Disappearance of Gas in the Electric Discharge. 695 



The second concerns the values of the glow potential. 

 If the molecular weight of phosphorus vapour is 124, the 

 glow-potential curve corresponding to fig. 4 should lie 

 between those of argon and mercury (cf. § 6). Only one 

 point on the curve is definitely known (^ = 0'014 mm., V^ = 

 46*5), but this point (and probably that for T = 273K.) lies 

 well above the argon curve and close to that for CO. This 

 discrepancy suggests interesting considerations which will 

 be raised when the general theory of the glow potential is 

 discussed in a later communication ; but it may be pointed 

 out that an explanation would be obtained if it could be 

 established that the ionization of the phosphorus molecule 

 splits it into its constituent atoms, so that the weight of the 

 phosphorus ion is 31. At present, however, this suggestion 

 is merely speculative*. 



The third peculiarity lies in the great difference between 

 V^ and \ g \ the rising and falling glow potentials. It is 

 impossible to state the difference quantitatively without 

 measuring fully the glow-potential curves ; such measure- 

 ment is difficult, because it involves a rapid method of 

 determining the pressure of the phosphorus vapour present 

 in the vessel. But an indication is obtained by observing 

 by what amounts the potential must be raised, in order to 

 start the discharge once more when it has ceased, owing 

 to the disappearance of the vapour (i.e. by tracing out the 

 line NABijDE in fig. 3) ; this amount is the difference 

 between Y g and V^' at the same pressure. If in argon 

 (fig. 3) we start with a glow potential of 38 volts, the 

 successive values of V requisite to restart the discharge 

 when it has stopped will be 38, 54, 76, 105, 150, .... In 

 phosphorus vapour, the successive values are approximately 

 46'5, 78, 350. They are difficult to determine because,, 

 at the lower pressures, the glow is so transient that it 

 may be missed ; moreover, unavoidable impurities become 

 important ; but there is no doubt that, after the glow has 

 been started at a potential less than 100 volts, the restarting 

 of the glow after it lias stopped needs a potential very much 

 higher than in similar conditions in other gases. No ex- 

 planation is attempted at present of this difference ; hut it 

 will appear that the fact is of great practical importance. 



* Note added Feb. 9, 1921. — It has been discovered that these con- 

 clusions are doubtful. The vapour-pressure of white phosphorus in the 

 presence of hot tungsten is considerably lower than its normal vapour- 

 pressure. The hot tungsten converts some, but not all, of the vapour 

 into the red modification ; equilibrium is reached when the concentration 

 of the vapour present is less than the normal, but still finite. The high 

 glow potentials are doubtless due to this lowering of l.he equilibrium 

 vapour-pressure. 



