56 ABSORPTION AND EMISSION. 



rays are equally absorbed ; or, in other words, that the co- 

 efficient of transmission, a, is the same for all, and varies only 

 with the refrangibility of the ray, or the rapidity of the 

 vibrations of the molecules of the ether. It follows from 

 this, that these three effects cannot be separated from one 

 another by absorption, any more than by refraction, in any 

 ray of definite refrangibility. Hence also we draw the im- 

 portant conclusion, that the three properties of solar light 

 are not to be referred to separate agencies, but are only dif- 

 ferent effects of the same vibratory movement, varying with 

 its rapidity. 



(71) Translucent and colourless bodies are those which 

 transmit all the luminous rays in the same proportion. 

 There is, however, in nature, no body absolutely translucent. 

 Air, which is the most so, colours the light of the Sun at first 

 yellow, and afterwards red, when the thickness of the atmo- 

 sphere traversed is greatest, viz., in the horizon. And water 

 produces the same effects in a very decided manner, the light 

 which has traversed a tube filled with water becoming first 

 yellow, then orange, and finally red, as the length of the 

 tube is increased. The absorbing effects are, however, much 

 more marked beyond the limits of the visible spectrum, 

 nearly all translucent bodies stopping either the calorific or 

 the chemical rays outside these limits. The only known 

 exception to this rule is that of rock-salt, which is accordingly 

 the only known substance perfectly transparent. White quartz 

 possesses a similar property, although in an inferior degree. 

 On the other hand, alum and ice, which are both translucent, 

 powerfully absorb the calorific rays beyond the red extre- 

 mity of the spectrum. 



Black bodies absorb not only the luminous rays, but also 

 the chemical rays beyond the more refrangible end of the 

 spectrum, while they transmit the thermic rays beyond the 



