Disappearance of (ran in the Electric Discharge* ;V. ,( .) 



low glow potential obviously indicates that the discharge is 

 passing through the impurity as well as, or rather than, 

 through the gas. Thus it is well known that the presence 

 of mercury vapour is apt to mask the presence ol all other 

 gases, and that hydrogen is often concealed by CO. The 

 spectroscopic observations in these experiments have been 

 made so far with no instrument more effective than a small 

 direct-vision prism pointed at the main body of the lamp ; 

 but the great changes in the spectrum are visible to the 

 unaided eye. Thus the glow in the purest hydrogen is 

 almost invisible, much fainter than in any of the other gases 

 examined. The minutest trace of impurity at once brightens 

 the glow, and at the same time the spectrum of the impurity 

 (mercury or carbon monoxide) appears. The spectrum of 

 carbon monoxide is not easily suppressed ; it is clear even 

 when mercury at room temperature is present, though the 

 mercury lines are also bright. On the other hand such 

 mercury entirely suppresses the nitrogen spectrum, so far 

 as could be seen ; and the mercury lines could be seen in 

 the nitrogen spectrum even when the mercury was at 200°K. 

 As the mercury is allowed to warm up, the sudden cessation 

 of the red nitrogen glow and its replacement by the mercury 

 glow, much fainter (to the eye), are very striking. 



The Glow and Ionization. 



8. The facts that have been stated do not provide a complete 

 theory of the physical meaning of the glow potential, but 

 a few obvious conclusions may be noted. 



There is no doubt that the appearance of the glow is 

 accompanied by an increase of the ionization of the gas, but 

 it should be remarked that the sudden increase of current, 

 at the pressures of which curve III. is characteristic, does not 

 represent merely the addition of current carried by ions 

 from the gas. By far the greater part of this increase is due 

 to the neutralization of the space-charge and to the passage 

 between the electrodes of a greatly increased proportion of 

 the electrons liberated by the thermionic emission of the 

 cathode. 



That the glow is closely connected with the neutralization 

 of the space-charge is indicated also by the relation between 

 the molecular weight of a gas and its glow potential at a 

 given pressure. A given number of ions will be the more 

 effective in neutralizing the space-charge the less is their 

 velocity : their velocity with a given potential will be the 

 less the greater is their molecular weight. It is doubtless 



