52 THE CHEMISTRY OF THE SUN. [CHAP. 



" 3. The fact that the yellow light given out when salt is thrown 

 into burning spirits consists almost solely of the two nearly 

 dentical qualities which constitute that double bright line. 



" 4. Observations made by Stokes himself which showed the 

 bright line D to be absent from a candle flame when the wick was 

 snuffed clean, so as not to project into the luminous envelope, and 

 from an alcohol flame when the spirit was burned in a watch-glass ; 

 and, 



" 5. Foucault's admirable discovery, already referred to, that the 

 voltaic arc between the charcoal points is a medium which emits 

 the rays D on its own account, and at the same time absorbs them 

 when they come from another quarter." 



The conclusions at which Stokes arrived were : 



" 1. That the double line D, whether bright or dark, is due to the 

 vapour of sodium. 



"2. That the ultimate atom of sodium is susceptible of regular 

 elastic vibrations, like those of a tuning-fork or of stringed musical 

 instruments ; that, like an instrument with two strings tuned to ap- 

 proximate unison, or an approximately circular elastic disc, it has two 

 fundamental notes or vibrations of approximately equal pitch ; and 

 that the periods of these vibrations are precisely the periods of the 

 two slightly different yellow lights constituting the double bright 

 line D. 



"3. That when vapour of sodium is at a high enough temperature 

 to become itself a source of light, each atom executes these two 

 fundamental vibrations simultaneously, and that therefore the light 

 proceeding from it is of the two qualities constituting the double 

 bright line D. 



" 4. That when vapour of sodium is present in space across which 

 light from another source is propagated, its atoms, according to a 

 well-known principle of dynamics, are set to vibrate in either or 

 both of these fundamental modes, if some of the incident light is 

 of one or other of their periods, or some of one and some of the 

 other : so that the energy of the waves of these particular qualities 

 of light is converted into thermal vibrations of the medium and 

 dispersed in all directions, while light of all other qualities, 

 even though very nearly agreeing with them, is transmitted with 

 comparatively no loss. 



