﻿712 Prof. R. W.AYood on Selective Reflexion, Scattering 



weaken at a pressure of about 2 cm. and is practically gone 

 at 70 cm. 



Polarization Experiments. 



Inasmuch as I have discovered strong polarization in the 

 fluorescent light emitted by sodium, potassium, and iodine 

 vapour, I fully expected to find it in the case of the mercury 

 resonance radiation, as the mechanism of the emission would 

 appear to be much simpler in this case than in the other 

 cases where we have, in addition to the resonance radiation, 

 other associated frequencies emitted, which give rise to what 

 I have named resonance spectra. No trace of any polariza- 

 tion could be detected with a Babinet compensator mounted 

 in front of the resonance lamp. A small Foucault prism was 

 used as an analyser, as ordinary NicoPs prisms are opaque 

 to the ultra-A'iolet on account of the Canada balsam with 

 which they are cemented. 



Experiments with the Resonance Lamp. 



The radiation emitted from the exhausted quartz bulb is so 

 homogeneous, that a layer of mercury vapour 5 mm. thick 

 and at the pressure which it has at room temperature 

 (0*001 mm.) reduces its intensity by about one half. 

 Various investigations with the vapour at exceedingly low 

 pressure at once became possible. It is as if we had a gas 

 which appeared quite black even at pressures commonly 

 employed in vacuum-tubes. It will be possible to study the 

 rate at which the vapour diffuses into other gases at low 

 pressures, and it may be possible to tell in this way whether 

 the resonators are in reality mercury molecules or larger 

 aggregates. 



I made two photographs which illustrate what a sensitive 

 detector of small traces of mercury vapour we have in the 

 light of the resonance lamp. A quartz bulb having an 

 internal diameter of 1*5 cm., containing a drop of mercury, 

 was mounted in front of a photographic plate in a dark box 

 and illuminated with the light of the lamp. The bulb cast a 

 shadow as black as ink. The bulb was now opened, the 

 mercury drop removed, and the bulb washed out with nitric 

 acid and distilled water, heated nearly red-hot and a blast of 

 air blown into it, and again photographed. It still gave the 

 black shadow, though not as black as before. It was only by 

 prolonged heating to a red-heat and much rinsing out with 

 an air current that I was able to obtain a shadow picture; 

 which showed the flask transparent. The flask had been 



