The sensibility of Iodide of Silver upon Collodion is greatly superior to that of the same salt employed in conjunction with any other vehicle at present known. Hence the Collodio-Iodide film will supersede the paper and Albumen processes in all cases where objects liable to move are to be copied. The causes of this superior sensitiveness, as far as ascertained, may be referred to the state of _loose coagulation_ of a Collodion film and other particulars presently to be noticed. It must however be allowed that there are yet some points affecting the sensitiveness of Iodide of Silver, both mechanical and chemical, of the exact nature of which we are ignorant. The present Chapter may be divided into four Sections:--the nature of Collodion; the chemistry of the Nitrate Bath; the causes affecting the formation and development of the Image upon Collodion; the various irregularities in the development of the Image. SECTION I. _Collodion._ Collodion (so named from the Greek word κολλἁω, to _stick_) is a glutinous, transparent fluid, procured, as generally said, by dissolving Gun-Cotton in Ether. It was originally used for surgical purposes only, being smeared over wounds and raw surfaces, to preserve them from contact with the air by the tough film which it leaves on evaporation. Photographers employ it to support a delicate film of Iodide of Silver upon the surface of a smooth glass plate. Two elements enter into the composition of Collodion: first, the Gun-Cotton; second, the fluids used to dissolve it. Each of these will be treated in succession. CHEMISTRY OF PYROXYLINE. Gun-Cotton or _Pyroxyline_ is Cotton or Paper which has been altered in composition and properties by treatment with strong acids. Both Cotton and Paper are, chemically, the same. They consist of fibres which are found on analysis to have a constant composition, containing three elementary bodies, Carbon, Hydrogen, and Oxygen, united together in fixed proportions. To this combination the term _Lignine_ or _Cellulose_[12] has been applied. [12] Lignine and Cellulose are not precisely identical substances. The latter is the material composing the cell-wall; the former, the contained matter in the cell. Cellulose is a definite chemical compound, in the same sense as Starch or Sugar, and consequently, when treated with various reagents, it exhibits properties peculiar to itself. It is insoluble in most liquids, such as Water, Alcohol, Ether, etc., and also in dilute acids; but when acted upon by Nitric Acid of a certain strength it liquefies and dissolves. It has been already shown (p. 12) that when a body dissolves in Nitric Acid the solution is not usually of the same nature as an aqueous solution; and so in this case--the Nitric Acid imparts Oxygen first to the Cotton, and afterwards dissolves it.