Prof, Draper on the Chemical Action of Light. 379 



of decomposition by radiation. Thns a ray which has impinged 

 on a Daguerreotj'pe plate can no longer impress any change on 

 the surface of a second such plate, to which it may be reflected. 

 In bi-inging about a change in the ponderable medium, the ray 

 itself undergoes a change. The same is observed when a ray 

 has passed through a mixture of chlorine and hydrogen (Phil. 

 Mag., July 1844); it loses the power of acting on a second 

 portion of the same substance. So, too, one which has passed 

 through a solution of bichromate of potash can no longer change 

 a piece of paper made sensitive by being soaked in the same 

 solution. And such instances might be indefinitely multiplied. 



To show that the ray absorbed is in reality the active ray, I 

 made an examination by the prism in the case of the peroxalate 

 of ii-on. For this purpose the rays absorbed by the solution 

 were first determined, and then those bringing aboxit decompo- 

 sition were ascertained. They were found to be identical. 



In whatever manner we interfere with the absorptive quality 

 of a body, we also interfere \nth its tendency to decomposition, 

 increasing or diminishing its sensitiveness to the light. Thus 

 the i-ays which change the iodide of silver are included between 

 the fixed lines F and Q. But in the Daguerreotype plate the 

 absoiqitive power is under complete control ; and we can have 

 the iodide of almost any colour, yellow, blue, rose-red, steel-gray, 

 &c., by regTilating the exposure to the vapour of iodine, and 

 thereby changing the thickness of the film. As is well known 

 to those who are familiar with the process, the film when of a 

 yellow colour is most sensitive to the light, becoming less so 

 when its tint is blue, and very sluggish indeed when it is brought 

 to a steel-giay. A little consideration shows that all this is 

 owing to our modifying the reflecting power, and thereby the 

 absorption ; and that the yellow film is more sensitive, because 

 it absorbs the indigo rays, which are the active ones, and reflects 

 the yellow, which are without action; and that the steel-gray 

 film IS tardy to change, because its optical relations are such^ 

 that it reflects indiscriminately the incident light of all kinds, 

 and the active indigo ray is but little absorbed. 



From these and many other such facts it may be infeiTed, 

 that whenever a substance changes on exposure to light, rays of a 

 certain refrangihitity have been absorbed, and that without absorp- 

 tion no such chemical change can take place. 



But what ih it we mean by the term absorption of a ray ? If 

 light consists of a vibratory movement ])ro])agated in the fether 

 and if bodies have a certain tempcratm-e, or even become luminous 

 only because their moh'cules are (executing vibrations of a certain 

 am|)]itudc and ficcpu'ncy, what mechanical ideas are we to attach 

 to the term abborption ? 



