and the Absorption of the Tithonic Rays. 477 



fall in the yellow space, and the yellow, for that reason, be- 

 come the active ray in decomposing carbonic acid, and giving 

 a green colour to leaves? Is it for this cause, also, that re- 

 ceived into the eye, the yellow ray impresses us with the great- 

 est illuminating power? It would be a beautiful result to co- 

 ordinate phaenomena apparently so widely apart as the form- 

 ation of chlorophyll in a leaf, and the regulated destruction of 

 the retina in the chamber of the human eye in producing the 

 phaenomena of vision. In nature there are many results which 

 are apparently equally distinct, and which the progress of 

 knowledge has shown are intimately allied. That to our 

 organs of vision yellow light is the most brilliant, arises from 

 the incidental circumstance, that it is a carbonaceous com- 

 pound of which the changing nervous expansion is constructed. 

 Had it been possible for nature to have formed a retina in 

 which a salt of silver formed the basis, the maximum of bril- 

 liancy of light would have shifted, and the blues would have 

 been among the brightest rays. Is it in the optical peculiari- 

 ties of the carbon atom that all our ideas of harmony among 

 colours and beauty of external objects have arisen ? 



Experimental science will probably before long trace a close 

 connection between the physical properties of atoms and the 

 physical properties of rays. It will show that molecules of 

 a given weight can be moved most easily by aethereal waves 

 of a given length, as a stretched string is thrown into vibra- 

 tion by atmospheric undulations of proper dimensions; that 

 the transverse vibrations of the aethereal particles can agitate 

 in a corresponding way ponderable atoms of a proper magni- 

 tude and constitution. We shall then have no difficulty in 

 understanding how it was that among metallic substances, 

 those first detected to be changed by light, such as silver, gold, 

 mercury, lead, have all high atomic weights ; and why such as 

 sodium and potassium, the atomic weights of which are low, 

 appear to be less changeable. 



In this memoir the following facts have been brought for- 

 ward : — The photographic and tithonographic peculiarities of 

 the interference spectrum, and the propriety of using wave- 

 lengths and colour designations for the purposes of actino-che- 

 mistry. We have obtained a rough estimate of the mecha- 

 nical force of the tithonic rays, and have shown that a ray 

 cannot produce chemical effects without itself becoming deti- 

 thonized. From the decomposition of the chrysotype prepa- 

 ration and the bichromate of potash, we have deduced the 

 general laws of tithonic absorption, and seen how these may 

 be extended to thermic and photic absorption. From the 



