Now, when these electrons arrive at the end of the cathode wire, they are urged strongly to cross into an open space beyond the wire, partly by the crowds of electrons following them, and partly by the strong attraction which the anode opposite exerts upon them. But the atoms of the wire themselves attract and hold the electrons, and there is therefore always some difficulty to overcome before the electrons can leave the wire. Once they are free from the cathode, they are impelled forward by the repulsion from behind and the attraction in front. Like stones falling from a great height they rush on more and more quickly. The force which plays upon the electrons is enormously greater than the force of gravitation. It is more than a million times greater. On the other hand, the particles or electrons are very small, and, therefore, do not produce any very destructive effects, however high their speed may become. But their speed is something enormous. It is something like fifty thousand miles per second. No wonder that when these particles are stopped by a glass wall they produce some very curious effects. These effects become much more intense when they are concentrated upon a sheet of platinum placed opposite the cathode. That sheet becomes white hot and sends out the famous Röntgen Rays, which Röntgen himself called “X-rays.” Now as to the history of the discovery of these rays. Professor Röntgen was one day experimenting with such a vacuum tube, and trying to get the electrons to go out of the tube into the open air. This experiment had been done many times before him by using a little window made of very thin aluminium, which can be penetrated by electrons. [Illustration: Fig. 73.--Vacuum Tube for producing Röntgen Rays.] He was studying the effect of these electrons upon a sheet of some phosphorescent substance. He wished to find out whether the electrons would still come out if the vacuum tube were enclosed completely in black cardboard. He also wished to observe the effect in complete darkness. He was surprised to find that the phosphorescent screen was lighted up, and that some influence from the vacuum tube reached the screen through the black cardboard.