228 CONDITION OF A DAGUERREOTYPE SURFACE. [MEMOIR XVI. 



the film of iodide and leave the amalgam, can only be co- 

 ordinated with that by gum-water, in which the amalgam 

 is removed and the iodide left, by supposing that there 

 is not anything like a direct superposition in the case, 

 and that the particles of amalgam and iodide lie, as it 

 were, side by side. 



3d. That when a ray of light falls upon the surface 

 of this, etc. 



There is no difficulty in proving this directly, and the 

 indirect evidence is copious. If we lay a piece of paper 

 imbued with starch on an iodized plate, and expose it 

 to the sun, although the plate presently assumes a dark 

 olive-green color, the starch remains uncolored. 



This dark substance is probably a subiodide of silver; 

 the iodine therefore which has been disengaged from it, 

 not having been set free, must have necessarily united 

 with the adjacent metallic silver this, for very obvious 

 reasons, there is no difficulty in admitting. 



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 vapor, 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 me- 

 tallic 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: 



1st. Dry some mucilage of gum-arabic on a daguerre- 

 otype 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 sil- 

 ver, of the very same thickness as at first, as is proved 

 by its color, is left. 



2d. Dry upon the same plate a solution of Russian 



