ON A VIEW OF THE CONSTITUTION OF A LUMINOUS GAS SUGCtESTED 

 BY LORENTZ'S THEORY OF DISPERSION 



J. J. THOMSON. 



The refractive index /x of a gas for liglit wliose vibratiou frequency 

 is p was showii bij Lorkntz to be given bj the Equation 



where e is the charge aud >// the inass of an ion ; // the frequency of a free 

 vibration, JSl the number of ions vibrating with this frequency^ }\ the 

 velocity of liglit in a vacuum^ and the summation is to be taken for ail 

 the modes of vibration of the molécule^ i. e. for ail the lines in the spec- 

 truin of the gas. Por very long waves yj is approximately zero^ t/z- = A 

 where K is the speciiic inductive capacity of the gas. If A is the wave- 

 length corresponding to the frequency «, theu vin = ?J2 tt and équa- 

 tion (1) becomes 



In the denominator of the left haud side of équation (2)_, A' -[- ;? has 

 been put equal to 3 as A for ail gazes is approximately unity, 



Now from the spectrum of the gas we can détermine the varions 

 values of A and if we assume that ail tlie molécules eau give ont the 

 vibration whose wave-length A, we can fiud Ne . e\m is equal to 10 ' if the 

 vibrating ion is negatively charged. Now let us apply this formula to 

 some gas, we shall take Hélium; each Hue in the spectrum will contri- 

 bute to the right hand side of équation (2) so that if we only take into 



