Rectified Ether, sp. gr. ·750 6 fluid drachms. Spirits of Wine, sp. gr. ·836 2 " " Pyroxyline 2 to 4 grains. Iodide of Potassium or Ammonium 3 to 4 " If the operator wishes to prepare a stock of the plain Collodion, and to iodize as required, the last formula will stand thus:-- Rectified Ether, ·750 3 fluid ounces. Alcohol of ·836 2 fluid drachms. Pyroxyline 8 to 14 grains. Dissolve the Pyroxyline, and let the fluid stand for forty-eight hours to subside, then draw off clear, with a siphon. To each fluid ounce of this plain Collodion add about two fluid drachms of the following iodizing mixture:-- Alcohol, sp. gr. ·836 1 fluid ounce. Iodide of Potassium 16 grains. Of the two formulæ above given, the first is considered the best, but the second may be substituted for it when highly rectified spirits cannot be obtained. Iodide of Ammonium chemically pure is perhaps superior to any other Iodide for preparing a portrait Collodion, but Iodide of Cadmium, with addition of free Iodine, possesses better keeping properties, and gives very good results. A mixture of the two Iodides may also be used advantageously, or Iodide of _Potassium_ may be combined with Iodide of Cadmium: this preparation has been much recommended, but the Collodion will be liable to produce a spotted film unless the salts are quite pure. The exact quantity of Pyroxyline will vary with the temperature at which the preparation was made. The Collodion should flow smoothly on the glass and remain free from crapy lines on setting. When Iodide of Cadmium is used, the tendency to glutinosity will be a little greater than usual, which must be obviated by the directions given at page 83. The film, after dipping in the Bath, should appear opalescent and not too yellow and creamy. Pale-blue films yield very good Positives, but with more liability to failure than thicker films (p. 109). If the Positives are not perfectly clear and transparent in the shadows, dissolve 5 grains of Iodine in an ounce of Spirits of Wine (not methylated), and add a few drops until the Collodion assumes a golden-yellow colour. In hot weather advantage will be gained by somewhat increasing the quantity of Alcohol in Collodion; the evaporation of the solvents being retarded, and the film rendered less liable to become dry before development. _Anhydrous_ Alcohol of Sp. Gr. ·796, may be mixed with pure Ether of ·715, even to the extent of equal parts; but this is the extreme limit, and with the strongest spirit ordinarily obtainable, the Collodion will often become somewhat glutinous if the proportions (by measure) of 5 parts of Ether to 3 of Alcohol be exceeded.