D.—DIRECTIONS FOR TAKING PHOTOGRAPHS.

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of gas will escape freely, which will be found to bo oxygen. By en-closing pieces of liydriodated paper in a tube to darken, we discover,as might have been expected, some hydrogen is set free. If tho paperis then well dried, and carefully shut up in a warm dry tube, it remainsdark; moisten the tube or the paper, and the yellowness is speedilyrestored.

Tako a photograph thus formed, and place it in a vessel of water, ina feiv days it will fade out, and bubbles of oxygen will gather aroundthe sides. If the water is examined, there will be found no trace ofeither silver or iodine ; thus it is evident the action has been confinedto the paper.

Wo see that the iodide of silver has the power of separating hydrogenfrom its combinations. I cannot regard this singular salt of silver as adefinite compound: it appears to me to combine with iodine in uncertainproportions. In the process of darkening, the liberation of hydrogen iscertain; but I have not in any one instance been enabled to detect freeiodine ; of course it must exist, either in the darkened surface, or incombination with the unaffected under layer: possibly this may be theiodide of silver, with iodine in simple mixture, which, when light actsno longer on the preparation, is liberated, combines with the hydrogenof that portion of moisturo which tho hygrometric nature of the paperis sure to furnish, and as an hydriodatc again attacks the darkened sur-face, restoring thus the iodide of silver. This is strikingly illustrativeof the fading of the photograph.

The picture is formed of iodide of silver in its light parts, and oxideof silver in its shadows. As the yellow salt darkens under the influenceof light, it parts with its iodine, which immediately attacks the darkoxide, and gradually converts it into an iodide. The modus operandiof the restoration which takes placo in the dark is not quite so apparent.It is possible that tho active agent light being quiescent, the play ofaffinities comes undisturbed into operation—that tho dark parts of thepicture absorb oxygen from the atmosphere, and restore to the lighterportions tho iodino it has before robbed them of. A series of experi-ments on the iodide of silver, in its pure state, will still more strik-ingly exhibit this very remarkable peculiarity.

Precipitate with any hydriodate, silver, from its nitrato in solution,and expose the vessel containing it, liquid and all, to sunshine, the ex-posed surfaces of tho iodide will blacken; remove tho vessel into thodark, and after a few hours, all the blackness will have disappeared.We may thus continually restore and remove the blackness at pleasure.If we wash and then well dry the precipitate, it blackens with difficulty,and if kept quite dry, it continues dark, but moisten it, and the yellowis restored after a little time. In a watch-glass, or any capsule, place alittle solution of silver, in another, some solution of any liydriodic salt,connect the two with a filament of cotton, and make up an electric cir-cuit with a piece of platina wire, expose this little arrangement to thelight, and it will be seen, in a very short time, that iodine is liberated