208 THE PROCESS OF DAGUERREOTYPE. [MEMOIR XIV. 



unchanged surfaces radiate heat unequally. There may 

 be strong doubts as to the correctness of this, but is not 

 the daguerreotype due to the same cause, whatever it 

 may be ? 



We must separate carefully the chemical changes 

 which iodide of silver undergoes in the sunbeam from 

 the mechanical changes which happen to the sensitive 

 film : iodide of silver turns black in the solar ray the 

 whole success of the daguerreotype artist depends on 

 his checking the process before that change shall have 

 supervened. 



The coating of iodine is not immediately necessary to 

 the production of images by the mercurial vapor. The 

 condition seems to be traceable to the metallic surface. 

 If you take a daguerreotype, clean off the mercury, polish 

 the plate thoroughly with rottenstone, wash it with nitric 

 acid, and bring it to a brilliant surface, yet if it has not 

 been^exposed to heat, the original picture will reappear 

 on exposure to the mercurial vapor. Is not this a result 

 of the same kind as those just referred to ? 



As a polishing material for the daguerreotype plate, 

 common rottenstoue and oil answer very well. The 

 plate having been planished by the workman, is to be 

 rubbed down to a good surface, and as high a polish 

 given to it as possible; it is to be heated and washed 

 with nitric acid, as indicated in the French account, and 

 finished by being rubbed with whiting (creta prcepa- 

 rata) in the state of a very fine powder, going over it for 

 the last time with a piece of clean dry cotton ; this 

 gives an intensely black lustre, that cannot be obtained 

 by rottenstone alone, and thoroughly removes any film 

 which nitric acid may have left. 



To coat w r ith iodine, I make use of a box about two 

 inches deep, in the bottom of which that substance in 

 coarse flakes is deposited ; no cloth intervenes, but the 



