Photography. The Daguerreotype process consists in exposing a silvered copper plate to vapour of iodine, whereby a film of iodide of silver is formed on the surface; then placing it in the camera, so that the light may act upon it in certain parts, and reduce the iodide of silver to the metallic state ; and afterwards exposing it to vapour of mercury. The mercury adheres in minute globules to those parts on which the silver has been reduced. Lastly, the plate is washed with a solution of hyposulphite of sodium, to remove the iodide of silver, not decomposed by the light. In the Calotype process, the picture is taken upon paper. The paper is prepared by immersion first in a solution of nitrate of silver, then, after drying, in a weak solution of iodide of potassium ; after which it is soaked in water for a quarter of an hour, and then dried, first with blotting paper, and afterwards before the fire. The paper thus prepared is called iodized paper; it is not sensitive to light. When required for use, it is washed with a liquid prepared by dissolving 100 grains of crystallized nitrate of silver in two ounces of distilled water, adding to the solution its own volume of strong acetic acid, and mixing the solution thus formed with from one to twenty volumes of a saturated solution of crystallized gallic acid in cold distilled water. These operations must be performed in the dark. The paper thus prepared is highly sensitive, and is fit far taking pictures in the camera. The picture thus obtained is a negative one, being darkest in the parts where the strongest light has acted on it. When first produced it is generally invisible, but may be brought out by again washing the paper with a mixture of one part of the silver solution with three parts of the saturated solution of gallic acid. It is then to be fixed by washing it with water, lightly drying between blotting-paper, and washing with a solution of bromide of potassium. From the negative picture thus produced, positive copies may be obtained by placing it with its face against a piece of ordinary photographic paper (prepared by washing good writing-paper with a solution of common salt, then wiping it dry, and washing it with nitrate of silver), pressing it into close contact by a board above and below, and exposing it for a short time to sunshine. Finally, it is to be fixed by immersion in a hot solution of hyposulphite of sodium. Such are the general principles of these processes.