A Manual Of Photography

ON THE PRODUCTION OE POSITIVE PHOTOGRAPHS. 293 tional time required—about one-sixth longer—is, however, in most eases, of little consequence. The picture being formed by the influence of light, it is re- quired, to render it unchangeable by any further action of the luminous fluid, not only that the hydriodic salt be entirely re- moved from the paper, but that the iodide of silver which is formed be also dissolved out of the drawing. By well washing the drawing in warm water, the hydriodate is removed, and the pictures thus prepared have been stated to be permanent; and if they are kept in a portfolio, and only oc- casionally exposed, they are really so : for I shall show presently, that they have the property of being restored in the dark to the state in which they were prior to the destructive action of light. A drawing which I executed in June, 1839, which has often been exposed for days successively to the action of sunshine, and has altogether been very little cared for, continues to this date (October 1852), as perfect as at first. These photographs will not,however, bear long-continued exposure without injury—about three months in summer, or six weeks in winter, being sufficient to destroy them. As this gradual decay involves some very curious and interesting chemical phenomena, I shall make no excuse for dwelling on the subject a little. The drawing fades first in the dark parts, and as they are per- ceived to lose their définedness, the lights are seen to darken, until at last the contrast between light and shadow is very weak. If a dark paper is washed with an hydriodate and exposed to sunshine, it is first bleached, becoming yellow ; then the light again darkens it. If, when quite dry, it is carefully kept from the light, it will be found in a few days to be again restored to its original yellow colour, which may be again darkened by ex- posure, and the yellow colour be again restored in the dark. The sensitiveness to the influence of light diminishes after each exposure, but I have not been enabled to arrive at the point at which this entirely ceases. If a dark paper, bleached by an hydri- odate and light, be again darkened, and then placed in a bottle of water, the yellow is much more quickly restored, and bubbles of gas will escape freely,which will be found to be oxygen. By en- closing pieces of hydriodated paper in a tube to darken,we discover, as might have been expected, some hydrogen is set free. If the paper’is then well dried, and carefully shut up in a warm dry tube, it remains dark ; moisten the tube or the paper, and the yellowness is speedily restored. Take a photograph thus formed, and place it in a vessel of water : in a few days it will fade out, and bubbles of oxygen will