170 POPULAR LECTURES AND ADDRESSES. 



it to be accurate to an astonishing degree of 

 minuteness. 



(3) The fact that the yellow light given out 

 when salt is thrown on burning spirit consists 

 almost solely of the two nearly identical qualities 

 which constitute that double bright line. 



(4) Observations made by Stokes himself, which 

 showed the bright line D to be absent in 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 (L 1 lusfitiit, 

 Feb. 7, 1849) that the voltaic arc between 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, theoretical and practical, which 

 Stokes taught me, and which I gave regularly 

 afterwards in my public lectures in the University 

 of Glasgow, were : 



(i) That the double line D, whether bright or 

 dark, is due to vapour of sodium. 



