118 Mr. A. A. Campbell Swinton. On the Luminosity 



below, but was approximately about 0*00005 atmosphere, the maxi- 

 mum luminosity being obtained when the dark spaces of the two 

 cathodes just crossed at the centre of the bulb. Owing to the large 

 amount of gas occluded by the mantle, a proper degree of permanent 

 exhaustion was very difficult to arrive at, and required continuous 

 pumping for many hours with the cathode rays turned on at intervals. 

 Even then the conditions of maximum luminosity were exceedingly 

 unstable, owing to the further liberation of occluded gas on the one 

 hand, and on the other to the rapid increase in the degree of exhaus- 

 tion owing to absorption of the residual gas by the electrodes. That 

 such absorption probably took place in the aluminium electrodes, and 

 not in the mantle, was demonstrated by other experiments with a tube 

 in which there was no mantle, but only two electrodes of aluminium 

 wire. 



After the cathode rays had been allowed to bombard the mantle for 

 a short time, the latter was found to have become discoloured where 

 bombarded. That portion which was composed of pure thoria became 

 dark blue, while the thoria plus ceria became brown. This effect, 

 which appears to be analogous to those observed by Goldstein with 

 lithium chloride and sodium chloride,* seems to be due to a partial 

 reduction of the oxides by the cathode rays. The admission of a very 

 minute quantity of air to the tube while the cathode rays are acting on 

 the mantle, and the latter is in parts incandescent, causes the dis- 

 coloration to disappear instantaneously on the incandescent, but not 

 upon the cool portions, probably by re-oxidation of the partially 

 reduced oxides, while the discoloration also slowly vanishes in a day 

 or two with the mantle cold if air at ordinary atmospheric pressure is 

 admitted to the tube. By continuing to bombard the mantle with 

 cathode rays, and alternately allowing the vacuum to increase and 

 letting in small quantities of air, the discoloration can be made to 

 appear and to disappear over and over again as often as desired. At 

 the moment of admitting the air, the amount of light was found 

 momentarily to increase, this being probably due to the increased 

 temperature due to the re-oxidation of the partially reduced oxides. 



After repeating this process of letting in small quantities of air, and 

 allowing them to be absorbed, several times, it was found that the 

 degree of exhaustion which gave the maximum incandescence had 

 altered from 0-000047 to 0*000112 atmosphere, as measured by the 

 McLeod gauge. Similar effects were obtained with a tube containing 

 no mantle, but only aluminium wire electrodes, the inference being 

 that some change takes place in the residual gas which renders it less 

 conducting. 



At a higher degree of exhaustion than that which produced incan- 

 descence of the mantle, the pure thoria was found to fluoresce blue, 

 * ' Wied. Ann.,' 1895, No. 54, p. 371. 



