Heated in a crucible the salt melts, and when poured into a mould and solidified, forms the white _lunar caustic_ of commerce. At a still higher temperature it is decomposed, and bubbles of Oxygen Gas are evolved: the melted mass cooled and dissolved in water leaving behind a black powder, and yielding a solution, which is faintly alkaline to test-paper, from the presence of minute quantities of Nitrite or basic Nitrite of Silver.[2] [2] Nitrite of Silver differs from the Nitrate in containing less Oxygen, and is formed from it by the abstraction of two atoms of that element; it is described in the vocabulary, Part III. THE CHEMISTRY OF THE CHLORIDES OF SILVER. _Preparation of Protochloride of Silver._--The ordinary white Chloride of Silver may be prepared in two ways,--by the direct action of Chlorine upon metallic Silver, and by double decomposition between two salts. If a plate of polished silver be exposed to a current of Chlorine Gas,[3] it becomes after a short time coated on the surface with a superficial film of white powder. This powder is Chloride of Silver, containing the two elements Chlorine and Silver united in single equivalents. [3] For the properties of the element "Chlorine," see the third division of the Work. _Preparation of Chloride of Silver by double decomposition._--In order to illustrate this, take a solution in water of Chloride of Sodium or "common salt," and mix it with a solution containing Nitrate of Silver; immediately a dense, curdy, white precipitate falls, which is the substance in question. In this reaction the elements change places; the Chlorine leaves the Sodium with which it was previously combined, and crosses over to the Silver; the Oxygen and Nitric Acid are released from the Silver, and unite with the Sodium; thus Chloride of Sodium _plus_ Nitrate of Silver _equals_ Chloride of Silver _plus_ Nitrate of Soda. This interchange of elements is termed by chemists double decomposition; further illustrations of it, with the conditions necessary to the proper establishment of the process, are given in the first Chapter of Part III. The essential requirements in two salts intended for the preparation of Chloride of Silver, are simply that the first should contain Chlorine, the second Silver, and that both should be soluble in water; hence the Chloride of Potassium or Ammonium may be substituted for the Chloride of Sodium, and the Sulphate or Acetate for the Nitrate of Silver. In preparing Chloride of Silver by double decomposition, the white clotty masses which first form must be washed repeatedly with water, in order to free them from soluble Nitrate of Soda, the other product of the change. When this is done, the salt is in a pure state, and may be dried, etc., in the usual way.