602 Research Staff of the Gr. E. C, London, on the 



" cleaned-up " by the discharge so completely that no 

 further discharge will occur until the much greater poten- 

 tial E is applied. On the other hand, if the initial pressure 

 is slightly less than C, the available potential will not start 

 the discharge, and no " clean-up " can be obtained ; if at 

 any subsequent time the potential is raised slightly above C 

 the discharge will occur. It may easily happen that too 

 low an initial pressure in the lamp is prejudicial to the 

 obtaining o£ a complete " clean-up," and that the clean-up 

 is more complete if initially a little extra gas is introduced. 



Chemical Effects of the Dischakge. 

 The Disappearance of Carbon Monoxide. 



10. The changes just described will not occur unless the 

 circumstances are such that the gas disappears only when 

 the glow discharge passes, and not when the potential between 

 the electrodes is insufficient to produce the glow. Thev 

 would not occur i£ the gas disappeared when there was no 

 potential between the electrodes, or when the potential, 

 though sufficient to produce some ionization, was less than 

 Yff. There undoubtedly are such circumstances ; there are 

 conditions well known in technical practice in which the 

 "clean-up" of the lamp is accompanied by, and inseparable 

 from, a marked glow, and the experiments of Langmuir give 

 clear evidence of their existence ; for he shows that in 

 some gases — notably nitrogen and carbon monoxide — when 

 the temperature of the filament is below that at which 

 notable vaporization occurs, the gas disappears only " elec- 

 trically " and in consequence of the passage of a discharge. 

 It should be observed that in all his experiments there must 

 have been some " discharge," for there was a difference of 

 potential between the ends of his filament ; and if that 

 potential exceeded (as it often did) the ionization potential 

 of the gas, it must have been accompanied by some ioniza- 

 tion of the gas, and cannot have been a pure electron current. 

 The fact, therefore, that he distinguishes between conditions 

 in which there was a discharge and those in which there* 

 was not, shows that it is only some special form of discharge 

 that is effective : the experiments described here, in con- 

 junction with his observations, leave little doubt that this 

 form was the glow. 



The matter has been studied more closely in the case of 

 carbon monoxide, nitrogen, and hydrogen, both in the 

 presence and in the absence of phosphorus vapour. It will 



