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INTRODUCTORY. M OST photographers, in a general way, know how the wet collodion plate is prepared and used, although the process has now so largely gone out of use for ordinary photography, and in dealing with collodion emulsion, it will assist towards a better under standing of the subject if we recall some of the features of the old process, and trace the steps which led up to the introduction of emulsion processes. The employment of collodion as a substratum for the silver haloid was first suggested by Gustav le Gray, in 1850, and was made known in a practicable form by Scott Archer in 1851. Collodion (so named from the Greek word Koaw, to stick) is prepared by dissolving pyroxyline in a mixture of ether and alcohol. Pyroxyline is cotton or linen which has been altered in composition by treatment with a mixture of nitric and sulphuric acids. The action of these acids is such that the cotton becomes soluble in various liquids, such as a mixture of ether and alcohol. Cotton nitrated in this way is often inaccurately called gun-cotton, but, as a matter of fact, the nitration is stopped short of the production of gun-cotton. Pyroxyline is more soluble than the latter, and is also not so inflam mable nor so explosive. The collodion formed by dissolving pyroxyline in ether and alcohol is a transparent, glutinous liquid, which