d. _Impurities in the soluble Iodides._--Commercial Iodide of Potassium often contains _Iodate_ of Potash, which is found to have a retarding effect upon the action of light; also Carbonate of Potash, which, in Collodion, produces Iodoform,[14] and in the paper processes, where "Aceto-Nitrate" is used for sensitizing, forms Acetate of Silver. Iodoform has a marked influence in diminishing the sensitiveness of Iodide of Silver; Acetate of Silver may perhaps increase it a little by securing the absence of free Nitric Acid (p. 117). Iodide of Potassium prepared by the process in which Sulphuretted Hydrogen and Alcohol are used, and having a smell of Garlic, contains probably Xanthate of Potash, and is nearly useless for Photography. [14] See the Vocabulary, Part III., Art. Iodoform. Commercial Iodide of Cadmium is a purer salt than the Iodide of Potassium, and may be advantageously substituted for it; but it possesses the property of coagulating Albumen, and hence cannot be employed in conjunction with that substance. e. _Presence of free Iodine._--Both in the waxed paper and the Collodion processes, the solutions often contain a small quantity of free Iodine. This Iodine, in contact with the Nitrate of Silver of the Bath, produces a mixed Iodide and _Iodate_ of Silver, and liberates Nitric Acid. It thus retards the sensitiveness of the film in proportion to the quantity of Iodine present. Collodion of a full yellow colour is perceptibly less sensitive than the same rendered colourless; and when enough Iodine has been liberated to give a red or brown tint, double the original exposure will probably be required. If brown Collodion be much used, the Nitrate Bath may by degrees become sufficiently contaminated with free Nitric Acid to interfere with the sensitiveness of the film; but if colourless or lemon-yellow tinted Collodion be employed, this evil need not be anticipated. Certain substances may be added to coloured Collodion which possess the property of counteracting the retarding influence of the free Iodine, such, for instance, as the Oils of Cloves, Cinnamon, etc.; they probably act in virtue of their affinity for Oxygen, by preventing the formation of Iodate of Silver. In colourless Collodion they produce little or no effect, neither do they remove the insensitiveness of the film when dependent upon a too acid condition of the Nitrate Bath.