Collodion, Pos′itive

(Hardwich.) To form the PYROXYLIN :—Take sulphuric acid, sp. gr. 1·845, at 60° Fahr., 12 fl. oz.; nitric acid, sp. gr. 1·45, at 60°, 12 fl. oz.; water, 3 1 ⁄ 2 fl. oz.; mix, and allow the temperature to fall to 140°; then immerse cotton, 300 grains. (If the cotton is found to gelatinise or di olve in the acid mixture, the quantity of water is too great, and may be reduced to 3 fl. oz.) The cotton should be well pulled out in pieces, weighing about 30 grains each; and should be left in the acid for about 8 minutes, the ve el being covered over. It is taken out with a gla spatula, squeezed to remove acid, washed for at least 24 hours by a stream of water, then squeezed in a cloth, and pulled out to dry. To form the PLAIN COLLODION :—Shake up the dry pyroxylin, 48 grains, with alcohol, sp. gr. ·805, 1 1 ⁄ 2 fl. oz., and then add ether, sp. gr. ·725, 4 1 ⁄ 2 fl. oz. The solution should be allowed to rest for a week or ten days, when the clear fluid should be decanted from the sediment. To prepare the IODISING SOLUTION :—Take of iodide of ammonium, 1 1 ⁄ 2 dr.; iodide of cadmium, 1 1 ⁄ 2 dr.; bromide of ammonium, 40 grains; powder, and di olve in alcohol, sp. gr. ·805 to ·816, 10 fl. oz. The collodion is iodised by adding the solution to it in the proportion of 1 part solution to 3 parts collodion. The iodised collodion should be kept for at least six weeks before using. If required for immediate use, add a few drops of an alcoholic solution of iodine, formed by di olving 5 grains of iodine in 1 fl. oz. of alcohol. Obs. Mr Hardwich recommends that the cotton, before being converted into pyroxylin, should be cleansed by boiling for two hours in a solution of caustic pota a (2 oz. to the gallon), and by being afterwards repeatedly washed and dried. The purest nitric acid, sp. gr. 1·45, should be employed, but the ordinary commercial sulphuric acid (oil of vitriol) is sufficiently pure for use. To purify the ETHER and to get rid of a certain ozonised principle which would decompose the iodising solution, Mr Hardwich recommends the following proce :—Take the best washed ether of commerce and agitate it thoroughly with a small portion of dilute sulphuric acid, and then introduce it into a retort, and distil over one third. The alcohol used is of the strength of that sold for absolute alcohol; it should be pure.