of the cathode in a discharge tube. 185 



becomes more sharply defined with time of running, seems to 

 suggest that the cathode rays emitted by the fresh cathode have 

 various velocities, but that as the running is continued, the faster 

 and more penetrating rays cease to be emitted in any quantity, 

 until at last when the cathode is quite fatigued the rays are very 

 homogeneous. 



Having reduced the lower carbon to this fatigued state 

 attempts were made to restore it to an equality with the standard. 

 To this end it was caused to absorb hydrogen, air, carbon dioxide 

 [and helium] by cooling in liquid air and admitting the gas. 



No appreciable return to equality with the standard could be 

 detected. 



It should be mentioned that during the absorbing process the 

 upper standard carbon electrode was kept hot to prevent any 

 absorption taking place there. This meant that both oxides of 

 carbon were being absorbed by the lower electrode during the 

 admission of carbon dioxide, for carbon monoxide would be con- 

 tinuously produced by the union of the incandescent upper carbon 

 with the surrounding carbon dioxide. 



Platinum electrodes were next tried, the heating in this case 

 being produced by the passage of an electric current. No certain 

 results could be obtained owing to the erratic behaviour of the 

 platinum after heating. The only point that could definitely be 

 established was that, as in the case of the carbon, running as 

 cathode could liberate gas which mere heating could not. 



Aluminium electrodes were next tried but without any heating 

 arrangement. The same fatigue effect though rather less marked 

 than with carbon was observed. Again it was found impossible 

 to restore the cathode to the original (standard) state by causing 

 the metal to absorb various gases. The method of causing gas 

 absorption in this case was to heat up the metal to about 400° C. 

 in a glass tube containing the gas required and then allowing it 

 to cool. In this way some little of the gas was always absorbed. 

 A considerable volume of hydrogen could be introduced into the 

 body of the metal in quite a different way. The cathode was made 

 one of the electrodes in an electrolytic cell containing dilute 

 sulphuric acid and the current passed so as to deposit hydrogen 

 against the aluminium. Although this caused considerable 

 absorption of hydrogen yet the fatigued metal showed no return 

 to the standard state. 



It is well known that the green phosphorescence excited on 

 glass by cathode rays falls off in intensity as the bombardment is 

 continued. 



This effect was eliminated by using a horizontal discharge 

 tube with the electrodes as before facing each other at either end, 

 in the centre of which was suspended a Willemite screen carrying 



13—2 



