=156. Vacuum-Tubes.= As electricity passes through rarefied gases much easier than through ordinary air, regular tubes, called _vacuum-tubes_, are made for such study. Fig. 217 shows a plain tube of this kind, platinum terminals being fused in the glass for connections. These tubes are often made in complicated forms, Fig. 218, with colored glass, and are called _Geissler tubes_. They are often made in such a way that the electrodes are in the shape of discs, etc., and are called _Crookes tubes_, Fig. 219. A slight amount of gas is left in the tubes. [Illustration: Fig. 220.] [Illustration: Fig. 220-A.] =157. Cathode Rays.= The _cathode_ is the electrode of a vacuum-tube by which the current leaves the tube, and it has been known for some time that some kind of influence passes in straight lines from this point. Shadows, Fig. 219, are cast by such rays, a screen being placed in their path. =158. X-Rays.= Professor Roentgen of Würzburg discovered that when the cathode rays are allowed to fall upon a solid body, the solid body gives out still other rays which differ somewhat from the original cathode rays. They can penetrate, more or less, through many bodies that are usually considered opaque. The hand, for example, may be used as a negative for producing a photograph of the bones, as the rays do not pass equally well through flesh and bone. [Illustration: Fig. 221.] Fig. 220 shows a Crookes tube fitted with a metal plate, so that the cathode rays coming from C will strike it. The X-rays are given out from P. These rays are invisible and are even given out where the cathode rays strike the glass. Some chemical compounds are made luminous by these rays; so screens are made and coated with them in order that the shadows produced by the X-rays can be seen by the eye. Professor Roentgen named these the X-rays. Fig. 220-A shows a _fluoroscope_ that contains a screen covered with proper chemicals. [Illustration: Fig. 222.] [Illustration: Fig. 223.] =159. X-Ray Photographs.= Bone does not allow the X-rays to pass through it as readily as flesh, so if the hand be placed over a sensitized photographic plate, Fig. 221, and proper connections be made with the induction coil, etc., the hand acts as a photographic negative. Upon developing the plate, as in ordinary photography, a picture or shadow of the bones will be seen. Fig. 222 shows the arrangement of battery, induction coil, focus tube, etc., for examining the bones of the human body. Fig. 223 shows the bones of a fish. Such photographs have been very valuable in discovering the location of bullets, needles, etc., that have become imbedded in the flesh, as well as in locating breaks in the bones. CHAPTER XXIV. THE ELECTRIC MOTOR, AND HOW IT DOES WORK.