164 CATHODE RAYS. 



wlien the rays from oue cathode pass near the other they are repelled 

 from it. This is just what would happen if the dark space round the 

 electrode were an insulator, and so able to transmit electrostatic 

 attractions or repulsions. To show that the gas in the dark space 

 diifers in its properties from the rest of the gas, I will try the follow- 

 ing experiment. I have here two spherical bulbs connected together 

 by a glass tube; one of these bulbs is small, the other large 5 they 

 each contain a cathode, and the pressure of the gas is such that the 

 dark space round the cathode in the small bulb completely fills the 

 bulb, Avhile that round the one in the larger bulb does not extend 

 to the walls of the bulb. The two bulbs are wound with wire, which 

 connects the outsides of two Leyden jars; the insides of these jars 

 are connected with the terminals of a Wimshurst machine. When 

 sparks pass between these terminals currents pass through the wire 

 which induce currents in the bulbs, and cause a ring discharge to 

 pass through them. Things are so arranged that the ring is faint in 

 the larger bulb, bright in the smaller oue. On making the wires in 

 these bulbs cathodes, however, the discharge in the small bulb, which 

 is filled by the dark space, is completely stopped, while that in the 

 larger one becomes brighter. Thus the gas in the dark space is 

 changed, and in the opposite way from that in the rest of the tube. 

 It is remarkable that when the coil is stopped the ring discharge on 

 both bulbs stops, and it is some time before it starts again. 



The defleetioD excited on each other by two cathodic streams would 

 seem to have a great deal to do with the beautiful phosphorescent fig- 

 ures which Goldstein obtained by using cathodes of different shapes. 

 I have here two bulbs containing cathodes shaped like a cross. They 

 are curved and of the same radius as the bulb, so that if the rays came 

 off these cathodes normally the phosphorescent picture ought to be a 

 cross of the same size as the cathode. You see that in one of the bulbs 

 the image of the cross consists of two large sectors at right angles to 

 each other, bounded by bright lines, and in the other, which is at a 

 lower pressure, the geometrical image of the cross instead of being 

 bright is dark, while the luminosity occupies the space between the 

 arms of the cross. 



So far I have only considered the behavior of the cathode rays inside 

 the bulb, but Lenard has been able to get these rays outside the tube. 

 To do this he let the rays fall on a window in the tube, made of thin alu- 

 minum about one one-hundredth of a millimeter thick, and he found that 

 from this window there proceeded in all directions rays which were 

 deflected by a magnet, and which produced phosphorescence when 

 they fell upon certain substances, notably upon tissue paper soaked iu 

 a solution of pentadekaparalolylketon. The very thin aluminum is 

 difficult to get, and Mr. McClelland has found that if it is not neces- 

 sary to maintain the vacuum for a long time oiled silk answers admi- 

 rably for a window. As the window is small, the phosphorescent patch 



