Disapjjearance of Gas in the Electric Discharge. 697 



These statements apply to the total quantity of gas which 

 can be made to disappear. What is far more important for 

 some purposes is the lowest pressure that can be reached, 

 even if the amount o£ gas initially present is very small. In 

 § 9 it was pointed out that the lowest pressure that can be 

 reached in the absence of phosphorus is that at which the 

 failing glow potential becomes equal to the potential which 

 can be applied between the electrodes. This pressure 

 depends largely on the form of the electrodes, and is in 

 general lower the more uniform the field between them. 

 In the lamp of fig. 1, with an applied potential of 150 volts, 

 the limits were for H 2 001 mm., for N 2 and CO 0-0012.. 

 (To reach this limit in hydrogen it is necessary to remove 

 the water formed.) On the other hand, in the presence of 

 phosphorus, with the same potential, the discharge and the 

 absorption of gas do not cease until the pressure has fallen 

 to 0'0002 mm., and when the discharge ceases at this 

 pressure it is not started again by raising the potential to 

 300 volts. 



An explanation of this effect of phosphorus in lowering the 

 limit of pressure that can be reached may be based on the 

 facts narrated already. The absorption of gas ceases when 

 the glow discharge ceases ; and the glow discharge ceases 

 when the falling glow potential becomes equal to the applied 

 potential. The admixture of phosphorus vapour with the 

 gas increases the pressure corresponding to any given partial 

 pressure of the gas, and thus decreases the glow potential 

 corresponding to that partial pressure. It therefore enables 

 the discharge and the absorption of gas to continue when 

 the partial pressure has fallen so low that, if the gas were 

 present alone and its partial pressure were the total pressure, 

 the potential applied could no longer be sufficient to maintain 

 the discharge. When the partial pressure of the gas has 

 fallen sufficiently, the phosphorus vapour, and not the gas, 

 begins to disappear ; and the disappearance of this vapour 

 proceeds until the pressure of the phosphorus has fallen so 

 low that the discharge can proceed no longer. And when 

 the discharge ceases because the falling glow potential has 

 become equal to the applied potential, it is not started again 

 except by a very great increase of potential, owing to the 

 wide difference between the falling and rising glow potentials 

 in this vapour (cf. § 9). 



But why does the gas disappear before the phosphorus 

 vapour (or before the greater part of it), as this explanation 

 demands? Since pure phosphorus vapour disappears much 

 more rapidly than pure gas ; it might be expected that the 



