Chapter XXIII

X-RAYS, AND HOW THE BONES OF THE HUMAN BODY ARE PHOTOGRAPHED.

=154. Disruptive Discharges.= We have seen, in the study of induction coils, that a spark can jump several inches between the terminals of the secondary coil. The attraction between the two oppositely charged terminals gets so great that it overcomes the resistance of the air-space between them, a brilliant spark passes, and they are discharged. This sudden discharge is said to be _disruptive_, and it is accompanied by a flash of light and a loud report. The _path_ of the discharge may be nearly straight, or crooked, depending upon the nature of the material in the gap between the terminals.

=155. Effect of Air Pressure on Spark.= The disruptive spark takes place in air at ordinary pressures. The nature of the spark is greatly changed when the pressure of the air decreases. Fig. 216 shows an air-tight glass tube so arranged that the air can be slowly removed with an air-pump. The upper rod shown can be raised or lowered to increase the distance between it and the lower rod, these acting as the terminals of an induction coil. Before exhausting any air, the spark will jump a small distance between the rods and act as in open air. As soon as a small amount of air is removed, a change takes place. The spark is not so intense and has no definite path, there being a general glow throughout the tube. As the air pressure becomes still less, the glow becomes brighter, until the entire tube is full of purple light that is able to pass the entire length of it; that is, the discharge takes place better in rarefied air than it does in ordinary air.

=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.

=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.

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.

=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.