﻿and Absorption by Resonating (Jus Molecules. 701 



intensity of the secondary radiation by an amount varying 

 from 25 to 30 per cent. This appears to be very strong- 

 evidence of the persistence of the luminosity of the molecule 

 after it leaves the region traversed by the exciting beam. 

 Direct evidence of the phenomenon is much to be desired, 

 but I have been unable to think of any method, for we 

 cannot make a screen which will transmit the moving 

 molecules but cut off the light. 



It is evident, however, that the primary cause of the 

 destruction of the intensity of the secondary radiation by 

 the presence of air at three or four millimetres pressure is 

 to be sought elsewhere, as fully 75 per cent, of the secondary 

 radiation results from the light of the primary. I shall now 

 show that the effect of the air in reducing the intensity of 

 the secondary radiation results from the introduction of 

 the factor of true absorption as contrasted with molecular 

 scattering. 



Molecular Scattering and True Absorption. 

 Ratio of the two quantities. 



It is easy to see that, if true absorption occurs as well as 

 scattering, the intensity of the secondary radiation will be 

 greatly diminished in intensity. When the vapour is in a 

 vacuum of less than *01 mm. it is probable that the energy 

 diverted from the primary exciting beam is all scattered, and 

 no true absorption occurs. We should of course find what 

 appeared to be an absorption line in the spectrum of the 

 transmitted light, and yet the molecules would not be 

 absorbing energy but merely diverting it from the primary 

 beam and sending it out in all directions. The molecules 

 lying in the path of the beam will glow with a certain 

 intensity, while those which lie outside of the path of the 

 beam will be illuminated by the radiating molecules which 

 are directly excited, and will in consequence emit a light of 

 a lesser intensity. Suppose now that by the introduction 

 of air at a pressure of 5 mm. the intensity of the light 

 emitted by the directly excited molecules is reduced to 1/3 

 of its original value. By means of a threefold increase in 

 the intensity of the exciting light we can raise this intensity 

 to its original value, so that the same amount of light is 

 available for the excitation of the secondary radiation as 

 before. The intensity of the secondary radiation excited 

 under these circumstances will, however, be only one-third 

 of its former value, since two-thirds of the energy received 

 from the directly excited molecules is transformed into boat 

 by the true absorption which has been introduced by the 



