Disappearance of Gas in the Electric Discharge. 693 



be found, but the vapour-pressure has been determined 

 recently in the chemical department of this laboratory by 

 Walker's method of drawing a neutral gas over the phos- 

 phorus and determining the weight of the material subse- 

 quently frozen out of the stream. The results will be 

 published elsewhere, but they were .sufficiently concordant 

 with the value just given, if the molecule of the vapour 

 is taken (in accordance with the accepted view) to be P 4 . 



20. The discharge in phosphorus vapour. — A side tube 

 containing white phosphorus was attached to the lamp 

 (fig. 1) and separated from it by a tap A*. The lamp was 

 also separated by a tap B from the remainder of the 

 apparatus. By exhausting with both taps open, closing 

 B and then A, the lamp could be filled with phosphorus 

 vapour at the pressure corresponding to room temperature ; 

 or, by cooling the side tube, at the pressure corresponding 

 to any lower temperature (T). 



There was no evidence that heating a tungsten filament 

 in the vapour to 2500° K. produced any chaime either in the 

 filament or in the vapour. But if the potential of the anode 

 was gradually raised, a definite glow potential, similar in all 

 respects to that discussed in § 5, was reached at 46*5 volts 

 (T = 290). At T = 273, V, was 58 volts. Thus the glow 

 potential, .as in other gases, is greater at the lower pressure. 

 The glow had a characteristic blue colour, but observations 

 with a pocket spectroscope did not disclose any recognizable 

 lines other than those of CO, which were doubtless derived 

 from the tap grease. 



The glow, and the increased current which accompanies it, 

 only lasts for a second or two. It very soon ceases, and is 

 not renewed unless the potential is raised further. The 

 phosphorus vapour disappears in the discharge, like most 

 other gases, but much more rapidly than any of the other 

 gases investigated. And it is easy to determine what has 

 happened to the vapour that has disappeared (or at least to a 

 great part of it). At the moment that the glow discharge 

 nasses, the walls of the lamp assume a faint yellow tint, 

 precisely that of the " yellow bulb," which is so familiar to 

 lamp makers. If further charges of vapour are admitted 

 and the process repeated, the vapour disappears as before, 

 and the coloration of the wall gradually deepens to brown. 



* Of course, taps always mean the presence of grease-vapour : and it 

 is impossible to be sure that this vapour has no effect on the result. Hut 

 all the evidence that could be obtained by varying the conditions in all 

 possible ways convinced us that the grease vapour had no effect what- 

 ever on the experiments about to be described. 



