﻿and Absorption by Resonating Gas Molecules. 691 



diverted from the primary beam by a molecule in exact 

 resonance with the light- waves which excite it. 



Lord Rayleigh has interested himself chiefly with the 

 scattering which may occur when the molecules are not in 

 resonance, as is the case in the scattering of the blue light 

 by the molecules of the atmosphere. This case has been 

 experimentally investigated by Abbott, who has measured 

 the atmospheric absorption from the summit of Mount 

 Wilson. Of greater interest, however, is the scattering of 

 molecules resonating under the influence of the light-waves. 

 But few cases of this are known, though this action has been 

 made the basis of the theory of absorption developed by 

 Planck. Some years ago I found that the non-luminous 

 vapour of sodium at a comparatively low temperature, when 

 illuminated by the light from a powerful sodium flame, 

 re-emitted light of the same wave-length in all directions. 

 This re-emission of light, without change of wave-length, by 

 molecules of an absorbing gas, I named resonance radiation, 

 to distinguish it from fluorescence, where there is in general 

 an increase in the wave-length. Sodium vapour is, however, 

 almost impossible to handle in a satisfactory manner, and is 

 in consequence unsuitable for a quantitative investigation of 

 the subject. 



About two years ago I discovered that mercury vapour 

 acts in the same manner, except that the phenomenon occurs 

 in the ultra-violet region at the absorption line 2536. I 

 first observed the resonance radiation of this vapour at room 

 temperature in an exhausted silica bulb, the pressure of the 

 mercury vapour being 0*001 mm. 



This vapour appeared to be the ideal medium for a careful 

 study of the subject in all of its aspects, except for the fact 

 that everything has to be done by photography. The work 

 has turned out even better than I had hoped, and I have 

 been able to pass gradually from the case where the scattered 

 light is radiated in all directions by the highly rarefied gas 

 to that in which it is regularly reflected from the surface of 

 the gas under high pressure. 



I have already described the selective reflexion of mono- 

 chromatic light by mercury vapour (this journal, vol. xviii. 

 p. 187, 1909), in a paper which I shall have occasion to refer 

 to later on. 



Although 1 first observed the resonance radiation of 

 mercury vapour nearly two years ago, I published nothing 

 on the subject, as no quantitative data had been obtained. 

 The observation was made in the following way. A small 

 drop of mercury was introduced into a tube of fused quart/, 



2 Z2 



