Disappearance of Gas in the Electric Discharge. 937 



wide difference between the rising and falling glow potentials 

 of phosphorus vapour. The other differences are probably 

 due to the fact (II., p. 698) that phosphorus once deposited 

 on the walls can be liberated and re-deposited under the action 

 of the discharge, while salt cannot. This re-deposition 

 doubtless accounts for the increase in absorption beyond 

 that corresponding to a single layer obtained when the 

 amount of phosphorus is increased beyond that required for 

 a single layer, even if the phosphorus is deposited on the 

 walls before the absorption begins (see the dotted line of 

 fig. 1). The form of the full-line for the larger quantities 

 of phosphorus is more difficult to explain in detail, for here 

 the first stage of Par. 13 as well as the second is concerned ; 

 it is not unreasonable on any theory to imagine that the 

 absorption will be greater when the gas is actually mixed 

 with a large proportion of the vapour being deposited. The 

 decreased absorption with the largest quantities of phosphorus, 

 of which we are sure experimentally, we attribute to the fact 

 that, when such large quantities are used, the number of 

 phosphorus molecules present in the lamp just after evapo- 

 ration is much greater than the number of gas molecules. 

 In these circumstances the phosphorus molecules rather 

 than those of the gas are ionized, and it is the former rather 

 than the latter which are deposited on the walls. 



21. Finally, there remains the question of absorption 

 without either phosphorus or salt. Is this also to be 

 attributed to an electrically polar layer on the glass surface? 

 We imagine that it is ; for all surface layers are bound to be 

 electrically polar to some extent, since the elementary 

 charges are not coincident. There is also the possibility that 

 the surface layer is rendered polar by the action of the 

 discharge, which also ionizes the molecules and makes them 

 able to adhere to it. Against this latter suggestion is the 

 fact, apparently ascertained, that the absorption takes place 

 on portions of the walls which have not been exposed to the 

 discharge, at least as readily as on those so exposed. But all 

 these matters require further investigation. 



