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

66 THE ADVANCE OF PHOTOGRAPHY blue iodide of starch, and thus colours the lines of the picture. There are several other methods of making the colour of the iron pictures darker. The pictures in Prussian blue gradually fade, for this dye is bleached by light (blue parasols rapidly lose their colour in the light). The same remark applies to pictures of iodide of starch ; the gold pictures are too pale, and their preparation too costly. The salts of uranium present the same phenomena as the salts of iron. Uranium itself is a rare metal, the salts of which are much used as colouring materials ; thus, there is a yellow oxide which, when burnt into porcelain, colours it dark green, and fused with glass, imparts to it a beautiful grass green (uranium glass). The best known salt of uranium is the nitrate, which in the presence of organic matter (for example, paper) is reduced by light to uranous nitrate. If a piece of paper be immersed in a solution of one part of the salt in five of water, dried in the dark, and exposed under a negative, a very faint picture is produced, which consists of uranous oxide. If the print is now immersed in a silver or gold solution, the picture becomes at once visible, since uranous oxide precipitates gold and silver as such from their solution (silver as a brown, gold a violet powder). Uranium is, however, too scarce and too expensive to be employed generally in photography. As will be perceived, the salts of iron and uranium are analogous to the salts of chromium, by only being sensitive to light in the presence of organic bodies. In a pure state salts of uranium and iron do not change in the light. Copper forms with chlorine a green salt, soluble in water — cupric chloride, — which is reduced to cuprous chloride in the light. Obernetter took advantage of this fact, by mixing chloride of copper and chloride of iron together, and saturating paper with them. This was