MKMOIR XVI.] CONDITION OF A DAGUERREOTYPE SURFACE. 223 



fected silver beneath. It follows that when such a plate 

 is withdrawn from the mercurial vapor, there is all over 

 it a uniform film of iodide of silver of the very same 

 thickness as at first, and that this has happened through 

 a direct corrosion of the silver by the iodine, while it 

 was undergoing the mercurial operation. 



I pass at once to the proofs of these several propo- 

 sitions, and, 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 it is as metallic mer- 

 cury, in the lights as silver amalgam. 



I took a plated copper three inches by four in surface, 

 and having prepared it with care, I exposed half of it to 

 the diffused light of the day, screening the other half; it 

 was then mercurialized at 170 Fahr., the iodide removed 

 by hyposulphite of soda, and washed. And now, a plate 

 on which a gold-leaf was spread was placed over it, but 

 separated, as shown in Fig. 34, in 

 the points a, b, c, by three slips of i i *i i *rr 



glass. By means of a spirit-lamp 

 the photographic plate d e was heated, and the gilded 

 plate g k kept cool by occasionally wetting it. On part- 

 ing the plates, it was perceived that faint but distinct 

 traces of whitening were visible all over the gold, as well 

 on that part which was over the whitened half of the 

 photograph as over that which was unchanged. 



But as it might happen that the mercury diffused 

 itself laterally past the imperfect obstacle Z>, I made the 

 following decisive trials : 



I iodized three silver plates, A, B, C, each three inches 

 by four in surface, conducting the processes for each in the 

 same way ; and having exposed each for two minutes to a 

 faint daylight, I laid them aside in the dark, to be pres- 

 ently used as test-plates in lieu of the gilded plate (g K). 



