_Alkaline condition of the Bath._--By "alkalinity" of the Bath is meant a condition in which the blue tint is rapidly restored to reddened litmus-paper. This indicates that an Oxide of some kind is present in solution, which, by combining with the acid in the reddened paper, neutralizes it and removes the red colour. If a small portion of caustic Potash or Ammonia be added to a strong solution of Nitrate of Silver, it produces a brown precipitate, which is Oxide of Silver. Ammonia + Nitrate of Silver = Oxide of Silver + Nitrate Ammonia. The solution however, from which the precipitate has separated, is not left in a neutral state, but possesses a faint alkaline reaction. Oxide of Silver and Carbonate of Silver are also _abundantly_ soluble in water containing Nitrate of Ammonia; which salt is continually accumulating in the Bath when compounds of Ammonium are used for iodizing. An alkaline Bath is perhaps of all conditions the one most fatal to success in photography. It leads to that universal darkening of the film on applying the developer to which the name of "fogging" has been given. Hence care must be used in adding to the Bath substances which tend to make it alkaline. Collodion containing free Ammonia, often sold in the shops, gradually does so. The use of Potash, Carbonate of Soda, Chalk, or Marble, to remove free Nitric Acid from the Bath, has the same effect; and hence, when they are employed, a trace of Acetic acid must afterwards be added. The mode of testing a bath for alkalinity is as follows:-- a strip of porous blue litmus-paper is taken and held to the mouth of a bottle of glacial Acetic acid until it becomes reddened; it is then placed in the liquid to be examined and left for ten minutes or a quarter of an hour. If Oxide of Silver be present in solution, the original blue colour of the paper will slowly but gradually be restored. _Occasional formation of Acetate of Silver in the Nitrate Bath._--In preparing a new Bath, if the crystals of Nitrate of Silver are acid, it is usual to add an alkali in small quantity. This removes the Nitric Acid, but leaves the solution faintly alkaline. Acetic Acid is then dropped in, which, by combining with the Oxide of Silver, forms Acetate of Silver. Acetate of Silver is not formed by the simple addition of Acetic Acid to the Bath, because its production under such circumstances would imply the liberation of Nitric Acid; but if an alkali be present to neutralize the Nitric Acid, then the double decomposition takes place, thus-- Acetate of Ammonia + Nitrate of Silver = Acetate of Silver + Nitrate of Ammonia.