84 Colours and their Relations. [January, 



the other with the yellow ray, when exposed to light. On 

 the withdrawal of the heat the salts again imbibe moisture 

 from the air, and their molecular vibrations, under the 

 stimulus of the incident light, have no longer these definite 

 rates. 



The temporary effects of heat on nitrous acid gas may be 

 classed under this same head. This gas, even at ordinary 

 temperatures, exerts on the incident light a strong absorp- 

 tive action, in virtue of which numerous dark lines are de- 

 veloped in the spectrum ; but raising the temperature of the 

 gas so increases this absorptive power as ultimately to con- 

 vert the whole of the incident light into dark radiant heat — 

 the gas becoming quite opaque. A fall of temperature 

 allows it to resume its transparency. In this case, the heat 

 tends to cause the molecules of the gas to take up the vibra- 

 tory energy of the incident light, and in virtue of this energy, 

 united to that of the applied heat, to perform vibrations of 

 so great an amplitude and so slow a rate that they do not in 

 their turn communicate to the ether back-waves of a 

 rapidity sufficient to affect the optic nerve. These back- 

 waves accordingly assume the form of dark radiant heat. 

 When the temperature is lowered again, the molecules per- 

 form vibrations of smaller amplitude and greater rapidity, 

 which in their turn propagate through the ether back- 

 waves of such rates as to develop colours belonging to the 

 red end of the spectrum. 



Part II. 



Intrinsic colours having been considered in the previous 

 part, the present shall be devoted to those called adventi- 

 tious. Of such, the most simple sort are those produced 

 by dispersion, or the separation of wave from wave of the 

 incident light. In this case, the medium by which the 

 separation is effected may itself be destitute of colour. 

 All that is requisite is that it should be shaped into the form 

 of a wedge or prism, so that the incident light shall pass 

 through varying thicknesses. The diverse waves, of which 

 the incident light consists, are thus subjected to the re- 

 tarding action of the medium for different periods of time; 

 and they are accordingly turned aside out of their direct 

 course, or refracted in unequal degrees. Those most easily 

 retarded become thus separated from those least easily 

 retarded, and the waves of different lengths reach the eye 

 in this separate condition, producing each its distinct im- 

 pression of colour. All refracted spectra are of this cha- 

 racter. The colours do not belong intrinsically to any 



