142 PHYSICAL SCIENCE 



the ring formula of benzene which is inferred from 

 ordinary chemical evidence. 



Direct evidence of the negative charge carried 

 by the cathode rays was given by experiments of 

 Perrin. He showed that, when the rays were 

 deflected by a magnet so that they fell on an in- 

 sulated metal cylinder placed within the discharge- 

 bulb and connected with an electrometer, a strong 

 negative electrification was imparted to the system. 

 When the rays fell on other parts of the bulb, this 

 electrification was not observed. 



A less direct but more interesting method was 

 used by Thomson in 1897, and led to one of the 

 great discoveries of modern science. In the glass 

 apparatus shown in Fig. 30, the left-hand terminal 

 of the induction coil is connected with the cathode, 

 the right-hand terminal with a thick metallic disc 

 which acts as the anode. Through the anode, 

 and through a second thick disc connected with 

 the earth by the wire going to the bottom of the 

 photograph, are bored in sequence two holes about 

 a millimetre in diameter. A thin pencil of cathode 

 rays is thus obtained beyond the second disc. 

 These rays pass between the two metallic plates, 

 seen in the wider part of the tube, which can be 

 connected with the poles of a voltaic battery by 

 means of the wires passing to the right. An 

 electric force of known amount can thus be applied 

 to the cathode rays. When that force is sufficient, 

 the path of the rays is deflected, and the magnitude 

 of this effect can be determined by observing the 

 deflection of the spot of fluorescent light on the 

 screen at the right-hand end of the apparatus. It 

 is well known that the cathode rays are deflected 



