Collodion (from the Gr. κόλλα, glue), 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 Ménard 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 pyroxylin it contains. Collodion in which 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. 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; iodized collodion that has been discoloured by the development of free iodine may be purified by the immersion in it of a strip of silver foil. 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 and covers the skin with a thin film which contracts as it dries and therefore affords both pressure and protection. Flexible collodion, containing Canada balsam and castor oil, does not crack, but, on the other hand, does not contract. It is therefore of less value. Collodion is applied to small aseptic wounds, to small-pox pustules, and occasionally to the end of the urethra in boys in order to prevent nocturnal incontinence. Collodion and crystals of carbolic acid, taken in equal parts, are useful in relieving toothache due to the presence of a carious cavity. Vesicating or Blistering Collodion contains cantharidin as one of its constituents. The styptic colloid of Richardson is a strong solution of tannin in gun-cotton collodion. Similarly collodion may be impregnated with salicylic acid, carbolic acid, iodine and other substances. Small balloons are manufactured from collodion by coating the interior of glass globes with the liquid; the film when dry is removed from the glass by applying suction to the mouth of the vessel. 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.
Collodion
