Collodion prepared by Formula No. 1, and iodized with Iodide of Cadmium, may be kept for weeks or months without much loss of sensitiveness; but when Alkaline Iodides are employed as in the second Formula, Iodine is liberated, and the fluid becomes at last brown and insensitive. THE NITRATE BATH. Nitrate of Silver 30 grains. Nitric Acid 1/20 minim, or Acetic Acid (glacial) 1/6 minim. Alcohol 15 minims. Distilled water 1 fluid ounce. Nitrate of Silver which has been melted, in order to expel Oxides of Nitrogen, is always the most certain in its action: but the heat must not be raised too high or the salt will be contaminated with _Nitrite_ of Silver. In the Vocabulary (see Part III.) directions are given for the preparation and purification of Nitrate of Silver; also for the testing of distilled water, and the best substitutes when it cannot be obtained. The Bath must be saturated with Iodide of Silver, and Nitric Acid neutralized if it be present. Nitrate of Silver however which has undergone fusion is free from Nitric Acid. Weigh out the total quantity of crystals of Nitrate required for the Bath, and dissolve in about two parts of water. Then take a quarter of a grain of Iodide of Potassium to each 100 grains of Nitrate, dissolve in half a drachm of water, and add to the strong solution; a yellow deposit of Iodide of Silver first forms, but on stirring is completely re-dissolved. When the liquid is clear, test for free Nitric Acid by dropping in a piece of blue litmus-paper. If at the expiration of two minutes the paper appears _reddened_, Nitric Acid is present, to neutralize which, add solution of Potash or Carbonate of Soda (not Ammonia) until a distinct turbidity, remaining after agitation, is produced (an excess does no harm). Then dilute down the concentrated solution with the remaining portion of the water, stirring all the time, and filter out the milky deposit. If the liquid does not at first run clear, it will probably do so on passing it again through the same filter. Lastly, add the Acetic Acid (previously tested for impurities, see Part III.) and the Alcohol to the filtered liquid. As the bulk of the Bath becomes lessened by use, fill it up with a solution containing 40 grains of Nitrate to the ounce, which will be found sufficient to maintain the strength nearly at the original point. The common practice of occasionally dropping Ammonia or Potash into the solution, to remove Nitric Acid liberated by free Iodine in the Collodion, is not recommended (see p. 89). When the Bath becomes old, and yields Positives which are highly intense or stained, and slightly foggy, with a deficiency of half-tone, it will be advisable to precipitate it with a Chloride and prepare a new one. THE DEVELOPING FLUIDS. Either of the three following formulæ may be used, according to the taste of the operator:-- FORMULA No. 1.