Absorption of Gas in a Crookes Tube. 515 



noticed that the amount of gas they contained was consider- 

 ably less than for similar aluminium wires: when this gas had 

 b^en driven off, the rate of absorption was equal to that found 

 in a similar tube with aluminium electrodes. 



After using the same end as cathode for several days, 

 a curious effect was noticed. The resistance of the tube 

 suddenly decreased, as did also its rate of running down, the 

 positive column doubled its length, and the heating at the 

 cathode was less marked. This was not due to the discharge 

 passing along the glass, for, on reversing the current, the 

 resistance and rate of absorption rose to their original values. 

 It is likely that some of the sputtered platinum close to the 

 cathode acted as electrode, when the latter would be a disk 

 rather than a point; this would lessen the resistance and also 

 the heating effect. If the absorption is due to a chemical 

 action, the latter effect might be expected to reduce it also, 

 since it would depend on the temperature o£ the gas. 



12. It has been frequently noticed that Rontgen-ray bulbs 

 after prolonged use show a bluish-violet coloration; this is not 

 merely a surface coloration but extends also to the volume of 

 the glass. Villard * believes this is due to Rontgen rather 

 than cathode rays. M. and Mme. Curie f have noticed the 

 same effects in glass flasks containing radio-active substances. 

 This coloration did not appear in my experiments except in 

 tubes that had been run down very low with an induction- 

 coil, so that the chemical action producing it does not have any 

 appreciable effect on the absorption of gas. It was thought 

 that this coloration might be due to alkali salts in the glass. 

 Some common salt placed in a tube in which the pressure was 

 a mm. was coloured the whole distance between the electrodes 

 after a few days' passage of a discharge; no change could be 

 seen in the glass. It was noticed when the salt was exposed 

 to the discharge and was cut off from the mercury pump, that 

 the rate of running down was extremely slow ; when the salt 

 was shielded from the discharge (see Sec. 10) it was found 

 not to influence the absorption. If the coloration is due to a 

 physical modification of the salt, this result would not be 

 expected; so that it must be looked on as supporting Wiede- 

 mann and Schmidt's view of a chemical action. 



13. Since the results of the experiments seemed to point 

 to a chemical combination between the gas and the glass, 

 various samples of the latter were made into tubes and the 

 rate of absorption tested. 



* Phil. Mag. Feb. 1900, p. 244. 

 f Ibid. p. 242. 



