FILM OF IODIDE TORN OFF MECHANICALLY. 147 



such an image on the mirror of my heliostat ; it could be revived by breathing on the 

 metal many weeks afterward, nor did it finally disappear until the end of several months. 



577. I do not at present know what is the reason of this result, but the analogy 

 between it and the arrangement of mercurial globules, which cover the surface of a 

 Daguerreotype, is too striking to be overlooked. It proves to us that surfaces may as- 

 sume such a condition as to affect the deposition of vapours upon them, so as to 

 give rise to the reproduction of appearances of external forms. I gave, therefore, 

 particular attention to this point, but eventually found that silver exists in an ordinary- 

 Daguerreotype, in connexion with the mercury, all over the plate, in a less proportion 

 in the shadows, and in a greater proportion in the lights. This result was, however, 

 only obtained after the following fact was discovered : that the mucilage of gum-arabic, 

 when slowly dried in a thin layer on the surface of a Daguerreotype, splits up in 

 shivers, bringing along with it the white portions of the picture, and leaving the plate 

 clean. 



578. Having, therefore, prepared three plates, D, E, F, exactly as before (573), I 

 poured on them a solution of gum, drained them so as to leave only a small quantity, 

 and let them dry slowly over the sand-bath. The gum separated readily, and lay in 

 chips on the surface of each plate ; it was easily removed to three sheets of paper, 

 by tapping with the finger on the back of the plate. Each was then treated alike, as 

 follows : 



579. The gummy matter was incinerated on a platinum leaf, and the remaining 

 ashes transferred to a test tube, half an inch in diameter. One drop of nitric acid 

 and one drop of water were added ; it was boiled over a small flame, and diluted 

 with a little water. Dilute muriatic acid was now added, and the chloride of silver 

 immediately fell. In repeating this, it is necessary to attend to the state of dilution of 

 the acid, for if too strong, it wholly dissolves the minute quantity of chloride of silver 

 generated. 



580. As, from the minuteness of that quantity, it was impossible to obtain a direct 

 quantitative analysis, I adopted the foregoing method, and added the dilute acid to all 

 three tubes at the same time. In D there was a faint opalescence, in E and F a cloud ; 

 but I could not always determine whether the deposite of E or F was most copious, 

 sometimes the one and sometimes the other appearing to have a slight advantage. 



581. I conclude, therefore, that while the whole surface of the plate is coated with 

 mercury, it exists as silver amalgam chiefly in the lights, and as uncombined mercury 

 chiefly in the shadows, and in a mixed proportion in the demitints ; and that when a 

 plate is solarized, both free mercury and amalgam are present. 



582. Such is the state of surface in a Daguerreotype recently formed. In the course 

 of time, however, a great portion of the mercury that is in the shadows, and also free 

 in the lights, evaporates away. When the picture has thus changed, the shadows are 

 metallic silver, and the lights silver amalgam. 



583. 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. 



