780 Prof. A. LI. Hughes on Dissociation of 



pressure was measured by a McLeod gauge. Hydrogen 

 was admitted through a palladium tube. The apparatus 

 was thoroughly outgassed by prolonged pumping and heating 

 to 400° C. When systematic observations were carried out, 

 the apparatus was subjected to an outgassing for 30 minutes 

 at 400° C. between each run, the filament being heated to a 

 white heat, and the nickel gauze being bombarded by elec- 

 trons. The runs tabulated below were obtained after several 

 weeks' preliminary testing, during which there had been 

 many short heat treatments with the pump in action, and 

 during which no gas had been admitted except hydrogen up 

 to pressures of from *001 to "1 mm. It is extremely unlikely 

 therefore that there could be any impurity in the hydrogen. 

 To keep mercury out of the experimental tube E, it was 

 customary, after heating both E and U, to surround the 

 U-tube with liquid air and to continue the heating and 

 pumping for some minutes to ensure that no appreciable 

 amount of mercury was left in E. 



Method of Experiment. — Before admitting the hydrogen, 

 a test was always made (1) on the amount of gas given 

 out when the filament was heated at the temperature to be 

 used in experiment on the hydrogen, and (2) on the amount 

 of o-as given out when electrons were driven across the tube. 

 (1) was in general not measurable, and (2) was usually very 

 small compared with the pressure changes observed when 

 the hydrogen was in the apparatus. Then hydrogen was 

 admitted to approximately the pressure desired. A test 

 was now made for the amount of hydrogen cleaned up by 

 the hot filament — the purely thermal effect discovered by 

 Langmuir. It was found that, at any temperature above a 

 dull red heat, there was a very appreciable clean up. The 

 experiments on the clean up due to the electron stream had 

 therefore to be carried out with a lower filament temperature. 

 Fortunately, the filament gave out an ample supply of 

 electrons when heated to a temperature at which it was 

 barely visible in the dark. The electrons were driven across 

 by the accelerating potential chosen, the electron stream 

 being held constant, if necessary, by slight adjustment of 

 the heating current. Pressures were read every four 

 minutes. It was impracticable to take gauge readings 

 oftener than this. At the end of 40 minutes the electron 

 current was stopped and the constancy of the pressure with 

 the filament still hot, but without an electron current, was 

 checked. The residual hydrogen was pumped out to a 

 pressure less than *00001 mm. Then the evolution of gas 



