Disappearance of Gas in the Electric Discharge. 933 



even by heating the glass to near its softening temperature. 

 The same appears to be true when hydrogen is absorbed ; 

 but, for the reasons explained, the conclusion is less reliable. 

 These observations indicate again that the removal of the 

 solid layer in which the gas is absorbed is necessar} r to the 

 restoration of the gas. There is similar evidence that, if 

 salt and phosphorus are evaporated together from the wire, 

 some of the phosphorus is so entangled in the salt that it is 

 permanently removed from the action. This is the reason 

 why quantitative experiments in which salt and phosphorus 

 are evaporated together are unreliable (cf. Par. 8). 



One further fact may be noted. The gas restored by 

 heating does not depend on the pressure of the gas in the 

 lamp during absorption. Even when tl lis pressure is as 

 great as 05 mm., if the unabsorbed gas is pumped out 

 completely and the lamp then sealed off and heated, the 

 oas restored is indistinguishable from that which would have 

 been obtained if the absorption had taken place from a 

 pressure so low that all the gas ,originally present could be 

 absorbed. Even at a pressure so high that the free path of 

 ions is very small compared with the dimensions of the 

 lamp, the absorbed gas is to be found on the walls of the 

 vessel. h 



16. Whitney has shown * that other substances than 

 phosphorus possess much the same specific action in 

 promoting absorption, namely sulphur, iodine, and arsenic. 

 The first two we have not investigated, but we can con- 

 firm his conclusions that the behaviour of arsenic is 

 indistinguishable from that of phosphorus in most of the 

 matters discussed here. All these substances resemble each 

 other in being elements and so not decomposed by the 

 discharge, and in having more than one allotropic modi- 

 fication. The bearing of the last property will be evident 

 from our suggestions in Par. 20. 



General Conclusions. 



17. We do not profess to offer a complete theory of all the 

 observations that have been described. But it seems to us 

 clear that the suggestions as to the action of phosphorus in 

 promoting absorption, which were offered in III. p. 704, are 

 generally confirmed. The action of this substance is due to 



* Cf. S. Dushman, General Electric Review, xxiv. p. 680 (1921). 



