A Manual Of Photography

ON THE PRODUCTION OE POSITIVE PHOTOGRAPHS. 295 Place a similar arrangement in the dark; iodine is slowly liberated. .Vo iodide of silver is formed, but around the wire a beautiful cyrstallization of metallic silver. Seal a piece of platina wire into two small glass tubes ; these, when filled, the one with hydriodate of potash in solution, and the other with a solution of the nitrate of silver, reverse into two watch- glasses, containing the same solutions; the glasses being connected with a piece of cotton. An exposure during a few hours to daylight will occasion the hydriodic solution in the tube to become quite brown with liberated iodine: a small portion of the iodide of silver will form along the cotton, and at the end dipping in the salt of silver. During the night the hydriodic liquid will become again colourless and transparent, and the dark salt along the cotton will resume its native yellow hue. From this it is evident that absolute permanence will not be given to these photographs until we succeed in removing from the paper all the iodide of silver formed. The hyposulphites dissolve iodide of silver ; therefore it might have been expected, a priori, they would have been successful on these drawings. If they are washed over witli the hyposulphite of soda, and then quickly rinsed in plenty of cold water, the drawing is improved, but no better fixed than witli cold water alone. If we persevere in using the hyposulphite, the iodide is darkened by combining with a portion of sulphur, and the lights become of a dingy yellow, which is not at all pleasant. No plan of fixing will be found more efficacious with this variety of photographic drawings, than soaking them for some hours in cold water, and then well washing them in hot water. It often happens that a picture, when taken from the camera, is less distinct than could be desired: it should not, however, be rejected on that account. All the details exist, although not visible. In many cases the soaking is sufficient to call them into sight ; if they cannot be so evoked, a wash of weak ammonia or muriatic acid seldom fails to bring them up. Care, however, must be taken not to use these preparations too strong, and the picture must be washed on the instant, to remove the acid or alkali. One very singular property of these photographs is, that when first prepared, and after the washing, they are not fixed or other- wise ; but when exposed to sunshine, they change in their dark parts from a red to a black. This peculiarity will be found by