'698 Research Staff of the G. E. 0., London, on the 



order of disappearance would be reversed. It is not because 

 the current is carried by the gas rather than the phosphorus 

 vapour, for there is evidence that, even in the presence of 

 gas, part of the discharge is carried by the phosphorus. 

 Such evidence cannot be based on spectroscopic observations, 

 for the spectrum of phosphorus is not readily discernible ; 

 but it is obtained from the observation that, if sufficient 

 phosphorus vapour is present, a ''yellow bulb" is formed 

 before all the gas has disappeared. The reason for the per- 

 sistence of the phosphorus is, we believe_, to be found in the 

 reversibility under the discharge of the reaction which leads 

 to the conversion of the white phosphorus vapour into the 

 red solid. Even when nothing but phosphorus is present, 

 the reaction is probably reversible, in accordance with our 

 general principles. But the equilibrium lies so far on the 

 side of red phosphorus that the residual pressure of white is 

 barely detectable*. But when the red phosphorus on the 

 walls is bombarded, not only by phosphorus molecules which 

 may be themselves converted into solid, but also by gas 

 molecules which cannot be so converted, the equilibrium is 

 pushed towards the vapour phase. So long as there is gas 

 present in considerable quantity, the conversion of white 

 into red is never complete ; there is always enough white 

 phosphorus re-evaporating to maintain the discharge : and 

 it is only when the gas has been greatly reduced in quantity 

 that the equilibrium moves once more towards the solid phase, 

 and a complete disappearance of all gaseous molecules is 

 obtained. 



Is THE ACTIOX OF PHOSPHORUS CHEMICAL ? 



22. \Ve have no doubt ourselves that this explanation of the 

 lower limit of pressure attainable in the presence of phos- 

 phorus vapour is right in essentials ; such uncertaintv as 

 affects its detail will probably be removed when the exact 

 mechanism by which the glow discharge causes the adhesion 

 of gas to the walls is better known. But it appears to be 

 believed very generally that the action of phosphorus in 

 aiding absorption by the discharge is chemical, and arises 

 from the formation of definite chemical compounds of phos- 

 phorus w T ith the gas ; such a theory seems to pervade all the 



* But it is detectable by that very sensitive test, the glow when air or 

 oxygen is admitted. If air is admitted to a bulb in which phosphorus 

 vapour has been absorbed by the discbarge, whether with or without 

 gas, the flash can always be seen. On the other hand, it is not seen if 

 powdered red phosphorus is placed in the bulb and freed from white by 

 long-continued exhaustion in contact with the vessel cooled in liquid air. 



