and on the chemical action of the Solar Spectrum. 275 



treme red to the green ; it fades in passing through the blue 

 and increases in intensity as it leaves the indigo, until, be- 

 yond the invisible chemical rays, it is nearly as strong as it is at 

 the calorific end of the spectrum. 



Does not this protected surrounding band appear to indi- 

 cate the existence of rays of a peculiar and unknown order, 

 proceeding from the extreme edge of the sun ? 



28. By lightly rubbing a daguerreotype picture of the 

 prismatic rays, it is obliterated, except over the space of the 

 yellow and red portion. This effect corresponds with my 

 experiments on media of these colours (11. 12. 13). 



Until we have more experience than we now have of the 

 effects of the solar rays individually and collectively, we can 

 offer no satisfactory explanation of the process in action, on 

 a daguerreotype plate, by which the subtle painter Light im- 

 presses such delicate designs. 



The existence of two iodides of silver, is, I think, certain. 

 In my photometric experiments I have always observed the 

 formation of an iodide which speedily darkens, and of another 

 portion which is unalterable by light*. 



The sensitive film on the silver plate appears to be the 

 former of these iodides. Throuohout the range of the che- 

 mical spectrum, particularly so called^ the iodide is I ima- 

 gine converted into an oxide of silver; that a partial oxidation 

 takes place numerous experiments have rendered certain ; 

 whilst the influence of the ra^^s of least refrangibility is to 

 form the unchangeable iodide of silver. Experiments, how- 

 ever, are wanting to prove this satisfactorily. 



An attentive consideration of the facts I have enumerated, 

 will, I think, satisfy all, that we can no longer with propriety 

 attach the name of chemical to the most refrangible rays only. 

 Every ray has its particular chemical office, either of compo- 

 sition or of decomposition ; and although Seebeck has attri- 

 buted the acquirement of a rose hue by chloride of silver 

 when put into the red ray, to the heating power of that por- 

 tion of the spectrum, it is now proved to be dependent upon 

 some other influence, for where it has been shown the most 

 calorific rays exist this salt undergoes no change. 



Devonport, February 29, 1840. 



* [See Mr. Talbot's account of the processes employed in Photogenic 

 Drawing, Lond. and Edinb. Phil. Mag., vol. xiv. p. 210 (2).— Edit.] 



T2 



