_On the Photographic Properties of the Salts of Silver._ In addition to the Salts of Silver described in the first Section of this Chapter there are many others well known to chemists, as the Acetate of Silver, the Sulphate, the Citrate of Silver, etc. Some occur in crystals which are soluble in water, whilst others are pulverulent and insoluble. The Salts of Silver formed by colourless Acids are white when first prepared, and remain so if kept in a dark place; but they possess the remarkable peculiarity of being darkened in colour by exposure to Light. Action of Light upon the Nitrate of Silver.--The Nitrate of Silver is one of the most permanent of the Silver salts. It may be preserved unchanged in the crystalline form, or in solution in distilled water, for an indefinite length of time, even when constantly exposed to the diffused light of day. This is partly explained by the nature of the acid with which Oxide of Silver is associated in the Salt; Nitric Acid, possessing strong oxidizing properties, being opposed to the darkening influence of Light upon the Silver compounds. Nitrate of Silver may, however, be rendered susceptible to the influence of Light, by adding to its solution _organic matter_, vegetable or animal. The phenomena produced in this case are well illustrated by dipping a pledget of cotton-wool, or a sheet of white paper, in solution of Nitrate of Silver, and exposing it to the direct rays of the sun; it slowly darkens, until it becomes nearly black. The stains upon the skin produced by handling Nitrate of Silver are caused in the same way, and are seen most evidently when the part has been exposed to light. The varieties of organic matter which especially facilitate the blackening of Nitrate of Silver are such as tend _to absorb Oxygen_; hence pure vegetable fibre, free from Chlorides, such, for instance, as the Swedish filtering-paper, is not rendered very sensitive by being simply brushed with solution of the Nitrate, but a little grape sugar added soon determines the decomposition. _Decomposition of Chloride, Bromide, and Iodide of Silver by Light._--Pure moist Chloride of Silver[4] changes slowly from white to violet on exposure to light. Bromide of Silver becomes of a grey colour, but is less affected than the Chloride. Iodide of Silver (if free from excess of Nitrate of Silver) does not alter in appearance by exposure even to the sun's rays, but retains its yellow tint unchanged. Of these three compounds therefore _Chloride_ of Silver is the most readily acted on by light, and papers prepared with this salt will become far darker on exposure than others coated with Bromide or Iodide of Silver. [4] The Chloride here spoken of is the compound prepared by adding a soluble Chloride to a solution of Nitrate of Silver: the product of the direct action of Chlorine upon metallic Silver is sometimes insensitive to light.