﻿THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



j 

 . 



MA Y 1912. 



LXVIL Selective Reflexion, Scattering and Absorption by 

 Resonating Gas Molecules. By R. W. Wood, Professor 

 of Experimental Physics, Johns Hopkins University, 'and 

 Adams Research Fellow of Columbia University *. 



[Plates X.-XII.] 



IN the present paper I shall deal with phenomena which, 

 up to the present time, have never been made the 

 subject of experimental investigation ; and though elaborate 

 mathematical treatments have been given by Planck, Lord 

 Rayleigh, Schuster, Lamb, and others, no experimental data 

 have ever been obtained. Through the discovery that the 

 vapour of mercury, at room temperature, emits a brilliant 

 resonance radiation when stimulated by monochromatic ultra- 

 violet light of wave-length corresponding exactly to that of: 

 its so-called absorption line (X = 2536), I have been enabled 

 at last to obtain quantitative data on the subject. 



It has been found that when the vapour is in a high 

 vacuum there is no true absorption, the energy diverted 

 from the primary beam of light being wholly scattered, as 

 imagined by Planck in his treatment of the theory of ab- 

 sorption. The presence of a small quantity of air or other 

 gas has been found to introduce the factor of true absorption, 

 or the conversion of the energy of the light-waves into lit at, 

 and the ratio of the amount of energy scattered to that- 

 absorbed as a function of the pressure of the gas with which 



* Communicated by the Author. 



Phil. May. S. G. Vol. 23. No. 137, May 1912. 2 X 



