416 H. V. Gill — Electric Discharge in Geissler Tubes. 



It may be of interest to note an effect produced on the pow- 

 der daring the " sensitive discharge." As has been shown by 

 Spottiswoode and Moulton,* in order to produce the sensitive 

 state it is only necessary to put a small spark gap in the circuit. 

 The tube already described was connected with an induction 

 coil, a small spark gap being introduced at the extremity. The 

 powder was distributed evenly all along the tube. When the 

 spark gap was at the positive pole the discharge was sensitive ; 

 on touching the tube with the finger the powder inside the 

 tube was observed to (if we may use the expression) boil up, 

 little craters being produced in it at the point touched, as if 

 there were a discharge taking place from the sides of the tube 

 through the layer of powder. No distribution into lines was 

 observed. Near the extremity with the gap the powder was 

 not affected. When the spark gap was at the negative terminal 

 the effect was hardly observable at all ; but if the poles were 

 suddenly reversed, on touching the tube the powder was affected. 

 This effect was also noticed on touching the tube after the dis- 

 charge had ceased. For some days after, the powder in the 

 tube adhered to the side so that it could not be removed by 

 tapping the tube ; after some time, however, it became some- 

 what detached. This effect on the powder was only produced 

 when there was a spark gap, and never in other cases. These 

 results confirm the explanation which has been given, and we 

 simply mention them in passing. 



Such is the theory we propose in order to explain the strati- 

 fied discharge in Geissler tubes. We have endeavored to 

 express it as simply as possible without at the same time insist- 

 ing too much on details. Our experiments seem to confirm the 

 hypothesis on which we started, and we see that our results 

 lead back naturally to this starting point. Again, we think any 

 one repeating these experiments will agree that thej' show the 

 oscillatory nature of the discharge in a vacuum tube. 



These experiments might be confirmed by several others 

 w r hich would present but little difficulty to one having at his 

 disposition sufficient apparatus. For instance, Spottiswoode's 

 experiment might be repeated with greater care. Since any 

 system of longitudinal waves can only be propagated by means 

 of rarefactions and condensations, it would follow that two con- 

 secutive strata would be in opposite phases. Now we have 

 seen that when the strata are examined in a rapidly rotating 

 mirror, the image of each stratum presents a series of short 

 detached lines. If two strata are in opposite phases it follows 

 that the bright lines composing the image of one stratum should 

 correspond to the spaces separating the bright lines of the next 

 stratum, which spaces should correspond to the bright lines of 



* Phil. Trans., 1879-80. 



