Collodion is an ethereal solution of a substance almost identical with Gun-Cotton. On evaporation it leaves a transparent layer, resembling gold-beater's skin, which adheres to the glass with some tenacity. M. Le Grey of Paris originally suggested that this substance might possibly be rendered available in Photography, but our own countryman, Mr. Archer, was the first to carry out the idea practically. In a communication to 'The Chemist' in the autumn of 1851, this gentleman gave a description of the Collodion process much as it now stands; at the same time proposing the substitution of _Pyro_-gallic acid for the Gallic acid previously employed in developing the image. At that period no idea could have been entertained of the stimulus which this discovery would render to the progress of the Art; but experience has now abundantly demonstrated, that, as far as all qualities most desirable in a Photographic process are concerned, none at present known can excel, or perhaps equal, the Collodion process. CHAPTER II. THE SALTS OF SILVER EMPLOYED IN PHOTOGRAPHY. By the term Salt of Silver we understand that the compound in question contains Silver, but not in its elementary form; the metal is in fact in a state of chemical union with other elements which disguise its physical properties, so that the Salt possesses none of the external characters of the Silver from which it was produced. Silver is not the only metal which forms Salts; there are Salts of Lead, Copper, Iron, etc. Sugar of Lead is a familiar instance of a Salt of Lead. It is a white crystalline body, easily soluble in water, the solution possessing an intensely sweet taste; chemical tests prove that it contains Lead, although no suspicion of such a fact could be entertained from a consideration of its general properties. Common Salt, or Chloride of Sodium, which is the type of the salts generally, is constituted in a similar manner; that is to say, it contains a metallic substance, the characters of which are masked, and lie hid in the compound. The contents of this Chapter may be arranged in three Sections: the first describing the Chemistry of the Salts of Silver; the second, the action of Light upon them; the third, the preparation of a sensitive surface, with experiments illustrating the formation of the Photographic image. SECTION I. Chemistry of the Salts of Silver. The principal Salts of Silver employed in the Photographic processes are four in number, viz. Nitrate of Silver, Chloride of Silver, Iodide of Silver, and Bromide of Silver. In addition to these, it will be necessary to describe the Oxides of Silver. THE PREPARATION AND PROPERTIES OF THE NITRATE OF SILVER. Nitrate of Silver is prepared by dissolving metallic Silver in Nitric Acid. Nitric Acid is a powerfully acid and corrosive substance, containing two elementary bodies united in definite proportions. These are Nitrogen and Oxygen; the latter being present in greatest quantity.