SILVER CHLORIDE.
When metallic silver is covered with sufficient nitric acid, the silver will be dissolved, and on evaporating this solution, flaky transparent crystals will result. This N silver nitrate, to which chemists give the formula it being a compound of silver (argentum), nitro gen, and oxygen in the proportions indicated—that is, 1 atom of silver, 1 atom of nitrogen, and 3 of oxygen. If the silver nitrate is dissolved in distilled water, which should always he used for gold, silver or oxalates, a perfectly clear solu tion will result. Place a small quantity of this in a test tube, and in another test tube dissolve a pinch of common table salt, known chemically as chloride of sodium, and having the formula .NaCl. Pure distilled water should again be used, so that one tube contains sodium, chlorine, and water, and the other the three elementary substances mentioned above. Now if a portion of the salt solution is added to the silver nitrate (AgNO,), there will immediately be a heavy precipitate of silver chloride (:gel) which is of a cream colour. The
silver chloride is insoluble in water, and therefore sinks to the bottom. What has taken place ? + NaC1 = AgCl + or silver nitrate added to sodium chloride forms silver chloride and sodium nitrate, this being the man ner in which the change (known as '' double decomposition," or " mutual chemical exchange '') is expressed by chemists. The silver having a greater affinity for the chlorine than it has for the other substances, readily combines with it. If this silver chloride is exposed to sunlight, and allowed to remain for a time, it will slowly darken to a violet colour, whilst parts shielded from the light will remain white. Such was the crude form of printing in the earliest photo graphic processes ; its limitations will be at once apparent.