TROWBRIDGE. 



ELECTRICAL DISCHARGES. 



449 



tive discharges by the X-ray vacuum, the resistance of the X-ray 

 tube was measured in the process of pumping wheu it had reached a 

 pressure of 2 mm., at which stage the rarefied air appears to have the 

 greatest conductibility, aud it was next measured at the X-ray vacuum, 

 the gap C being illuminated by the X-rays. These measures of resist- 

 ance were made by using an additional air gap in the circuit, and by the 

 method of dampening of oscillations. Nine oscillations could be obtained 

 through the rarefied air at 2 mm. pressure, while under the same condi- 

 tions eleven oscillations were obtained in the X-ray vacuum. This shows 

 that, when this high vacuum is broken down, it exhibits less resistance 

 than air at 2 mm. pressure. 



Figures 10 to 14. 



In another experiment the gap O was placed in a tube which was 

 separate from that in which the X-rays were produced, in order to pre- 

 vent any disturbing effect of the current which produced the X-rays. 

 The same breaking down effects were observed, altliougli they were 

 lessened by the necessary weakening of the X-rays, since they had to 

 pass through two surfaces of glass separated by a layer of air. 



In order to confirm the statement that this breaking down effect is 

 exerted at the cathode in the tube through which a battery current is 

 led, one of the terminals was covered with a piece of thick barometer 

 tubing which projected bejond the metal of the terminal B' (Fig. 13). 

 This thick glass served to diminish the effect of the X-rays on this 

 VOL. XXXIII. — 29 



