84 THEORY OF IDEAL COLORATION. 



CHAPTER X. 



THEORY OF IDEAL COLORATION. 



CONTENTS : Former Observations on Colours in the Chemical Rays. Nomenclature 

 derived from it. Case of the Chrysotype. Case of Bichromate of Potash. Laws 

 deduced. Control of Optical Forces over Chemical Effects. Application to Spectrum 

 ~Stains. Herschel's Law for Light.- Explanation of Variable Effects in Films of 

 different Thickness. Mode of Action of the Tithonic Rays. 



326. IN the year 1837, while the study of the chemical agencies of light was yet in 

 its infancy, from phenomena connected with the decomposition of carbonic acid, the 

 synthesis of chlorine and hydrogen, and the decomposition of chloride of silver, I came 

 to the conclusion that there were among the chemical rays intrinsic differences, of the 

 same order as the differences in colour among the rays of light, and that for those rays 

 the doctrine of invisible coloration would have to be admitted (Ap., CH. X.). 



327. More recently, M. MELLONI, to whom science is indebted for originating and de- 

 veloping this beautiful thought, in the case of rays of heat, has independently come to 

 the same conclusion. So intimately is this idea bound up with the explanation of the 

 phenomena, that, in the case of radiant heat, that eminent philosopher proposes to use 

 it as the foundation for the whole science, and for its nomenclature. {Taylor's Sc. Me- 

 moirs, vol. iii., p. 12.) 



328. In what follows, I shall not attempt to introduce the doctrine in an analytical 

 way, or to trace the arguments and experiments which lead to its conclusions ; but, assu- 

 ming it at once as true, show with what facility and ease it may be brought to explain 

 a number of remarkable phenomena. Calling to mind the facts and views which have 

 been given in the preceding chapter, and more especially to the general fact, that when- 

 ever a ray produces a chemical effect it undergoes a change, the sensitive or changea- 

 ble medium absorbing some one or more of its constituents, it remains now, in addition 

 to the considerations there given respecting the characters of the rays, to introduce an- 

 other element the element of variable refrangibility. 



329. Asserting, therefore, that for the chemical rays there exist peculiarities analo- 

 gous to the different colours of light peculiarities which, by reason of the invisibility 

 of those rays, are known to us only by certain chemical phenomena it remains, in 

 bringing forward this doctrine with clearness and precision, to adopt some provisional 

 nomenclature, and, for want of a better, one founded on those terms heretofore intro- 

 duced (242) will serve our present purposes. 



330. In developing the doctrine of ideal coloration applied to the tithonic rays, I 

 shall therefore resort to what appears to be the simplest rule, and designate rays of dif- 

 ferent orders of refrangibility by the same nomenclature which has been used for light : 

 thus we shall have 



