Disappearance of- Gas in the Electric Discharge. 925 



fluorides and chlorides, especially those containing sodium 

 and lithium, silica, glass (the powdered material of the 

 bulb), and sodium silicate. On the other hand, it has not 

 been found with oxides, or with any of the halogen salts of 

 potassium. Sulphates and some other salts cannot be tried 

 because they destroy the filament. We have been unable to 

 discover any relation between this property of salts and other 

 properties, but we have some suspicion that the ionization 

 potential of the ionized salt may be a determining factor, 

 for slight traces of impurity, such as grease vapour or other 

 hydrocarbons, may inhibit the effect even of those salts 

 which normally display it. We are inclined to compare their 

 effect with that of hydrocarbons or mercury vapour in 

 inhibiting absorption of gas ; the action ceases because 

 the impurity of low ionization potential is ionized in pre- 

 ference to the salt vapour. However, this explanation is 

 speculative. 



In addition to salts, metallic tungsten from the filament 

 itself may show the effect. If the lamp is burnt while it 

 contains just enough gas to allow the glow discharge to pass, 

 so that the walls become blackened with sputtered tungsten 

 from the cathode end of the filament, the increased absorp- 

 tion in the presence of tungsten is obtained. In II., p. 689, 

 we attributed the absorption of nitrogen in the absence of 

 phosphorus to the sputtered tungsten ; we believe "that the 

 presence of this material is the reason why the absorptive 

 power o£ the walls of the vessel is changed apparently by 

 the process of absorption (cf. II., p. 703) so that it is necessary 

 to use a new vessel for every experiment. 



It should be mentioned that the enhanced absorptive 

 power due to the salt is exhausted by absorption. If a 

 vessel in which salt has been " evaporated " and gas subse- 

 quently absorbed is re-filled with gas, after the usual baking 

 and evacuation, the absorption on the second occasion will 

 be much less than on the first, and on the third will not be 

 different from that of a normal glass vessel. This fact is not 

 inconsistent with the view suggested in the last paragraph 

 of the action of sputtered tungsten ; for in that case fresh 

 sputtered tungsten is being deposited while the previous 

 layers are being used up. 



10. Measurements have been made of the amount of salt 

 necessary to produce the increased absorption. The 

 following figures give the results of a series of trials with 



