# Page:Popular Science Monthly Volume 22.djvu/411

TAKING the case of a daguerreotype plate which has been exposed, and which we are about to develop by the action of mercury, I should like you to understand exactly what takes place in the plate when it is exposed and developed. On the surface of the plate we have a mixture of silver iodide and bromide; but, for simplicity's sake, I will suppose that it is simply silver iodide. When light acts on such a compound, the result is the liberation of iodine and the formation of a new salt, which we call silver subiodide, Ag2I2 ${\displaystyle =}$ Ag2I ${\displaystyle +}$ I. The iodine is taken up by the silver plate at the back of the sensitive film. To develop the picture, mercury-vapor is caused to condense on the subiodide, and leave the iodide intact. In the Talbotype process, the picture, which has been taken on a paper that has been washed with nitrate of silver, iodide of potassium, and nitrate of silver again, is developed by washing with gallic acid and silver nitrate. The picture begins to appear on washing after a very short exposure to the light, and becomes gradually more visible as the washing goes on. A paper process is a most fascinating process, because you can dabble about, and do exactly what you like; it is not like the gelatine plates of the present day, which you have to leave to come out mechanically. With paper, if you want to bring out a little better detail in one place,