For a long time the nature of these cathode rays was in dispute. German physicists held that they were of the same character as ordinary light, while English scientists, headed by Sir William Crookes, maintained that they were streams of extremely minute particles of matter in a peculiar fourth state. That is to say, the matter was not liquid, or solid, or gaseous in the ordinary sense, but was _ultra-gaseous_, and Crookes gave it the name of _radiant matter_. Most of us have been taught to look upon the atom as the smallest possible division of matter, but recent researches have made it clear that the atom itself is divisible. It is believed that an atom is made up of very much more minute particles called _electrons_, which are moving about or revolving all the time with incredible rapidity. According to Sir Oliver Lodge, if we imagine an atom of hydrogen to be as big as an ordinary church, then the electrons which constitute it will be represented by about 700 grains of sand, 350 being positively electrified and 350 negatively electrified. It is not yet definitely determined whether these electrons are minute particles of matter charged with electricity, or whether they are actually atoms of electricity. The majority of scientists now believe that the cathode rays consist of a stream of negative electrons repelled from the cathode at a speed of 124 miles per second, or not quite 1/1000 of the velocity of light. In November 1895, Professor Röntgen, a German physicist, announced his discovery of certain invisible rays which were produced at the same time as the cathode rays, and which could penetrate easily solids quite opaque to ordinary light. He was experimenting with vacuum tubes, and he found that certain rays emerged from the tube. These were not cathode rays, because they were able to pass through the glass, and were not deflected by a magnet. To these strange rays he gave the name of the “_X_,” or unknown rays, but they are very frequently referred to by the name of their discoverer. It was soon found that the Röntgen rays affected an ordinary photographic plate wrapped up in black paper so as to exclude all ordinary light, and that they passed through flesh much more easily than through bone. This fact makes it possible to obtain what we may call “shadow-graphs” of the bones through the flesh, and the value of this to the medical profession was realized at once. The rays also were found to cause certain chemical compounds to become luminous. A cardboard screen covered with one of these compounds is quite opaque to ordinary light, but if it is examined when the Röntgen rays are falling upon it, it is seen to be brightly illuminated, and if the hand is held between the screen and the rays the bones become clearly visible. [Illustration: FIG. 38.--X-Ray Tube, showing paths of Cathode and X-Rays.]