Absorption of Gas in a Crookes Tube. 513 



judged from the distance apart of the stria?, and an induction- 

 coil discharge was passed for two days, at the end of which 

 it was giving out Rontgen rays. 



The absorption cannot be accounted for by supposing the 

 electrodes to combine with or occlude the gas, for, as has 

 already been shown, an electrodeless discharge also brings it 

 about. 



An experiment of Prof. J. J. Thomson's, which he is good 

 enough to allow me to use, shows further that the gas does 

 not leak out either through the walls or along the electrodes. 

 A tube was used which communicated with the exterior 

 through two taps, the volume between the taps being known; 

 it was exhausted to about *5 mm., cut off from the pump, and 

 weighed. A discharge from a coil was passed for some days, 

 known amounts of gas being admitted when necessary. Bv 

 repeated weighings it was shown that the weight of the tube 

 increased by amounts equal to the weights of air that had 

 been admitted, although after admission the discharge had 

 caused the pressure to fall so much that .Rontgen rays were 

 given out by the tube. 



Difficulties in accepting either of the alternatives advanced 

 above are not wanting'. Thus against the chemical view it 

 mav be noted that air, nitrogen, and hydrogen give nearly 

 the same rates of absorption in a soda-glass tube ; that 

 throughout the experiments no differences in the behaviour of 

 the first two were noticed ; and that surfaces of soda-glass, 

 lead-glass, mica, silver, or aluminium showed little differ- 

 ence, at least when nitrogen and air were used. 



If the view be adopted that the ions are shot into the glass 

 and held there, from their greater velocity the negative ions 

 are the more important. In that case differences in the rate 

 of running down might be expected, according as facilities 

 were or were not afforded the negative ions of reaching the 

 glass. The experiments in Sec. 5 show that this is not the 

 -case. This view would ; however, explain the result of Sec. 7, 

 that the rate of absorption was much faster when the cathode 

 was surrounded by the anode than when the current runs in 

 the opposite direction; for in the former case the negative 

 ions have a greater chance of being shot into the walls (in 

 this case the anode) than in the latter. 



To test this view other experiments were performed. 



9. Since negative ions are shot off from the cathode at 

 right angles to its surface, the shape of the electrode will 

 influence the number which reach the walls. With a disk 

 most of them will be shot up the tube, while with simple wire 

 electrodes more will be sent to the sides. 



