THE CHEMICAL RAYS ARE ABSORBED. 149 



589. Now, therefore, when a photogenic impression existing on the surface of a 

 plate in an invisible state is brought out by the action of mercury vapour, we easily 

 understand how this is effected. No iodine is ever evolved. But each atom of iodide 

 of silver that has been acted on by the light yields to the attraction of the mercury 

 its atom of silver, and the iodine thus set free unites with the metallic silver particles 

 around it, reproducing the same yellow iodide by a direct corrosion of the plate : the 

 proofs that we have of this are two in number. 



590. 1st. Dry some mucilage of gum-arabic on a Daguerreotype just brought from 

 the mercury-bath ; when it has split up, we perceive that the white amalgam of silver 

 is removed, and a uniform coat of yellow iodide of silver, of the very same thickness 

 as at first, as is proved by its colour, is left. 



591. 2dly. Dry upon the same plate a solution of Russian isinglass, and, when it 

 has split up, it will be seen that it uniformly rends off with it the yellow iodide, leaving 

 the metallic plate with an exquisite polish ; and wherever the light has touched, there 

 it is corroded. 



592. These two facts, taken together, prove that in mercurializing a plate no iodine 

 is evolved, but that a new film of iodide of the same thickness is formed, at the 

 expense of the metallic surface. 



593. From these facts we readily gather that on the presence of the metallic silver 

 the sensitiveness of this preparation mainly depends, for to the tendency which the 

 light has impressed on the elements of the iodide to separate is added the strong 

 attraction of metallic silver for nascent iodine. 



594. This corrosion or biting in of the silver plates, by the conjoint action of the 

 mercury and iodine, gives rise to etchings that have an inexpressible charm. Could 

 any plan be hit upon of forcing the iodine to continue its action, the problem of pro- 

 ducing engraved Daguerreotypes would be solved. By another process, which will 

 be described hereafter, I have succeeded in producing deep etchings from Daguerreo- 

 types. 



595. I now commence with the proofs of the leading prooositions set out with in 

 this communication. 



And, 1st. That the chemical action produced by the rays of light depends upon the 

 ABSORPTION of those rays by sensitive bodies, just as an increase of temperature is pro- 

 duced by the absorption of those of heat. 



596. Without embarrassing myself here with any considerations of the tints of thin 

 plates, or the colours of natural objects, I shall use the term absorption as expressive of 

 a loss of radiant matter, whether that loss arises from a direct union of the luminous 

 molecules with ponderable matter, or is rather a disappearance of effect, caused by the 

 interference of systems of undulations. 



597. I iodized a plate to a golden-yellow colour, and exposed it to the diffused 

 light of day, setting it in such a position that it reflected specularly the light falling 

 upon it through the window to the objective of a camera-obscura, which formed an 

 'mage of it upon a second sensitive plate. The beams falling upon the sensitive plate 

 of course exerted their usual influence upon the iodide, which, after the lapse of a short 



