everyday life to the Roentgen or X-rays, and to the invisible violet or ultra-violet rays. The X-rays are utilized today in hundreds of practical ways, and are vastly important also in surgery, medicine, dentistry, and in biological investigations. It is perhaps not too much to say that the discovery of the so-called X-rays should be numbered among the two or three most important revelations of modern science. This will be clearly demonstrated in the course of the chapters to follow.