Absorption of Gas in a Crookes Tube. 511 



nitrogen. The change of gas produced no appreciable effect 

 in the rate of running down. 



It was found with nearly every tube that, when the elec- 

 trodes had been cleared of gas, the fall of pressure was 

 abnormally great when a fresh charge of gas had been newly 

 admitted from C (fig. 1) to the experimental tube, and this 

 took place although it had been drying for some days. 



The difference between the rates of running down accord- 

 ing as A was cathode or the reverse was very marked at 

 pressures above *5 mm. ; in some cases the current could be 

 passed for several hours with A anode, and the pressure did 

 not alter by more than one division on the gauge ; while if it 

 were reversed and kept at the same value, there was a de- 

 crease of about 25 divisions per half-hour. 



At pressures below *2 mm. the alteration amounted to four 

 divisions per half-hour with A the anode, a reversal of the 

 current raising this to 60 or 70 divisions in the same time. 

 The resistance of the tube was very much greater when A 

 was cathode than when it was anode. 



The measurements were very irregular with hydrogen gas, 

 notwithstanding some interesting results were obtained. The 

 pressure did not decrease so quickly as with air and nitrogen 

 when A was cathode, but with a reverse current the hydrogen 

 disappeared much faster than did the other gases under 

 similar circumstances. With hydrogen as with the others, 

 the absorption was always more marked when A was cathode 

 than when it was anode. 



Hydrogen further differed from air and nitrogen in that 

 when the apparatus was allowed to stand most of the gas 

 reappeared. Thus, after running for several hours and then 

 allowing to stand, the pressure generally rose during the first 

 half-hour by about ten divisions on the gauge; it then re- 

 appeared more and more slowly, after a couple of hours 

 rising about two divisions per hour ; after several hours' 

 standing it was found that about two-thirds of the gas had 

 again appeared, and this gradual reappearance went on for 

 several days. 



Other peculiarities also showed themselves with hydrogen. 

 When B was cathode, the voltage required to maintain (not 

 start) the discharge gradually decreased as the pressure de- 

 creased down to about *6 mm., after which it rose again. 

 This is in general agreement with the results obtained with 

 other tubes and gases; but when A was cathode the voltage 

 required increased as the pressure was lowered from 

 1*25 mm. 



The resistance of the tube when A was cathode was also 



