COLLODION (Gr. kollao, to stick). This substance, on its first introduction, employed in surgery as a preservative of wounds, etc., from contact of air, by means of the tena cious and transparent film which it leaves on evaporation—and now, also, in a slightly modified form, remarkable as the basis of a highly sensitive process for taking pictures by the agency of be defined as a solution of pyroxyline in a mixture of alcohol and ether, to which is added, for photographic operations, a small quantity of some soluble iodide, bromide, or chloride.
The first step in making C. is the preparation of pyroxyline: Take 10 fluid oz. of sulphuric acid, specific gravity 1.84, in a dish; add 12 fluid drams of water, and 10 fluid oz. of nitric acid, specific gravity 1.45; and raise the temperature to 140° by immersing. the dish in boiling, water. One oz. of pure linen or paper should now be immersed in small pieces at a time, keeping the mixture in motion until the liquid is nearly absorbed by the linen or paper.
The action having continued for about ten minutes, the contents of the dish should be plunged into a vessel containing a large quantity of water, which should be speedily poured of and repeatedly changed, until the linen or paper is so thoroughly washed as not to give the slightest indication of acid to the most delicate test-paper, after which it should be slowly dried at a low temperature, and preserved in a glass bottle for use.
One hundred and four grains of the pyroxyline thus prepared are dissolved in a mixture of 16 oz. of pure sulphuric ether, and 2 oz: alcohol, specific gravity .840; and to this mixture are added 48 grains iodide of cadmium, 30 grains iodide of potassium, and 25 grains bromide of cadmium, dissolved in 4 oz. of alcohol, of specific gravity .840. C. thus prepared should, after becoming clear by subsidation, yield a rich creamy film of iodide and bromide of silver on immersion in the nitrate bath.
The foregoing formula yields a C. very suitable for taking what are termed negative impressions in the camera; but when it is desired to take positive pictures, cotton wool should be substituted for linen or paper in the preparation of the pyroxylin°, and the iodizing solution should contain iodide of ammonium instead of iodide of potassium. It is important also that the alchol employed should be free from organic impurities in the shape of fusel and grain oils, small traces of which mar the purity of the high light in a positive, though their presence is comparatively harmless in a negative pho tograph. For the details of manipulation involver in, and apparatus required for, the practice of the C. process, the reader is referred to the excellent treatise by Mr. Hard wich on that subject.