_Remarks._--In the preparation of this sensitive collodion, it is necessary to be cautious and not add too much of the iodide of potassium and silver, for in that case the coating would flake off, and falling into the silvering solution, the operator would be obliged to filter it before he could silver his plate with safety as regards spotting it. The method I employ is to add the plain collodion, bromide and iodide of potassium and silver, iodide of ammonium and hydro-bromic acid, and then cautiously add the double iodide of potassium and silver from five to ten drops at a time, trying the collodion from time to time by pouring a little on a narrow strip of glass, which I dip into the silvering solution, and let it remain for two minutes. If the coating assumes the proper color (a cream color), I shake the contents of the bottle, and then stand it aside to settle: it is better after it has stood for a week or two. This collodion I have used after it has been made eight months, and produced fine and satisfactory results, and use this nearly altogether in practice. Since the first edition of this work has been issued, I have sold over two thousand pounds of this preparation, and the demand is on the increase. I will append another preparation (No. 2) which I have successfully employed, and some operators prefer. Bromo-Iodized Collodion for Positives.--No. 2. Plain collodion 10 ounces. Iodide of potassium 30 grains. Bromide of ammonium 20 " Enough of the double iodide of potassium and silver to give the coating a cream color when it comes from the silvering solution. It will take from one to three drachms. Or this last may be omitted, and a few drops of a saturated solution of dry iodine in alcohol may be added. Either of these plans have been successful in my practice. _Remarks._--The iodide of potassium being insoluble in the collodion, it should be first dissolved in as little water as possible; _i. e._, take the quantity, 30 grains, put it into a one-ounce graduate, and with a glass rod stir it, adding water, drop by drop, only until all of the salt is dissolved. Then it may be poured into the collodion, and there will be a white powdery precipitate. The bromide of ammonium will dissolve in the collodion, and can be put into it. When all of the accelerators are in, it should be well shaken, and then allowed to settle and become clear. When wanted, a sufficient quantity may be poured into a vial (see Fig. 34) for use, and the main or stock bottle should not be disturbed oftener than necessary. This last collodion is not as durable as the first, but is less trouble to prepare. Bromo-Iodized Collodion for Negatives. [Sidenote: IODIZED COLLODION FOR NEGATIVES]