1897.] the compression of certain rarified Oases. 299 



Pressure-glow, or luminosity accompanying compression. 



Up to this point the present note deals with phosphorescence 

 excited electrically and I now turn to the observations which 

 suggested the title of the note. 



15. The Hagen-Topler pump and tubes were filled with a 

 mixture of gases capable of exhibiting the yellow phosphorescence 

 that I attribute to oxygen with a trace of nitrogen in it. The 

 pressure was reduced below the value for which electrically excited 

 phosphorescence was a maximum for that mixture, namely below 

 mm, 4 or mm -3. Then the gas in the bulb of the pump itself was 

 compressed in the usual process of pumping, preparatory to its 

 being driven out of the pump. As the compression proceeded, the 

 gas was observed to become luminous throughout that part of the 

 pump-bulb which was not occupied by the rising mercury. Its 

 brightness increased considerably and then gradually faded away. 

 The maximum brightness was enough to enable one to read 

 ordinary small print with ease. The amount of compression 

 necessary for maximum brightness was found to depend upon the 

 initial pressure. 



16. After the gas has been compressed and has given out the 

 'pressure-glow,' then if it be allowed to return to its original 

 large volume and low pressure and be again compressed in the 

 pump-bulb, the gas does not glow, unless a certain operation has 

 taken place at low pressure ; it consists in (i) lowering the 

 mercury in the pump-bulb enough to open connection between the 

 pump and the tube used for the electrodeless discharge, and 

 (ii) producing an electrical discharge in this tube. 



17. It would appear that before compression the gas can be 

 put into a state of combination or grouping, which is stable at the 

 lower pressure ; and when compression has raised the pressure to 

 a certain value, the original grouping of atoms or molecules is no 

 longer stable and the gas passes into a new state of grouping 

 with an evolution of energy, part of which would account for the 

 ' pressure-glow.' In some way the electrodeless discharge supplies 

 the energy necessary to the formation of the combination which 

 is stable at low pressure. Inasmuch as the mixture of gases, 

 in which pressure-glow occurs, consists of oxygen with only a 

 trace of nitrogen (and possibly some other gases which would 

 ordinarily be treated as impurities), it immediately occurs to one 

 that we have to deal with the formation of ozone at the low 

 pressure. It is however not clear what part is played in the 

 phenomena by the impurities, though it seems certain that their 

 presence is of importance, if not essential for the production of the 

 phosphorescence. 



