178 POPULAR LECTURES AND ADDRESSES. 



vibration. Hence the spectrum of a highly 

 attenuated gas consists of one or more perfectly 

 sharp bright lines, with a scarcely perceptible 

 continuous gradation of prismatic colour. I IT 

 denser gas each atom is frequently in collision, 

 but still is for much more time free, in inter- 

 vals between collisions, than engaged in collision ; 

 so that not only is the atom itself thrown sensibly 

 out of tune during a sensible proportion of its 

 whole time, but the confused jangle of vibrations in 

 every variety of period during the actual collision 

 becomes more considerable in its influence. Hence 

 bright lines in the spectrum broaden out somewhat, 

 and the continuous spectrum becomes less faint. 

 In still denser gas each atom may be almost as 

 much time in collision as free, and the spectrum 

 then consists of broad nebulous bands crossing a 

 continuous spectrum of considerable brightness. 

 When the medium is so dense that each atom is 

 always in collision, that is to say never free from 

 the influence of its neighbours, the spectrum will 

 generally be continuous, and may present little or 

 no appearance of bands, or even of maxima of 



