COLLODION, a colourless, viscid fluid, made by dissolving gun-cotton and the other Varieties of pyroxylin in a mixture of alcohol and ether. It was discovered in 1846 by Louis Nicolas Menard in Paris, and independently in 1848 by Dr. J. Parkers Maynard in Boston. The quality of collodion differs according to the proportions of alcohol and ether and the nature of the pyroxy lin it contains. Collodion in which there is a great excess of ether gives by its evaporation a very tough film; the film left by col lodion containing a large quantity of alcohol is soft and easily torn. Under the microscope, the film produced by collodion of good quality appears translucent and colourless. To preserve collodion it should be kept cool and out of the action of the light. For the iodizing of collodion, ammonium bromide and iodide, and the iodides of calcium and cadmium are the agents employed. (See PHOTOGRAPHY.) Collodion is used in surgery since, when painted on the skin, it rapidly dries to a thin contractile film, affording both pressure and protection. Flexible collodion, con taining Canada balsam and castor oil, does not crack, but, on the other hand, does not contract. M. E. Gripon found (Compt. rend., 1875) that collodion membranes, like glass, reflect light and polarize it both by refraction and reflection; they also transmit a very much larger proportion of radiant heat, for the study of which they are preferable to mica.