﻿and Absorption by Resonating Gas Molecules. 71*5 



previously heated until the mercury vapour in it had a 

 pressure of several atmospheres, and I imagine that the inner 

 surface may have adsorbed some of the vapour, which was 

 not removed bv the acid. A chemist would undoubtedly 

 have called the flask clean, after the first treatment which 1 

 gave it. The two photographs are reproduced on PI. XII. 

 fig. 8. 



I next drilled a shallow cavity in the end ot a brass 

 cylinder, warmed it to a temperature of perhaps ten degrees 

 above the temperature of the room, and placed a drop of 

 mercury in the cavity, the drop standing up above the level 

 of the end of the cylinder. This was photographed in the 

 dark box by the light of the lamp, and the picture showed 

 the black column of mercury vapour carried up by the con- 

 vection current of warm air (PL XII. fig. 9). 



I am at present setting up a Michelson interferometer with 

 a fiuorite plate coated with a cathode deposit of gold for 

 the purpose of ascertaining how large a difference of path is 

 possible in the case of interference fringes formed by the 

 light of this lamp. I imagine that the light may turn out 

 to be more homogeneous than that of any other source with 

 which we are acquainted. Of course it is possible that the 

 line itself is complex, in which case matters wall be different. 

 The dispersion of the vapour in the vicinity of the 2536 line 

 will also be investigated with the same apparatus. It will 

 be interesting also to place the resonance lamp in a powerful 

 magnetic field, and study the Zeeman effect for a gas 

 radiating by resonance instead of under the action of an 

 electrical or other stimulus. 



It even appears possible that the electric analogy of the 

 Zeeman effect can be detected with the resonating mercury 

 vapour, since it can be brought to a high luminosity in a 

 vacuum which is practically non-conducting, that is if the 

 vapour does not become conducting when excited by the 

 ultra-violet light, which I doubt ; for a roughly carried out 

 experiment with a small gold-leaf electroscope showed no 

 conductivity when the vapour in the bulb between two 

 electrodes was excited. We cannot infer from this what it 

 would do with a potential sufficient to give say a 20 cm. spark 

 in an alternating spark-gap, but I have a feeling that the 

 vacuum would support it. 



In fact a very wide field of investigation appears to be 

 opened by the discovery of the very remarkable behaviour 

 of the vapour of mercury at very low pressures. 



This [taper forms the third of a series upon the optical 



