=418. Discharges in Rarefied Air.=--Fig. 417 represents a glass tube 60 or more centimeters long, attached to an air pump. Connect the ends of the tube to the terminals of a static machine or of an induction coil, _a-b_. At first no sparks will pass between _a_ and _f_, because of the high resistance of the air in the tube. Upon exhausting the air in the tube, however, the discharge begins to pass through it instead of between _a_ and _b_. This shows that an electrical discharge will pass more readily through a partial vacuum than through air at ordinary pressure. As the air becomes more and more exhausted, the character of the discharge changes. At first it is a faint spark, gradually changing until it becomes a glow extending from one terminal to the other and nearly filling the tube. [Illustration: FIG. 417.--An Aurora tube.] _Geissler tubes_ are tubes like the above. They are usually made of different kinds of glass twisted into various shapes to produce beautiful color effects. The _aurora borealis_ or northern light is supposed to be electric discharges through rarefied air at the height of from 60 to 100 miles above the earth's magnetic poles. (See Fig. 418.) [Illustration: FIG. 418.--Aurora Borealis.] =419. Cathode Rays.=--When the tube in Art. 420 is exhausted to a pressure of 0.001 mm., or a little less than one millionth of an atmosphere, the character of the discharge is entirely changed. The tube becomes filled with a yellowish green phosphorescent light. This is produced by what are called cathode rays striking the glass walls of the tube. These rays are called cathode rays because they come from the cathode of the tube. They are invisible and that they travel in straight lines is shown by the shadow obtained by using a tube with a screen (Fig. 419). [Illustration: FIG. 419.--A cathode ray tube.]