Magnetic Rays. 313 



was moved along it, as indicated in fig. 2. Many observa- 

 tions were made of these phenomena, and both the pressure 

 and magnetic field were varied so as to get all possible com- 

 binations. In general, the rays were more sensitive to 

 changes o£ pressure than to variations in the magnetic 

 intensity. For a given pressure, the magnetic field could 

 be changed within wide limits without any other apparent 

 effect than to increase or diminish somewhat the intensity 

 and length of the rays ; but with a given field, changes of 

 pressure would soon destroy the rays altogether. In our 

 apparatus the magnetic rays appeared quite suddenly at 

 what may be called the critical pressure. This pressure was 

 approximately 0*03 cm. As the current of the magnet was 

 increased the rays gradually grew longer and having 

 attained a maximum began to shorten until, with a 

 sufficiently intense field, they suddenly ceased. The lowest 

 pressure in which the rays could exist was about 0'006 cm. ; 

 and, in general, as the pressure decreases the strength of the 

 field must be increased. 



The tube was next filled with illuminating gas, thoroughly 

 dried. This gas is a natural gas obtained from wells and of 

 unknown but probably complex composition. When the 

 electric discharge was observed without a magnetic field, the 

 cathode rays were less dense and of a paler blue than in air. 

 The action of the magnetic field showed a decided difference* 

 The magnetic field, apparently, was unable to contract the 

 diffuse cathode rays into a well defined magnetic ray column. 

 Even when a field was used sufficiently intense to destroy 

 the action in air, the diffuse light filled nearly half the tube ; 

 the axial portion was slightly brighter, somewhat more blue 

 in colour and extended only a short distance into the large 

 tube, but no well defined column could be observed. This 

 inner core showed the characteristic deflexions of magnetic 

 rays when an auxiliary magnet was used. But the induced 

 column of light at the end of the magnetic ray was lacking 

 and could not be produced with any pressure and field. That 

 the gas used in the tube should have such an influence on 

 the nature of the magnetic rays is most difficult to explain 

 by Professor Eights hypothesis. 



Various simple gases were used successively. Hydrogen, 

 oxygen, nitrogen, and carbon-dioxide gases were prepared 

 in as pure a state as possible and were thoroughly dried 

 before being admitted to the tube, and the tube was filled 

 and emptied many times to get rid of other gases, especially 

 air. In no case, with any of these simple gases, could the 

 discharge characteristic in air be obtained. The pressure 



