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.
Preparation of Pyroxyline.—If, instead of treating Cotton with Nitric Acid, a mixture of Nitric and Sulphuric Acids in certain proportions be used, the effect is peculiar. The fibres contract slightly, but undergo no other visible alteration. Hence we are at first disposed to think the mixed Acids ineffectual. This idea however is not correct, since on making the experiment the properties of the cotton are found to be changed. Its weight has increased by more than one-half; it has become soluble in various liquids, such as Acetic Ether, Ether and Alcohol, etc., and, what is more remarkable, it no longer burns in the air quietly, but explodes on the application of flame with greater or less violence.
This change of properties clearly shows, that although the fibrous structure of the material is unaffected, it is no longer the same substance, and consequently chemists have assigned it a different name, viz. Pyroxyline.