_Preparation of Soluble Cotton._--I put into a Wedgewood mortar, twelve ounces, by weight, of dry and finely pulverized nitrate of potash (I use Dupont's refined), and add to it twelve ounces, by measure, of good commercial sulphuric acid; I mix these well together by the use of a glass rod and pestle, so that it forms a paste; I then add, in small quantities at a time, about 325 grains of good coarse cotton (this is according to your process), and _knead_ the mass well for from three to five minutes; and then cover the mortar with a piece of glass, and let it stand for twenty or thirty minutes, by which time it will have been sufficiently acted upon. Then the cotton is to be plunged into a quantity of clean water and thoroughly washed in a number of changes of water, so that when it is squeezed between the folds of blue litmus-paper it will not redden it; this indicates that the acid has been washed out; I then place the cotton in a clean strong towel and wring out all the water I can, then put it into alcohol, then wring it again, and adopt your plan of not letting it become thoroughly dry. The cotton being now ready for use, I dissolve it in the following mixture, in a bottle of proper size:-- Sulphuric ether 10 ounces. Alcohol, 95 per cent. 5 ounces. Soluble cotton, enough to make it about as thick as cream. The above should be well shaken, and then allowed to stand for one or two days to settle. This constitutes my Plain Collodion, and should be poured off into another bottle, leaving the sediment behind. _Iodizing._--For this purpose I employ the following preparation:-- _A._--_Iodide of Silver._--Dissolve fifty grains of nitrate of silver, crystals, in two ounces of pure water, and forty grains of iodide of ammonium in two ounces of pure water; then pour the two together, shaking the mixture well; let it settle, which it will do in a few minutes; then pour off the water carefully, leaving behind all the yellow iodide of silver; pour again a fresh quantity of water over the precipitate, and continue this washing for at least six changes of water; then drain off the water as close as possible and pour on two ounces of alcohol, which, when drained off, will leave the powder sufficiently clean for the purpose. _B._--I now dissolve forty grains of bromide of ammonium and one hundred grains of iodide of ammonium in two ounces of 95 per cent, alcohol, and then add the iodide of silver (preparation _A_), and shake the whole well together, giving a saturated solution. This is to be filtered through cotton or paper, when it will be ready for use.