The advance of photography : its history and modern applications (1911)

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THE CHEMICAL ACTION OF LIGHT 65 substances sensitive to light is much greater than appears at first sight ; and in fact, on close investigation, it would probably be found that all substances are more or less sensitive. Even in the first period of photography, in the year 1840, Herschel observed the sensitiveness of salts of iron, Burnett that of salts of uranium, and Kratochvila prepared successful daguerreotypes on copper plates, in a manner analogous to that for silver plates. It has long been known that a solution of ferric chloride, (a yellow substance composed of chlorine and iron) in ether, is bleached by light, the ferric becoming ferrous chloride, a colourless salt which contains less chlorine. The same change is produced in contact with paper ; thus, if clean paper be soaked in a solution of one part of ferric chloride in six of water, dried in the dark, and exposed under a negative, the yellow paper becomes white beneath the transparent parts of the negative, because the ferric chloride there becomes ferrous chloride. The faint picture thus produced may be easily coloured intensely dark by immersion in a solution of red prussiate of potash (Potassium Ferricyanide). This salt combines with the ferrous chloride to produce Turnbull's blue, but leaves the ferric chloride unchanged. In this manner a blue picture is obtained. If such a picture be plunged in a solution of gold, it becomes of a light blue colour, because the ferrous chloride produces a precipitate of metallic gold. And, similarly, all substances which form dark precipitates with ferrous chloride will serve to develop such pictures. Another process consists in transforming the iron pictures into iodine pictures. A positive (for example, a drawing) is printed on a piece of paper sensitized with ferric chloride ; the print comes out in yellow lines of unchanged ferric chloride, on a white ground. If this is now immersed in a solution of iodide of potassium and starch, the ferric chloride decomposes the iodide, and sets free iodine, which combines with the starch, forming the dark