COLLODION (from κoλλa, glue), a colourless, viscid fluid, made by dissolving gun-cotton and the other varieties of pyroxylin, or cellulotrinitrin, C g H 7 O 2 (NO 3 ) 3 , in a mix ture of alcohol and ether. It was discovered in 1848 by Maynard in Boston. The quality of collodion differs according to the proportions of alcohol and ether and the nature of the pyroxylin it contains. Collodion in wsliich there is a great excess of ether gives by its evaporation a very tough film; the film left by collodion containing a large quantity of alcohol is soft and easily torn; but in hot climates the presence of an excess of alcohol is an advantage, as it prevents the rapid evaporation of the ether. Pyroxylin for the making of collodion for photographic purposes is prepared by immersing cotton-wool ten minutes in a mixture of nitric and sulphuric acids at a temperature of 140° Fahr. When tolerably strong acids at a low temperature are employed, the nitric acid being in by far the larger quantity, the pyroxylin made requires an amount of alcohol equal to only about ⅜th or τ ⅛th that of the ether in bulk. According to Μ. Miallie, the most explosive kinds of gun-cotton are not the best adapted for the preparation of collodion; a pyroxylin very soluble in ether is in his process made from 2 parts by weight of carded cotton to 40 parts of nitre and 60 of concentrated sulphuric acid. Under the microscope, the film produced by collodion of good quality appears translucent and colourless, the cotton being perfectly dissolved; old collodion that does not give good photographic impressions sufficiently quickly exhibits liquid globules of modified ether. The film from collodion which is too alcoholic has the microscopic appearance of cellular tissue; and when water has been present, ...