MEMOIR XVI.] CONDITION OF A DAGUERREOTYPE SURFACE. 229 



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 wher- 

 ever the light has touched, there it is corroded. 



These two facts, taken together, prove that in mercu- 

 rializing 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. 



From these facts we readily gather that on the pres- 

 ence of the metallic silver the sensitiveness of this prep- 

 aration mainly depends, for to the tendency which the 

 light has impressed on the elements of the iodide to sep- 

 arate is added the strong attraction of metallic silver for 

 nascent iodine. 



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 ac- 

 tion, the problem of producing engraved daguerreotypes 

 would be solved. By another process, which will be 

 described hereafter, I have succeeded in producing deep 

 etchings from daguerreotypes. 



UNIVERSITY OF NEW YORK, September, 1811. 



