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PHOTOGRAPHIC IMPORTANCE OF CHROMIUM COMPOUNDS 81
chromium is formed. In this case the picture appears fainter.
Ponton's experiment remained a mere curiosity until the inventor of photography on paper containing silver salts discovered another property of chromate of potash, which led to the most extensive applications.
Action of Chromium Compounds on Glue. — This property consists in the action of the combinations of chromium on glue.
Glue in its purest form, known by the name of gelatine, is insoluble in cold water, but it absorbs cold water like a sponge, and thereby swells. If it is warmed with water, it dissolves ; but on cooling the solution solidifies to a jelly. This property is used to thicken soups. If to the warmed solution of glue be added alum, or a salt of the oxide of chromium, or chrome alum, the glue becomes insoluble in water, and forms a precipitate. On this is based the well-known system of white tanning ; for in the tanning of a piece of leather the alum combines with the gelatine contained in the leather, and this becomes thereby insoluble, and at the same time durable.
Bichromate of potash and glue can be dissolved together in warm water in the dark, without the glue suffering any change from the chromate. If a plate or a sheet of paper be covered with such a solution of gelatine and bichromate of potash, and the film be allowed to dry, it becomes firm, and yet remains soluble in water as long as it is kept in the dark. But as soon as the film is exposed to the light, the bichromate of potash is reduced to oxide of chromium, and this tans the film of gelatine — that is, makes it insoluble in water.
Talbot's Discovery. — This observation was made by Fox Talbot in 1852, and, as a careful observer, he knew directly how to turn it to account. He coated a steel plate with a solution of chromate and gelatine, let it dry in the dark, and then exposed it under a drawing or a positive glass
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