222 CONDITION or A DAGUERREOTYPE SURFACE. [MEMO xvi. 



MEMOIR XVI. 



ON THE CHEMICAL CONDITION OF A DAGUERREOTYPE 

 SURFACE. 



From the Philosophical Magazine, September, 1841. 



CONTENTS: Mercury exists all over a daguerreotype surface. There is 

 no superposition of the parts. The shadows have metallic mercury ; 

 the lights silver amalgam. No iodine is ever evolved from the plate. 

 Action of a solution of gum and one of gelatine in tearing off the 

 films. The starch experiment. The etching of daguerreotypes. 



As many of the results to be given in the next Mem- 

 oir depend on the use of daguerreotype tablets, I shall 

 in this examine the chemical and physical condition of 

 those surfaces and offer proof of the following facts : 



1st. That metallic mercury exists all over the surface 

 of an ordinary daguerreotype in the shadows as well 

 as in the lights in the shadows as metallic mercury, 

 in the lights as silver amalgam. 



2d. That in an iodized daguerreotype as taken from 

 the mercury-bath there is no order of superposition of 

 the parts, that is to say, the iodide is neither upon nor 

 beneath the mercury, but both are, as it were, in the same 

 plane. 



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

 this preparation, through all the intervening steps, and 

 up to the point of maximum action, no iodine is evolved 

 from the plate, but that in the common daguerreotype 

 the light communicates a tendency to the atoms of the 

 iodide to yield up to the mercurial vapor their silver, 

 while the iodine retires and combines with the unaf- 



