A simple solution of an Alkaline Iodide in Alcohol and Ether does not, however, become so quickly coloured as Iodized Collodion; and hence it is evident that the presence of the Pyroxyline produces an effect. It may be shown that Alkaline Iodides slowly decompose Pyroxyline, and that a portion of Peroxide of Nitrogen is set free: this body, containing loosely combined oxygen, tends powerfully to eliminate Iodine, as may be seen by adding a few drops of the yellow commercial Nitrous acid to a solution of Iodide of Potassium. The _stability_ of the particular Iodide used in Iodizing Collodion, influences mainly the rate of coloration, though elevation of temperature and exposure to light are not without effect. Iodide of Ammonium is the least stable, and Iodide of Cadmium the most so; Iodide of Potassium being intermediate. Collodion iodized with _pure_ Iodide of Cadmium usually remains nearly colourless to the last drop, if kept in a cool and dark place. As the presence of free Iodine in Collodion affects its photographic properties, it may sometimes be necessary to remove it. This is done by inserting a strip of Silver-foil; which decolorizes the liquid, by forming Iodide of Silver, soluble in the excess of Alkaline Iodide (p. 42). Metallic Cadmium, and metallic Zinc, have the same effect. When Methylated Spirits are employed in the manufacture of Collodion, the Iodine first liberated is afterwards either partially or entirely reabsorbed, the liquid acquiring at the same time an acid reaction to test-paper. SECTION II. _The Chemistry of the Nitrate Bath._ The solution of Nitrate of Silver in which the plate coated with iodized Collodion is dipped, to form the layer of Iodide of Silver, is known technically as _the Nitrate Bath_. The chemistry of Nitrate of Silver has been explained at page 13, but there are some points relating to the properties of its aqueous solution which require a further notice. _Solubility of Iodide of Silver in the Nitrate Bath._--Aqueous solution of Nitrate of Silver may be mentioned in the list of solvents of Iodide of Silver. The proportion dissolved is in all cases small, but it increases with the _strength_ of the solution. If no attention were paid to this point, and the precaution of previously saturating the Nitrate Bath with Iodide of Silver neglected, the film would be dissolved when left too long in the liquid. This solvent power of Nitrate of Silver on the Iodide is well shown by taking the excited Collodion plate out of the Bath, and allowing it to dry spontaneously. The layer of Nitrate on the surface, becoming concentrated by evaporation, eats away the film, so as to produce a transparent, spotted appearance.