298 Mr Newall, On Luminosity attending [Apr. 26, 



indications of the spectrum of the bright discharge preceding 

 phosphorescence, I am led to connect 



white phosphorescence with mixtures containing oxygen and 



carbon compounds, 

 yellow phosphorescence with mixtures containing oxygen and 



nitrogen, 

 blue phosphorescence with mixtures containing oxygen and 

 sulphur. 



Pressure conditions. 



12. So far as my experiments have gone, the phosphorescence 

 appears in rarified gases between the pressures 6 and mm 01. 

 It rises in brightness as the pressure is reduced and reaches a 

 maximum at a pressure of about mm '4s, and fades away into 

 invisibility as the pressure is further reduced. These pressures 

 vary within small limits according to the nature of the gaseous 

 mixture. 



13. To measure the pressure, the pump has been calibrated 

 in the well known method. The capacity of the pump-bulb is 

 close upon 500 c.c. ; and the bubble of air in the exit tube, measured 

 at atmospheric pressure (or, at a pressure of one-tenth of the 

 atmospheric pressure when very high exhaustion is to be dealt with), 

 affords means of estimating the pressure of the permanent gases 

 in the vacuum tube connected with the pump. A careful set of 

 experiments on the indications of the gauge has shown within 

 what limits they are trustworthy if certain precautions are 

 attended to, the chief being that sufficient time should be given 

 for the equalization of pressure throughout the tubes and bulb of 

 the pump. Unless the connecting tubes are of large diameter the 

 time required, when the pressure is of the order of 0*1 — 0*01, is 

 very great, something like 20 to 30 minutes, and it is consequently 

 probable that if the pump is worked at a rate greater than one 

 stroke in 20 — 30 minutes, the relative proportions in a mixture of 

 permanent gases will be altered considerably by the inevitable 

 differences in the rates of transpiration of the components. The 

 connecting tubes used in my experiments are about 2 yards long 

 and have an internal diameter of about £ inch. 



14. Striking effects may be observed if the pressure of the 

 phosphorescent gas be altered whilst the gas is actually phos- 

 phorescing after a bright discharge has been passed. Thus 

 suppose the pressure of the gas is higher than that at which the 

 maximum brightness for the given mixture occurs ; then if 

 during phosphorescence the pressure is reduced, a wave of increased 

 brightness passes from the end of the tube near the pump to that 

 remote from it. 



