72 YELLOW LIGHT CONTROLS THE DIGESTION OF PLANTS. 



CHAPTER VIII. 



IT IS YELLOW LIGHT WHICH CONTROLS THE PROCESS OF DIGESTION IN PLANTS. 



CONTENTS : Examination as to which of the Principles mentioned in the preceding 

 Chapter is engaged in the Decomposition of Carbonic Acid. It is not Radiant 

 Heat. ^fellon^ on the Ideal Coloration of Heat. Analogies in the Case of Light. 

 Herschel's Results. 



It is not the Tithonic Ray. Maximum of Decomposing Action for Carbonic Acid and 

 Carbonaceous Compounds, like the Retina, is in the Yellow Ray. Hence the Maxi- 

 mum of Visible Illumination coincides therewith. 



264. In the last chapter, and in the APPENDIX, CHAPS. XIII., XV., and XVIII., we 

 have given proofs of the separate existence of a number of independent imponder- 

 able principles in the solar beams, of which only one produces a specific effect upon 

 the eye, the others being wholly invisible, and known to us by the chemical or mechan- 

 ical effects they produce. 



265. When, therefore, a beam of light falls upon a prism, and is decomposed by it, 

 and the resulting colours are received upon a screen so as to give rise to a spectrum, 

 such as that represented in the frontispiece, in each portion of that spectrum these dif- 

 ferent imponderable principles are present. For example, in the region between the 

 lines A and C there are red rays of light, red rays of heat, and red tithonic rays. They 

 are mingled together there, fortuitously, through the optical action of the prism. Their 

 existence is perfectly separate and independent, and any one of them may be removed 

 by proper processes, and the others left 



266. Confining our attention now to the yellow portion of the spectrum, in which 

 region the decomposition of carbonic acid takes place, it is obvious, upon these princi- 

 ples, that there are coexisting there yellow light, yellow heat, and yellow tithonic rays. 

 It remains for us to inquire to which of these three principles the decomposition of 

 carbonic acid and the production of green matter is due. The phosphorescent rays 

 may be left out of the discussion, though these, with the other three classes, are spread 

 all over the spectrum. 



267. First let us ascertain whether radiant heat, generally, has the quality of pro- 

 ducing decomposition. To rays coming from a brightly-burning fire, I exposed some 

 vegetable leaves in water holding carbonic acid gas in solution, and, to increase the 

 effect, converged the calorific rays by a large metallic concave mirror. That no doubt 

 might remain of the incapacity of heat to produce the phenomenon, the temperature 

 of the water, under the influence of the radiant heat, was allowed to run up to 140 

 Fah., a much higher point than is ever attained under natural circumstances. But 

 neither at low temperatures, nor at these elevated ones, did any visible decomposition 



