66 DESCRIPTION OF THE TITHONIC RAYS. 



coexisting in solar light, tithonic, calorific, and prosphorescent rays ; for the two for- 

 mer the phenomenon of coloration has been distinctly traced, as will appear in a sub- 

 sequent chapter; for the latter, analogy would lead us to suppose that the same thing 

 holds. 



237. In yellow light, such as can effect the decomposition of carbonic acid, a vari- 

 ety of other principles exist yellow tithonic rays, yellow calorific rays, and perhaps 

 yellow phosphorescent rays. Prismatic experiments (Ap., 790) can only indicate the 

 refrangibility of the ray which produces a given effect. Experiments by absorbent me- 

 dia are finally required to point out its exact nature. 



238. In order that the reader may possess clear views of the evidence which is to 

 be brought forward, showing that it is yellow LIGHT, and not tithonic, calorific, or phos- 

 phorescent yellow rays which superintend the formation of organic molecules out of 

 inorganic matter, it is necessary to set forth in a more prominent way the distinctive 

 characteristics which appertain to each of these principles. 



239. Of the calorific rays, or rays of heat, nothing, however, need here be said. Their 

 physical independence of light, and separate existence, were completely established by 

 MELLONI (170), who also discovered in them modifications analogous to coloration. 



'240. The existence of the tithonic rays was first ascertained during the last century. 

 They passed in the books under the name of chemical rays a name still continued by 

 many chemists. As a multitude of phenomena with which they are connected became 

 known, it has been found absolutely essential to give to them a specific name, such as 

 can be moulded conveniently for the purposes of science, and combine readily to form 

 those different words which are necessary to give names to instruments in which they 

 are involved, or to phenomena with which they are connected a name which, how- 

 ever, involves no hypothetical idea of their nature or action. 



241. The most striking fact in connexion with these rays is their constant associa- 

 tion with light. Though the two principles are separable from each other by artificial 

 processes, yet, under natural circumstances, they always occur together. In sunlight, 

 in gas-flames, from candles, and even in the moonbeams, in which no heat is found, the 

 chemical and luminous rays exist together. They undergo reflexion, refraction, polar- 

 ization, and interference, apparently under the very same laws. But while light can 

 affect the eye, and bring us into relation with all the phenomena of the material world, 

 these rays fail to affect our organs of vision they are invisible. 



242. In view of this constant association of light and the chemical rays, and to ex- 

 press the idea of their near alliance to each other, and as the wants of science imper- 

 atively called for a specific designation, the name of tithonic rays has been suggested for 

 them. It is drawn from the fable of Aurora and Tithonus. This name forms compound 

 words with facility. It does not involve us in any speculative considerations, but merely 

 points out the fact that the principle to which it is given is almost always associated with 

 light. As has been just stated, all the mechanical laws which regulate the reflexion, 

 transmission, polarization, and interference of light, obtain also for this. 



243. The mathematical theory of light is based upon the postulate of undulations 

 taking place in an ethereal medium. This theory, like the theory of universal gravi- 



