Disappearance of Gas in the Electric Discharge. 927 



low (say less than *005 mm.), the pressure never falls below 

 that corresponding to the glow potential, and a complete 

 clean-up is never indicated by the bridge method of Par. 4, 

 even though the amount of gas that can be absorbed, starting 

 from a higher pressure, is much greater than that corre- 

 sponding to a pressure of '005 mm, During the first minute 

 of the burning, the pressure falls by an amount which 

 depends somewhat upon the original pressure, but is about 

 *02 mm. ; subsequent changes are much slower. In some 

 cases the pressure begins to increase again very slowly, and 

 will continue to increase until the filament finally fails ; 

 in other cases it will continue to fall very slowly, the fall 

 alwa} r s being accompanied by a marked blackening of the 

 bulb and being due presumably to sputtered tungsten (cf. 

 Par. 9). The cause of the difference has not been fully 

 ascertained, but it is suspected that the rise occurs when the 

 gas is more highly contaminated with grease vapour, some of 

 which may be admitted during filling. But in both cases 

 there is evidence that gas is being evolved as well as 

 absorbed, for even if no water vapour is initially present, a 

 trace of it, can often be found in the later stages, in spite of 

 the presence of the incandescent tungsten. 



If the wire is originally coated with one oE the active 

 substances mentioned in Par. 9, an appreciable absorption 

 of gas occurs during the first instant of burning and while 

 the glow lasts, which is the sign of the " evaporation ; ' 

 of the salt. This absorption may amount to *03 mm., and 

 is as much as *015 mm. in argon — a gas which shows 

 no absorption comparable with this in the other circum- 

 stances of these experiments. After this initial stage, or 

 if the salt is evaporated from the wire before the lamp 

 is filled with gas, the absorption during the burning of 

 the lamp is definitely greater than it is when no salt is used. 

 Moreover, the absorption is never reversed. It is still true 

 that a perfect clean-up, indicated by the bridge, is never 

 attained, but otherwise the salt acts in much the same way 

 as it does in the presence of phosphorus. In fact, after salt 

 has been deposited, the amount of gas that can be absorbed 

 is approximately independent of the original pressure (as it 

 is when phosphorus is present) ; and the amount of gas that 

 can be absorbed is not very different from that corresponding 

 to the step of fig. 1 which, when salt is present on the walls, 

 gives the absorption for almost all quantities of phosphorus 

 below a certain limit ; it appears, however, that the presence 

 of a little phosphorus is necessary to give the full value of 

 •0023 mgm. in fig. 1. 



