In the solution of Iodide of Silver by Nitrate of Silver a _double salt_ is formed, which corresponds in properties to the double Iodide of Potassium and Silver in being _decomposed_ by the addition of water. Consequently, in order to saturate a Bath with Iodide of Silver it is only necessary to dissolve the total weight of Nitrate of Silver in a small bulk of water, and to add to it a few grains of an Iodide; perfect solution takes place, and on subsequent dilution with the full amount of water, the excess of Iodide of Silver is precipitated in the form of a milky deposit. _Acid condition of Nitrate of Silver._--A solution of _pure Nitrate of Silver_ is neutral to blue litmus-paper, but that prepared from the commercial Nitrate has usually an acid reaction; the crystals having been imperfectly drained from the acid mother-liquor in which they were formed. Hence, in making a new Bath it is often advisable not only to saturate it with Iodide of Silver, but to neutralize the free Nitric acid it contains. There is also a peculiar condition of Nitrate of Silver crystallized from a solution of the metal in Nitric Acid, which renders it quite unfit for photographic purposes (see p. 101). It is thought to depend upon the presence of an oxide of Nitrogen, possibly of Nitrous Acid, and the remedy is to dry the crystals very strongly, or, better still, to fuse them at a moderate heat: mere neutralization with Carbonate of Soda does not suffice. In melting Nitrate of Silver great care should be taken not to raise the heat so high as to decompose the salt, or a basic Nitrite will be formed, which affects the properties of the solution (p. 13): fused Nitrate of Silver ought, when cold, to be quite white, and to dissolve perfectly in water without leaving any residue. The only objection to the employment of Nitrate of Silver in this form is the facility with which it may be adulterated with Nitrates of Potash and Soda, the presence of which would lessen the available strength of the Bath. The Nitrate Bath, although perfectly neutral when first prepared, may become _acid_ by continued use, if Collodion containing much _free Iodine_ be constantly employed. In that case a portion of Nitric Acid is liberated, thus:-- Nitrate of Silver + Iodine = Iodide of Silver + Nitric Acid + Oxygen. When Collodion is iodized entirely with alkaline Iodides, it liberates Iodine by keeping; and hence the occasional use of Ammonia may be required to remove acidity from the Bath. But since the introduction of the Iodide of Cadmium, which preserves the Collodion nearly or quite colourless, the necessity for neutralizing Nitric Acid in the Bath has ceased.