924 Research Staff of the G. E. C, London, on the 



the filament is coated with a suitable amount o£ certain salts, 

 of which sodium fluoride is typical, in the same manner as it 

 is coated with phosphorus for the experiments of Par. 7 ; and 

 if, after the lamp has been baked and evacuated, the filament 

 is heated for a moment to bright incandescence ; then it will 

 be found that the amount of gas that can be absorbed with 

 a given amount of phosphorus is very notably increased. 

 Unless two filaments are sealed into the lamp, one coated 

 with the salt and the other with phosphorus, it is necessary 

 in making these experiments to introduce the phosphorus 

 in the form of vapour. That method was adopted in all 

 the measurements described below, and the amount of 

 phosphorus which could be introduced was limited to about 

 *06 mgm. But it was found qualitatively that, if salt were 

 mixed with phosphorus on a single filament, so that both 

 were " evaporated " together, the amount of gas that could 

 be absorbed was greater than if the same amount of phos- 

 phorus were present without the salt. But for reasons 

 which will appear in the sequel, this method is not well 

 adapted for quantitative investigation. 



There is no doubt that the salt produces this increased 

 absorption because it is deposited on the walls. It certainly 

 is so deposited, because, if a large enough quantity is 

 used, their lustre is changed. Moreover, if, after the 

 filament has been heated, the lamp is opened, left exposed 

 to the air for some hours, and then re-exhausted, the salt is 

 still showing its presence by increased absorption. But if 

 deposition of the salt on the walls by this method increases 

 absorption, why does it not have the same effect when 

 the deposition is effected by evaporating a solution ? The 

 answer is that the passage of the salt from the filament 

 to the walls is not one of mere evaporation and conden- 

 sation. Ionization is also involved ; for a visible glow is 

 often seen to accompany the " evaporation," and electrical 

 measurements show the passage of a current between cathode 

 and anode, even when the greatest P.D. between them is 

 less than 10 volts. There is also chemical resolution ; for 

 the presence of free sodium, absorbing oxygen in the cold, 

 can be detected after the heating of the salt ; but the 

 presence of the sodium cannot be the determining factor, 

 because the exposure to the air after " evaporation " does 

 not affect the process. 



9. Not all salts have been found to produce this increased 

 absorption. Those that show it unmistakably are many 



