Chemists have proved that no substance is sensitive to light when perfectly pure and kept by itself in a dry place. If silver chloride is placed in a glass tube, the air exhausted, and the tube hermetically sealed, it may be exposed to sunlight, and will remain unchanged in color. The action of light on the silver chloride is to separate the chlorine from the silver, but there must be some substance with which the chlorine will combine, or the light will not decompose the chloride. (Decompose is to separate the parts composing a compound body.) For purposes of photography some substance must be used which will combine very quickly with the silver, and such a substance is found in silver nitrate. The chlorine set free by the action of light combines at once with the pure silver in the nitrate of silver. The chemical nature of the dark-colored substance produced by the action of light on the silver chloride is not yet fully determined, but most chemists agree that the silver chloride, when decomposed by light, produces silver sub-chloride and chlorine. (A sub-chloride is a chloride which contains more of the base than the acid. A molecule of silver chloride contains one atom of silver and one of chlorine, while a molecule of sub-chloride contains _two_ atoms of silver and one atom of chlorine.) This silver chloride is white, but passes through different shades of coloring, from a reddish-violet to a deep purplish-black, according to the length of time it is exposed to the light. When paper coated with a sensitive silver solution is placed under a negative and exposed to the sun, the light reaches the paper through different degrees of thickness, or density, in the gelatine film. In the part of the negative which represents the sky the film is quite thick, while in the part which represents the deep shadows it is thin, sometimes being almost transparent. The part of the paper which is under the portion of the negative representing the sky is scarcely affected by the light, but in that part representing the shadows the light acts at once, and quickly decomposes the silver chloride. When the paper is taken from the printing-frame it contains different grades of the deposit formed by the action of light on the silver chloride. NOTE.--The first article in the series of "Chemistry of Photography" was published in No. 867 (June 9). This article was on the chemical elements, and contained the following paragraph: