Origin of Spectra. 217 



by hydrogen. These experimenters could get no absorption 

 of light by the gas in the normal condition, but found distinct 

 absorption o£ the red and other lines when light was passed 

 through a long tube containing the gas at a pressure of a few 

 centimetres, conveying an electric current. Evidently these 

 lines are due to something which does not exist in the gas in 

 the normal state, but is produced in it by the passage of the 

 discharge. Similar results have been obtained with mercury 

 vapour. The absorption of this was first investigated by 

 Lockyer *, and later by Strutt f ; neither observer was able 

 to obtain any absorption in the luminous spectrum, but more 

 recently Wood J has shown that there is absorption in the 

 ultra-violet. That ionized mercury vapour does produce 

 absorption has been demonstrated by Ktich and Retschinsky§, 

 and also by Pniiger ||. The method consists in having two 

 mercury arc lamps placed one behind the other, and in mea- 

 suring the percentage absorption of the light from the first 

 lamp in passing through the second, in the case of certain 

 definite lines of the spectrum. The results showed that ionized 

 mercury vapour possesses the power of absorbing those rays 

 which it emits, whereas the vapour in the normal condition 

 produces no absorption. 



We thus see that ionized gases and vapours contain electronic 

 systems which are not present in them in the normal state. 

 These systems are the " molecules " of new substances formed 

 from the original gas or vapour by the passage of the electric 

 discharge. I call them "new substances'" because it is 

 probable that they possess chemical properties different from 

 those of the original substance. A simple experiment due to 

 Dr. H. Brereton Baker H shows that ionized mercury vapour 

 has at least one different chemical property from mercury 

 vapour in the ordinary atomic condition ; for if oxygen is 

 allowed to enter a mercury lamp immediately after the current 

 has been cut off, it is found that a considerable quantity of 

 mercuric oxide is formed, although the temperature is much 

 lower than that at which mercury vapour in a normal condition 

 combines with oxygen. 



The recent experiments of Sir J. J, Thomson** have shown 



* Lockyer, Roy. Soc. Proc. xxii. p. 374 (1874). 



t Strutt, Phil. Mag. vi. p. 76 (1903). 



X Wood, Astrophys. Journ. xxvi. p. 41 (1907). 



§ Kiich and Retschinsky, Ann. d. Phys. xxii. p. 852 (1907). 



|| Pfliiger, Ann. d. Phys. xxiv. 3, p. 515 (1907). 



H Baker, Nature, lxxxiv. p. 388 (1910). 



** Thomson, Phil. Mag. xxi. p. 225 (1911). 



