Collodion, Neg′ative

(Hardwich.) To form the PYROXYLIN :—Take of sulphuric acid, sp. gr. 1·845, at 60°, 18 fl. oz.; nitric acid, sp. gr. 1·475, at 60°, 6 fl. oz.; water, 5 1 ⁄ 4 fl. oz.; cotton, 300 grains. Mix, and allow the temperature to fall to 150° Fahr. The weight of the pyroxylin ought to be 375 grains. To form the PLAIN COLLODION :—Take alcohol, sp. gr. ·806, 1 ⁄ 2 gallon; ether, sp. gr. ·725, 1 gall.; pyroxylin, 1900 grains. Saturate the pyroxylin with the alcohol, then pour in half a gallon of the ether, agitate for 3 or 4 minutes, and repeat the proce in adding the remainder. Decant the clear liquid from the sediment after a week or ten days’ rest. The following forms for IODISING SOLUTIONS are recommended:— a. (Pota ium Iodiser.) Iodide of pota ium, 135 grains; alcohol, sp. gr. ·816, 10 fl. oz. Powder and di olve in the alcohol, previously heated to 140°.— b. (Cadmium Iodiser.) Iodide of cadmium, 170 grains; alcohol, sp. gr. ·816, 10 fl. oz. Di olve in the cold, and filter.— c. (Bromo-iodiser.) Bromide of ammonium, 40 grains; iodide of ammonium, 90 grains; iodide of cadmium, 90 grains; alcohol, sp. gr. ·816, 10 fl. oz. Pulverise and di olve in the cold. To sensitise the collodion, add to three parts one part of either a , b , or c . Obs. Most of the practical directions given under the head of POSITIVE COLLODION apply equally to NEGATIVE COLLODION . Nothing but patient and intelligent practice will ever lead to succe in preparing collodion for photographic purposes. Although formulæ of undoubted excellence may be used, it continually happens that the results are entirely nugatory from some trifling cause. See Photography .