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10 INTRODUCTORY. when poured on to the surface of a clean glass plate leaves, by the evaporation of the solvents, a beautifully uniform and highly transparent film, which adheres firmly io the glass. In order to prepare the collodion to form a photo graphically sensitive surface, some soluble iodide or bromide salts, or a mixture of the two are dissolved in it, and the plate coated with this “ salted ” collodion is sensitized by being immersed, before the solvents have fully evaporated, in a bath of nitrate of silver. The silver nitrate reacts with the bromide or iodide in the film, and gives a layer of silver bromide or iodide on the surface of the collodion. The plate has to be exposed in a moist state. If the plate were allowed to dry after being taken out of the bath, the silver nitrate upon the surface becoming concentrated by evaporation would crystallize, and in so doing would disintegrate or eat away the iodide of silver, producing transparent spots giving a freckled appearance to the film, which would be fatal to its photo graphic efficiency. The inconvenience of having to use the collodion film in its wet state led to many attempts being made in the early days of photography, to prepare the film for use in a dry state. To obtain this condition, the excess of silver nitrate on the surface was removed by washing, but as this practically destroyed the sensitiveness of the plate, it was necessary to coat the surface of the film with some substance which will restore the sensitiveness- Such substances are technically known as “ preservatives,” because they preserve the sensitive surface from immediate contact with the air, very much in the same way as a varnish. This coating maintains the collodion in a porous condition, so that it may be acted upon by developers, which would otherwise fail to penetrate it. The substances chosen for these preservatives were somewhat curious, consisting of tea, coffee, sugar, honey, albumen, beer, dextrine, gallic acid, tannin, etc. They, however, did something more than act as a preservative