A Manual Of Photography

PRACTICE OF PHOTOGRAPHY. 294 gather around the sides. If the water is examined there will he found no trace of either silver or iodine. Thus it is evident the action has been confined to the paper. We see that the iodide of silver has the power of separating hydrogen from its combinations. I cannot regard tins singular salt of silver as a definite compound: it appears to me to com- bine with iodine in uncertain proportions. In the process of darkening, the liberation of hydrogen is certain ; but I have not in any one instance been enabled to detect free iodine : of course it must exist, either in the darkened surface, or in combination with the unaffected under layer : possibly this may be the iodide of silver, with iodine in simple mixture, which, when light acts no fonder on the preparation, is liberated, combines with the hydrogen of that portion of moisture which the hygrometric nature of the paper is sure to furnish, and as an hydriodate again attacks the darkened surface, restoring thus the iodide of silver. This is strikingly illustrative of the fading of the photograph. The picture is formed of iodide of silver in its light parts, and oxide of silver in its shadows. As the yellow salt darkens under the influence of light, it parts with its iodine, which immediately attacks the dark oxide, and gradually converts it into an iodide. The modus operandi of the restoration which takes place in i dark is not quite so apparent. It is possible that Aheactive agent being quiescent, the play of affinities comes undisturbed into operation ; that the dark parts of the picture absorb oxygen from the atmosphere, and restore to the lighter portions the iodine it has before robbed them of. A series of experiments on the iodide of silver in its pure state will still more strikingly exhibit this very remarkable peculiarity.. Precipitate with any hydriodate, silver, from its nitrate in solution, and expose the vessel containing it, liquid and all, to sunshine ; the exposed surfaces of the iodide will blacken : remove the vessel into the dark, and, after a few hours, all the blackness will have disappeared. We may thus continually restore and remove the blackness at pleasure. If we vash and then well dry the precipitate, it blackens with difficulty, and if kept quite dry it continues dark ; but moisten it, and the yellow is restored after a little time. In a watch-glass, or any capsule, place a little solution of silver ; in another, some solution of any by driodic salt ; connect the two with a filament of cotton, and make up an electric circuit with a piece of platina wire: expose tins little arrangement to the light, and it will be seen, in a very ^short time, that iodine is liberated in one vesse, and the yellow iodide of silver formed in the other, which blackens as quickly as it is formed.