﻿1110 Prof. K. W. Wood on Polarized 



The decrease in the percentage of polarization caused by 

 the admixture of small quantities of other gases, and its low 

 value with pressures of mercury vapour much in excess of 

 the value which it has at room temperature, is of consider- 

 able theoretical importance, for if there is a brief interval 

 of time between the absorption of energy by the mercury 

 molecule and its re-emission, and the depolarizing factor 

 is a rotation of the molecule, it is probable that the magni- 

 tude of these quantities can be experimentally determined 

 by making a careful and exact determination of the percent- 

 ages of polarization corresponding to different pressures and 

 temperatures. This matter is now under investigation. 



In the case of the green fluorescent light emitted by 

 mercury vapour at higher densities, when stimulated by 

 ultraviolet light in the spectral range well below the 2536 

 line, the time interval between absorption and emission is so 

 (long that it can be shown experimentally by methods 

 which I described in the Proc. Roy. Soc. for 1921. The 

 vapour was distilled in an exhausted quartz tube in the 

 form of an inverted U, the rising column of vapour 

 remaining dark within the narrow beam of exciting rays, 

 but bursting into fluorescence at a distance of several 

 millimetres above this region, in the form of a pointed 

 green flame, concave on its under side. The resonance 

 radiation, excited by a narrow pencil of rays of wave-length 

 2536, traversing the U-tube, was confined wholly within 

 the illuminated region, showing that the time interval 

 between absorption and emission, if it exists, is very much 

 •briefer in this case. 



The action of argon and helium in augmenting the 

 intensity of the resonance radiation is of considerable 

 interest. With argon at 0*5 mm. pressure the intensity of 

 the resonance radiation is noticeably greater than in a highly 

 ■exhausted tube. The same is true with argon at 60 mms. 

 pressure, the intensity being more than doubled by the 

 ^presence of the gas. 



Helium at 330 mms. increased the intensity to fully four 

 times its value in an exhausted tube. At first sight this 

 seems very surprising in view of the fact that in all cases 

 previously observed, so far as I know, the introduction of a 

 foreign gas decreases the intensity of the fluorescence, for 

 example the introduction of helium or argon into fluorescing 

 iodine vapour. A little consideration, however, made it 

 :seem probable that, in this case, the argon and helium 



