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



vapour; it seems, therefore, as stated before (II., p. 699), 

 that the evaporation of the deposited phosphorus is necessary 

 for that restoration. In addition to phosphorus, hydrogen, 

 water, and C0 2 always appear, even if the vessel was originally 

 filled with nitrogen ; if it was so filled, nitrogen appears *. 

 The presence of C0 2 is an almost certain proof that these 

 gases are derived from the glass. Since the}' are produced 

 in greater quantity than if the discharge has not passed, it 

 may be concluded that they are produced by the discharge, 

 the hydrogen being reduced from water by the filament. 

 The liberation of gas from the glass, even while absorption 

 is going on, seems inevitable ; it doubtless accounts in 

 part for the fact that, if the absorption is very slow, the 

 amount of gas that can be absorbed is less (cf. Par. 7). 

 PH 3 appears fitfully in small amounts, never exceeding 15 

 per cent, by volume of the " restored ,? gas. It is known 

 that PHg dissociates at the temperatures used in restoration, 

 and it was thought at first that its small and variable amount 

 might be due to varying extents of dissociation ; but when 

 it appears, its quantity is not greatly changed by a further 

 maintenance at the highest temperature reached during 

 restoration ; it appears to be in equilibrium with the P and 

 H also present. NH 3 has sometimes been detected when 

 nitrogen has been absorbed ; but its amount is again small 

 and variable. 



When hydrogen has been absorbed, it is difficult to tell 

 under what conditions it is all restored, owing to the 

 admixture of hydrogen compounds from the glass. But 

 when nitrogen has been absorbed, this complication does not 

 enter, and it can be established within an error of some 10 

 per cent, that the gas restored by heating for a few minutes 

 to 300° is equal to that absorbed, so long as the absorption 

 has been effected by phosphorus. If, however, the nitrogen 

 has been absorbed by evaporating salt from the wire, as 

 described in Par. 12, little, if any, of the gas can be restored, 



* In II., p. 700, it was stated that nitrogen absorbed in the presence 

 of phosphorus could not be restored by baking. But in those experiments 

 the vessel was always filled with more nitrogen than the phosphorus 

 would absorb, and a glow discharge continued through the remaining 

 gas for some time after the absorption was really complete. In these 

 circumstances there is much sputtering of the cathode, and, as sus- 

 pected before, the layer of phosphorus and absorbed gas becomes 

 coated with involatile tungsten. In these experiments, on the other 

 hand, the nitrogen originally present was completely absorbed in a 

 few seconds and there was no appreciable blackening of the bulb ; 

 accordingly the gas could be restored by baking. 



